control of reactive plasmas for low-k/cu integration
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Applied Surface Science 253 (2007) 6716–6737
Control of reactive plasmas for low-k/Cu integration
Tetsuya Tatsumi *
STDD, Semiconductor Business Group, Sony Corporation, 4-14-1 Asahi-cho, Atsugi-shi, Kanagawa 243-0014, Japan
Available online 12 February 2007
Abstract
We proposed models for controlling surface reactions during etching of SiOCH and organic material. The etch rate of each material can be
determined by the balance between the total atom fluxes of O, C, F, N, and H that were supplied from both the plasma and the etched material to the
reactive layer. Low-k films (SiOCH, porous SiOCH, and organic material) have narrow process windows for obtaining good etching properties,
such as selectivity, because the polymer and reactive layers on these films can be changed by only slight changes in the plasma parameters.
Therefore, the partial pressure and dissociation of parent gas molecules in fluorocarbon or N–H plasma as well as plasma–wall interaction must be
controlled. To create highly reliable interconnects, the interfaces between the metal and low-k must be optimized during the etching of stopper
material and ashing. The surface of Cu is very reactive, the remaining F induces degradation of Cu. SiOCH can easily be oxidized during ashing
processes, and the adsorption of H2O on damaged SiOCH causes interconnect failure during electrical tests. To suppress problems in the etching
and ashing processes, the balance of the total atom fluxes should be quantitatively and instantaneously controlled to the optimum point for each
material.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Plasma; Dry etching; Low-k; SiOCH; Cu; Damage
1. Introduction
In recent years, low-k dielectric films have been widely
used in high speed and low power consumption CMOS
devices. Historically, as a dielectric material, SiO2, which has
a unique structure (atom ratio of Si:O = 1:2 and film density
of 2.4 g/cm3), has been used because it has excellent thermal
and mechanical stability and is easy to fabricate. A huge
database of SiO2 etching processes and knowledge of the
controllability of fluorocarbon plasmas in various dry etching
systems has been accumulated. Low-k films, such as SiOCHs
[1–4], however, have C and H (such as methyl groups –CH3)
in the film network and have a relatively lower film density
than SiO2 to create a lower dielectric constant. Furthermore,
many kinds of ‘‘SiOCH’’ exist with different film composi-
tions and film densities that depend on the fabrication method
(spun-on or chemical vapor deposition (CVD)), precursor of
the films (gas chemistry, etc.) or curing method (thermal,
electron beam (EB, ultra-violet photon (UV), etc.). Conse-
* Tel.: +81 46 230 6568; fax: +81 46 230 5400.
E-mail address: Tetsuya.Tatsumi@jp.sony.com.
0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2007.02.008
quently, the etching properties of each SiOCH film are
different from SiO2. Dry etching plasma must be frequently
adjusted to the optimum condition to have good etching
results. This means that etching processes must be developed
or modified for each device, each interconnect structure, and
each material. As a low-k material, organic polymer is also
used, which does not include Si in the film [5–7]. In the
process for etching of C-polymer, N–H based plasma is used
to control the etched profile. Organic materials are also used
as multi-stacked resist masks, and strict control of the critical
dimensions is required. Using this ‘‘halogen-less’’ plasma
during the fabrication of multi-stacked structures makes
plasma control more difficult. The walls in a dry etching
chamber can be changed step-by-step by exposure to different
plasmas. The mechanisms of plasma processes for various
materials and structures should be systematically analyzed.
We studied the controllability of plasma and modeled a
surface reaction for each low-k material (SiOCH and organic
material) to clarify the difference between SiCO2 etching and
low-k etching. The change in the surface or interface should be
optimized to create a reliable interconnect. Therefore, we also
determined the change in low-k surfaces and their effects on
interconnect properties.
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6717
2. Etching of SiOCH films
Starting with 90 nm node devices, low-k film was applied to
the manufacturing of logic devices [8–11]. This manufacturing
faced many problems, such as etch stop, surface roughness, or
non-uniformity of etched profiles induced by using low-k
material. A typical process flow is shown in Fig. 1, specifically
how to fabricate dual damascene interconnects. First, a resist
pattern for a via hole was formed on the stacked structure of
SiO2/SiOCH (Fig. 1a). During etching of the SiOCH hole
pattern, high selectivity to SiC is required to reduce the loss of
the bottom stopper material (Fig. 1b). Next, the via hole is filled
with organic polymer, and a resist mask for trench pattern is
formed on the SiO2 layer (Fig. 1c). After that, the SiO2/SiOCH
trench is etched (Fig. 1d), the resist mask is removed by ashing
plasma (Fig. 1e), and the SiC stopper layer is finally etched
(Fig. 1f). Higashi et al. reported [8] the process technologies for
mass production of homogeneous interconnect structures using
Cu wiring with SiOCH as an inter-layer dielectric (ILD)
material. They pointed out that precise control of the etched
profile as well as damage (see Section 4) reduction during
etching and/or ashing are required to create a highly reliable
interconnect.
The process window for selective etching of SiOCH/SiC
structures is very narrow [12,13]. Slight fluctuations of the
plasma or the film condition can induce etch stopping, residue,
or changes in the bottom or top diameter of via holes. To clarify
the way to optimize such processes, understanding the
difference between the mechanisms of SiO2 etching and
SiOCH etching is very important. In general, fluorocarbon
(C–F) plasmas (such as C4F8/Ar/O2, CF4/Ar/O2, etc.) are used
for the etching of SiOCH materials. C–F plasmas have been
widely used for SiO2 etching of high aspect ratio contact holes
[14–16]. In this section, we will describe the controllability of
C–F plasma (Section 2.1), incident fluxes and surface reactions
on SiOCH (Section 2.2), and a model for controlling the
surface reactions on SiOCH and porous SiOCH materials
(Section 2.3).
Fig. 1. Process flow of via first dual damascene interconnect: (a) resist mask for via; (
(f) stopper etching.
2.1. Control of fluorocarbon plasma
2.1.1. Dissociation of fluorocarbon molecules
Fluorocarbon plasma, which is used for SiOCH etching, is
basically similar to that used in SiO2 etching. To generate the
reactive species, such as CF2 radicals, the parent fluorocarbon
molecules (C4F8, C4F6, etc.) must be dissociated by multiple
collisions with energetic electrons (Fig. 2). The amount of CFx
species is determined by the partial pressure of the parent
molecules, and the dissociation of this molecule is determined
by the balance between the generation and loss of each species
in steady state plasma. The density of the CF2 radical can be
determined by the following equation (of course, as dissociated
species, many other radicals or ions (C3F5, C2F4, CF3, CF, F,
and etc.) can be also generated [17]. However, to simplify the
understanding of gas phase reactions, we here use CF2 for all
the reactive species because the density of CF2 was generally
much larger than the other radical species in actual etching
plasma for contact hole etching):
d½CF2�dt
¼ ½C4F8�nehsvi � ½CF2�tpump
� ½CF2�twall
¼ 0 ðsteady-stateÞ
(2.1.1)
where [C4F8] is the density of C4F8 that depends on the partial
pressure of C4F8 gas, [CF2] the CF2 radical density, ne the
electron density, s the collision cross-section for dissociation,
and v is the electron velocity, tpump and twall are time constants
for the loss rate of CF2 radical by pumping out and by sticking
on walls, respectively. hsvi means integrated value of s multi-
plied by v from threshold energy (eth) for s to infinity as shown
in following equation ( f(e) is the distribution function):
hsvi ¼Z 1eth
sðeÞv f ðeÞ de (2.1.2)
tpump and twall can be described as follows:
tpump ¼PV
Q(2.1.3)
b) via etching; (c) resist mask for trench; (d) trench etching; (e) removal of resist;
Fig. 2. Dissociation of fluorocarbon molecules by multiple collisions with electrons (after multiple impacts with energetic electrons, the amount of radicals and/or
ions with smaller molecular weights increased while the amount of undissociated parent molecules decreased).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376718
and
twall ¼S
V
2s
2� s� 1
4½CF2�
ffiffiffiffiffiffiffiffi8kT
pM
r(2.1.4)
where P, V, Q, and S are the gas pressure, plasma volume, total
flow rate, and surface area, respectively. s is the reaction
probability and is considered to depend on the surface tem-
perature and the energy of the incident ion on the wall surface,
k, T, and M are the Boltzmann constant, the radical temperature,
and the mass of the radicals, respectively.
To evaluate the controllability of reactive species, we used a
capacitively coupled plasma (CCP) system for the etching of
dielectric materials. In general, CCP systems have a relatively
small plasma volume (V) and generally use a very high total
flow rate of an Ar diluted gas mixture (Q) and a relatively lower
pressure of several tens mTorr (P) [18,19]. Therefore, tpump is
considered to be smaller than twall. Morishita pointed out that
when the gas residence time was less than 10 ms, most CFx
radicals are pumped out, and, consequently, a term for sticking-
to-wall has little effect on the radical composition in the plasma
[20]. Typical conditions we used were V = 1000 cm3,
Q = 420 sccm, and P = 30 mTorr, and tpump was calculated
to be about 6 ms. Under such short residence time conditions
Fig. 3. Model for dissociation control (the dissociation degree of a C–F molecule is r
controlled by varying the power, pressure, total flow, electron temperature, and pl
(tpump� twall), the total amount of CFx species (which can be
related to the dissociation degree of parent molecules) can be
estimated by the number of collisions with electrons (j, see
Fig. 3):
j ¼ tpumpnehsvi (2.1.5)
For example, we varied the electron density by changing the
RF power from 400 to 2000 W (ne = 0.5–1.5 � 1011 cm�3), the
total gas flow rate (Q = 100–600 sccm), the gas pressure
(P = 20–100 mTorr), and the gap width (V = 18–26 mm), and
measured the dissociation degree of C4F8 using a quadrupole
mass spectrometer (QMS) [20–23]. Regardless of which
parameter was changed, the dissociation degree of C4F8 in
dual frequency (27/0.8 MHz) increased monotonically with
increasing j, as shown in Fig. 4. The absolute densities of
radicals also were evaluated using various in situ measurement
tools (infrared laser absorption spectroscopy (IRLAS) [24–26]
and achtinometry [27–29]). The densities of CF, CF2, CF3, and
F as a function of power (plasma density) and flow rate
(residence time) are shown in Fig. 5. The relative amount of
smaller molecules, especially the atomic F radical, was
remarkably increased with increasing ne or tpump. By changing
the gap width of the etching system, the electron energy
elated to the number of collisions with electrons (j = tpumpnehsvi) which can be
asma volume).
Fig. 4. Dissociation degree of C4F8 molecule in C4F8/Ar/O2 plasma as function
of number of collisions (the power, flow rate, gap width, and gas pressure were
varied in the same chamber).
Fig. 5. Radical densities as functions of (a) plasma density and (b) total flow
rate (gas residence time) (an increase in the atomic density of F under high
power or long residence time conditions increases the dissociation of parent
molecules).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6719
distribution function (EEDF) was also changed. Kinoshita
reported that when the electron temperature in an ultra high
frequency (UHF) plasma source was varied by changing the gap
width, the dissociation of fluorocarbon molecules was
promoted because hsvi was increased [30]. As shown in
Fig. 6, when hsvi was larger, the relative atomic F density in
C4F8/Ar/He/O2 plasma was increased. The EEDF also can be
changed by changing the plasma source, the gas pressure, or the
ratio of the etching gas flow rate [31,32]. The total amount of
reactive species from both the partial pressure of parent
molecule, which can be quantitatively estimated from both the
gas composition (flow rate ratio) and the pressure P, and the
dissociation degree, which depends on number of electron
collisions j, can be controlled.
2.1.2. Plasma–wall interaction
As discussed in Section 2.1.1, the amount of CFx radicals
depends on the dissociation of fluorocarbon parent molecules.
Moreover, when the wall is actively controlled, the composition
of reactive species (F/C ratio) can be controlled (see Fig. 7). To
reduce the concentration of F radicals, which reduce the
selectivity to mask or underlying materials, most CCP reactors
use Si as a top electrode to enhance the reaction based on
following reaction:
SiðwallÞ þ 4F! SiF4: (2.1.7)
According to Eq. (2.1.2), the effect of the plasma–wall
interaction becomes larger when twall is relatively small and
twall is large. By increasing the energy of incident ions to the Si
wall, the reaction probability s in (2.1.4) strongly increases. An
example of the effect of increasing ion energy in a parallel plate
etching system is shown in Fig. 8. In this experiment, two
frequencies of 500/13 MHz were applied to a top electrode.
500-MHz power was used for plasma generation and 13-MHz
power changed the incident ion energy on the Si top electrode.
Using this system, the plasma density (dissociation of C4F8)
and ion energy to the Si wall (plasma–wall reaction) were
independently controlled. When the Vdc on top Si electrode was
high, a remarkable decrease in the F radical density in the C4F8/
Ar plasma was observed. The potential of chamber walls also
can be used to control the composition of reactive species in
fluorocarbon plasma.
2.1.3. Control of incident fluxes
The total amount of CFx species is related to the partial
pressure of parent molecules and dissociation by collision with
electrons. Some of the reactive species also react with chamber
walls, and the absolute densities of radicals or ions have been
determined in steady state plasma. These reactive species are
carried onto wafer surfaces to react with etched materials, as
shown in Fig. 9. The amount and composition of reactive
species depend on the etching system or process conditions.
Internal plasma parameters, such as the plasma density, the
radical density, and the ion energy, must be quantitatively
Fig. 6. Radical densities as function of electron temperature in UHF plasma
system (an increase in the number of high energy electrons in the narrow gap
chamber also increased the dissociation of molecules).
Fig. 8. Radical densities as function of ion energy at top plate of parallel plate
type UHF-ECR reactor (F can react on Si walls under ion bombardment).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376720
adjusted to optimum conditions by controlling the recipe,
which is a set of external parameters. However, the external
parameter, such as the power, the pressure, and the flow rate, do
not have a one-to-one correspondence with internal parameters,
such as plasma density or electron temperature, because
internal plasma parameters also depend on the configuration of
the etching system or discharge mechanisms for each plasma
source, etc. Therefore, even when using the same power
conditions, the plasma density is different in different etching
systems. To promote a quantitative understanding of our
processes, the relationship between external parameters and
Fig. 7. Plasma surface interactions (the wall can be either a sink or a source of
reactive species).
each value of t, ne, and hsvi, as well as their spatial
distributions in the plasma chamber, must be clarified. In situ
monitoring or prediction of reactive species in a dry etching
system is expected.
Fig. 9. Reactions in plasma (the generation of reactive species depends on gas
phase reactions, such as electron impact dissociation, plasma–wall interactions,
or pumping. When the same amounts and energies of reactive species (radicals
and ions) are supplied to an etched surface, the surface reaction on the etched
surface must be the same).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6721
2.2. Surface reaction of SiOCH etching
Next, we will explain the relationship between incident
reactive species and etching reactions. The reaction on etched
surfaces is the same when the same amount of reactive species,
such as CF2 or CF+ ions and ion energy, are supplied on the
surface. We determined the reaction of these species on the
surfaces of various materials.
2.2.1. Total atom flux
The etching rate of SiO2 or SiOCH is determined by the rate
of an ion-assisted reaction of incident F atoms with Si atoms in
films. F is supplied to the etched surface with C atoms in the
form of CFx radicals and/or ions. These CFx radicals are
dissociated from the parent molecules (C4F8, etc.) through
multiple collisions with energetic electrons, as mentioned in
Section 2.1. Next, to clarify the relationship between plasma
and surface reactions, we determined the total amount of
Fig. 10. Etch rate of SiO2 as functions of incident fluxes: (a) CF+; (b) CF2; (c)
1.8 � 1011 cm�3, the ion energy was 1450 V, and the thickness of the surface polym
incident species (e)).
incident reactive species as well as the ion energy supplied to
the etched surface.
As determined in a study on SiO2 etching, the etch rate of
SiO2 depends on the total amount of F included in all the CFx
species dissociated or ionized from parent molecules [32,33].
The etch rate of SiO2 as functions of various incident species,
which are quantitatively measured under various gas conditions
(the total ion flux, ion energy, and thickness of the surface
polymer (<1 nm) were kept constant), is shown in Fig. 10. The
etch rate did not depend on a single incident radical or ion (F,
CF, CF2, CF3+, etc.). When we calculated the total amount of F
included in F, CF, CF2, and CF3 radicals, a clear relationship
can be seen between the total F flux and the etch rates, as shown
in Fig. 10e. This result suggests that ANY of the incident
radicals or ions can be etchants of SiO2. In this experiment, the
amount of the total F from C–F radical species was calculated
from radical fluxes because we used the conditions where the
radical density (on the order of 1013 cm�3) was much larger
than the ion density (1011 cm�3). When we used high-density
CF3+; (d) F; (e) total F included in all CHx species (the plasma density was
er was <1 nm. The etch rate depended on the total amount of F included in all
Fig. 12. CF, CF2, and CF3 radical densities as function of C4F8 partial pressure
in C4F8/Ar/O2(N2) plasma (the dissociation degree of C4F8 (96%), ion energy
(1400 V), and plasma density (1.8 � 1011 cm�3) were kept constant. The
radical densities linearly increased with the partial pressure of the parent
molecule).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376722
and low-pressure plasma, we also needed to be take the ionic
species into account in the calculation of the total F. The radical
flux can be roughly expressed using the following equations:
G F-total�X
x
2s
2� sG CFx � xffi
Xx
s� 1
4NCFx vCFx x; ðs� 1Þ
(2.2.1)
and
G C-total G F-total
a
b(2.2.2)
where NCFx and vCFx are the densities and thermal velocities of
the radicals (F, CF, CF2, and CF3), and a and b are the numbers
of C and F atoms in a parent-gas molecule (CaFb). For example,
when C4F8 gas was used, a and b were 4 and 8, respectively, s
was the surface reaction probability of reactive species, which
depended on the ion energy. The s under ion energy of 1500 V
was about 0.1, as shown in Fig. 11. As described above, the total
amount of reactive species should be controlled to evaluate the
surface reaction during the etching of low-k materials. An
example of incident fluxes as a function of the C4F8 flow rate
in C4F8/Ar/O2(N2) plasma in a dual frequency (60/2 MHz) CCP
system is shown in Fig. 12. We kept the dissociation ratio of
C4F8 constant at 96%, the pressure at 30 mTorr, and the plasma
density at 1.1 � 1011 cm�3. CF, CF2, and CF3 radical were
measured using IRLAS and were linearly increased with the
C4F8 flow rate. Similarly, the density and incident fluxes of the
total amount of O and N could also defined by their densities,
thermal velocities, and reaction probabilities. The dissociation
degree of O2 (or N2) molecules was also monitored, and we
confirmed the total amount or O (or N) radicals was almost
constant. The dissociation degree of N2 was generally lower
(about 1/20) than that of O2 because of the smaller dissociation
collision cross-section. In this way, we estimated the total
numbers of each type of atom (F, C, O, and N) introduced
to the steady state etched surface per unit time and per unit area.
Using this quantitatively known plasma, we evaluated the
etched surfaces of SiO2 and SiOCH films.
2.2.2. Control of surface polymer
We studied the surface reaction of SiOCH using both the
incident fluxes from the plasma and the outflux from the
Fig. 11. Surface reaction probability as function of ion energy (an increa
SiOCH. We calculated the outflux (number of atoms (Si, O, C,
and H) released from etched surface) using the etch rate, the
film density, and the atomic composition of SiOCHs. Using
both fluxes, the surface reaction on SiOCH is described as
follows.
The etched surface on SiO2 and SiOCH has basically two
layers (Fig. 13). One is the reactive layer where ion energy was
deposited and incident F can react with Si in SiOCH films
because energetic ions break the bonds in the SiOCH network
resulting higher reactivity. Above the reactive layer, a C–F
polymer exists in a steady state surface [34–36]. The thickness
of this polymer (TC–F) is determined by the balance between the
total C and the total removal ability (specifically, chemical
reactions such as C + O! CO, C + N + H! HCN)
[32,33,37]. As shown in Fig. 12, we varied the total number
of incident CFx fluxes by increasing the C4F8 flow rates in the
C4F8/Ar/N2 plasma. The steady state thickness of the C–F
polymer (TC–F) on the SiOCH surface was changed as follows.
(I) T
se in
C–F < 1 nm (!high etch rate)
(II) 1
< TC–F < 4 nm (!unstable/residue)(III) T
C–F > 4 nm (!low etch rate)ion energy strongly increases the reactivity of radical species).
Fig. 13. Surface reaction model for SiOCH etching (the surface consisted of two layers (a polymer layer and a reactive layer). The thickness of the polymer TC–F was
determined by the balance between the total atom fluxes supplied from both the plasma (incident flux) and the etched material (out flux)).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6723
When TC–F was less than 1 nm, the SiOCH etch rate
increased (I), reached a peak, and then decreased (II) to a
constant value (III). In region II, we observed the fluctuation of
the etch rate and residue on the etched surface. A significant
change in both the etch rate and TC–F can be seen at around the
peak, called a ‘‘critical point’’ (Pc, see Fig. 14a), which is a
condition between I and II [38,39].
Fig. 14. (a) Etch rate of SiOCH, SiO2, and resist as function of C4F8 flow rate;
(b) SEM and TEM photographs of surface of etched SiOCH. Ru (in TEM
image) was used as a marker to clarify the top surface of C–F polymer. The
surface reaction can be classified into three conditions (I: thin polymer and high
etch rate, II: unstable, III: thick polymer and low etch rate).
To understand these change in the etch rate and the surface
polymer, the total atom fluxes of F, C, N, and O were estimated,
as shown in Fig. 15. The position of Pc (in Fig. 14a) is related to
the condition where the incident total C was equal the C-
removal ability of O (from SiOCH) and N (from plasma). When
the C4F8 flow rate was lower than Pc, the incident C were
immediately removed by the chemical reaction with O or N,
and all of the incident F reacted with the Si in SiOCH
effectively. However, when the C4F8 flow rate was larger than
Pc, the thickness of the C–F polymer increased. As the thick C–
F polymer decreased the ion energy deposited on the reactive
layer, the etch rate remarkably decreased under conditions II
and III. The 4-nm thick polymer decreased the ion energy by
about 800 V, as shown in Fig. 16. The net energy deposited on
the reactive layer was determined by both the incident ion
energy and the energy loss (DE) by the C–F polymer [36,40].
We used etching condition I in Fig. 14 to solve problems with
residue or etch stop (photographs in Fig. 14b). The width of
region I (i.e. process window) for SiOCH was much narrower
than that for SiO2 because of the lower concentration of O in the
film. The etch rate of various films with different oxygen
Fig. 15. Incident fluxes of total atoms in C4F8/Ar/N2 plasma (same conditions
as Figs. 12 and 14) (when the C4F8 flow rate was higher than Pc (where
C = N + O), TC–F was thicker, and some part of the F and ion energy were spent
by removing excess C).
Fig. 16. Ion energy decrease as ion passed though C–F polymer (TC–F of the
4 nm thick reduced ion energy to about 800 V).
Fig. 17. Etch rate of SiOCH, SiOH, and SiO2 as function of C4F8 flow rate (the
position of Pc (peak etch rate) for each material depended on the oxygen
concentration of each film. SiOCH, which has the lowest amount of oxygen, has
the narrowest process window for etching).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376724
concentrations is shown in Fig. 17. The process window of
SiOCH with the lowest O content of 31% has the narrowest
process window for etching. This indicates that SiOCH is very
sensitive to changes in the incident CFx fluxes. The optimal
plasma condition for the etching of SiOCH is a ‘‘pin-point’’,
and slight changes in the plasma or composition of the films
cause problems, such as residue, unexpected profiles, or low
selectivity. This is why low-k etching is so difficult to optimize.
The sample structure also changes the amount of CFx fluxes.
In high aspect ratio structures, such as via holes, incident CFx
species are slightly lower than those in trench patterns because
some species react at the sidewalls. As a result, the optimal
C4F8 flow rate (Pc) was different for trench etching and via
etching, as shown in Fig. 18. To optimize the flux balance on the
Fig. 18. Etched profile of via and trench as function of C4F8 flow rate (the structur
smooth surface with thin a polymer, to (II) rough, to (III) a low etch rate smooth sur
each structure).
bottom of a via structure, the amount of CFx species in bulk
plasma as well as the sticking coefficient on the sidewall of the
via holes, which is a function of the wafer temperature, should
be controlled.
2.3. Porous SiOCH
Since the 45-nm generation, materials with lower film
densities (inter-layer or inter-metal dielectric material (ILD or
IMD), stopper materials, multi-layer masks, etc.) have needed
to be used [41–43]. However, most of these materials still
consist of Si, O, C, H, and N, and can be etched by using
fluorocarbon plasma. Hence, the same model can be used to
control both the plasma and the surface reaction.
The etch rates of SiOCHs with the same film composition
(Si:O:C:H = 18:31:14:37) but different film densities are shown
in Fig. 19a. The etch rate was higher when the film density was
lower, whereas the Pc did not change. This is because the
number of Si atoms per unit cell was fewer in lower density
films while the incident total number of F atoms was the same
under the same condition. In the etching of porous materials,
the thickness of the surface polymer on porous SiOCH can be
changed more by small changes in radical fluxes. This means
e of the surface layer depended on the aspect ratio and was changed from (I) a
face and a thick polymer. The amount of reactive species must be optimized for
Fig. 19. Etch rate of various p-SiOCH films using C4F8/Ar/O2 plasma: (a) film
density varied; (b) H/C ratio varied (small changes in either film property
(atomic composition or film density) and the plasma induce large changes in the
surface reaction).
Fig. 20. Model for controlling etch rate of SiOCH (Region I (lower flow rate of
C4F8 than Pc) must be used to avoid etch stop and surface roughness. Pc can be
predicted from the incident and out fluxes.
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6725
the sensitivity to the change in incident fluxes was increased.
The effect of pore size on etching properties has been discussed
in Ref. [41]. The lower etch rate was observed in the etching of
SiOCH films with a higher porosity of a larger pore size. The
etch rates of porous SiOCHs with different H/C ratios are
shown in Fig. 19b. When an excess supply of CFx radicals
(higher C4F8 flow rate than Pc) was used, the etching reaction of
p-SiOCH, which had the lowest H/C ratio, was stopped.
Although surface roughness was observed, p-SiOCH with a
higher H/C ratio had a high etch rate. These results indicate that
H in films help to remove excess C from the steady state
polymer on the etched surface.
The model to control the surface reaction on various SiOCH
films is summarized in Fig. 20. First, the total amount of
incident CFx and O (or N) must be controlled by varying the
partial pressure and dissociation of parent molecules. Then the
atomic composition and density of the SiOCH film must be
known. The optimum etching condition Pc for each material
can be determined from both the incident flux from the plasma
(C, F, O, and N) and the out flux from the film (Si, O, C, and H).
The process window (width of region I) was determined by the
balance between C and O (N). When the film has a relatively
large amount of oxygen or when the additive O2 (or N2) flow
rate ratio in the fluorocarbon plasma is increased, Pc shifts to
the right in this figure, meaning the process window is wider.
The etch rate depends on the reaction between F and Si. When
we increased the total number of F atoms (without increasing
C), or when we decreased the film density, the etch rate
increased.
To have high selectivity in etching, the etching conditions
under which the polymer is thin on the etched material and
thick on the underlying material must be used. In the case of
SiO2 etching on Si (conventional contact hole etching), the
difference in the O content between these two materials is very
large, and a highly selective etching condition can easily be
determined. By contrast, in the case of low-k processes, the
SiOCH must be etched on SiC, which has a similar film
composition. For this reason, selective etching can only occur
under ‘‘pin-point’’ conditions. As shown in Fig. 21, the process
window in low-k processes was much smaller than that in SiO2
processes. Thus, the variation of film properties (density and
composition) as well as those of the structure (type of mask,
composition of underlying material, and aspect ratio) strongly
increase the difficulty in developing an optimal etching process
for SiOCH. To produce reliable dry etching processes for multi-
layer structures, the Pc for each material must be precisely
determined. However, the unevenness of the pattern width must
be suppressed to within several nanometers. Accordingly, a
large database of etching recipe for various materials must be
created but also one containing simulation techniques for
predicting the etching properties of various materials. The
results of the selectivity calculation using our surface reaction
model are showed in Fig. 22. The high selectivity of SiOCH/
SiC can be obtained for only a very small area on this map [44].
3. Etching of organic polymer film
In a 65-nm node device, a stacked structure of pure organic
low-k material and SiOCH (a so-called hybrid structure) was
applied to BEOL integration [45,46]. An example of the
process flow of multi-layer interconnects, called a hybrid
structure, using a dual hardmask is shown in Fig. 23. First, a
Fig. 21. Window for selective etching (selective etching can be done in
conditions where the polymer is thin on the etched material but thick on the
underlying material. This process window depends on the difference of oxygen
concentrations between the two materials. In general, a low-k process has a very
narrow window for selective etching).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376726
resist pattern for the trench was formed on the stacked structure
of the Si3N4/SiO2 mask on PAr(poly-allylene)/SiOCH
(Fig. 23a). After the etching of the top Si3N4 layer
(Fig. 23b), a resist mask for the via hole pattern was formed
(Fig. 23c). Then, the SiO2/PAr was etched, and the resist mask
was also removed during etching of the PAr layer (Fig. 23d).
Then, using the Si3N4 trench mask, a SiO2/PAr trench was
etched (Fig. 23e). Then, the bottom SiC and top Si3N4 masks
Fig. 22. Calculation of etch rate of various materials under various conditions (th
were removed simultaneously (Fig. 23f). In the fabrication of
this structure, we used two kinds of plasma. For the etching of
the organic layer (trench level), oxygen or hydrogen based
plasmas were used [47–51], and the SiOCH layer (via level)
was etched by fluorocarbon plasma, as described in Section 2.
Fluctuation of the cross-section of the Cu interconnect should
be minimized in 65-nm node and beyond devices. After the
fabrication of the multi-stacked hardmask, the etching
processes for the PAr layer must be controlled precisely.
Organic low-k material, such as poly-allylene (PAr), has no
Si and can be etched using halogen free plasma. When the N–H
plasma is well controlled, very high selectivity to the
underlying SiOCH layer can be had, suppressing the loss or
erosion of the trench bottom (however, it causes some damage
on SiOCH (Section 4)). This high selectivity is one of the
advantages of a hybrid dual damascene structure, because the
same trench depth can be had regardless of the pattern width. To
have high performance organic low-k etching, the controll-
ability of the N–H plasma processes must be clarified. In this
section, we discuss the control of N and H radicals in the plasma
(Section 3.1) and show the surface reaction on organic material
as well as on mask material (Section 3.2).
3.1. Control of N–H plasma
3.1.1. Generation of N, H radicals
NH3 or H2/N2 plasma has been used to allow precise control
of the etched profile of organic materials. The absolute densities
of H and N radicals as well as the ion energy must be controlled
to control the etching properties. The basic reaction in the gas
phase to generate H and N radicals in N–H plasma is described
as follows:
d½H�dt¼ ½H2�nehsvi � ½H�
tpump
� ½H�twall
¼ 0 ðsteady-stateÞ (3.1.1)
d½N�dt¼ ½N2�nehsvi � ½N�
tpump
� ½N�twall
¼ 0 ðsteady-stateÞ (3.1.2)
These equations are similar to Eq. (2.1.2). To generate the N
and H radicals in N–H based plasma, a high electron density or
e etch rate and the process window for selective etching can be calculated).
Fig. 23. Process flow of hybrid dual damascene interconnect: (a) resist mask for trench; (b) trench hardmask etching; (c) resist mask for via; (d) via etching; (e) trench
etching (and removal of resist); (f) stopper etching (and removal of hardmask).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6727
high electron energy is required because the cross-section (s)
for ionization or dissociation is smaller than that of
fluorocarbon molecules. The plasma density drastically
changed by changing the parent N-H molecules or the ratio
of the gas mixture, while the plasma density in SiOCH etching
is not sensitive to small changes in the flow rates of
fluorocarbon molecules or additive O2 or N2, because reactive
gases were generally diluted by the large flow rate of Ar. The
change in plasma density using NH3 and N2/H2 plasma is
shown in Fig. 24 [52]. When NH3 was used, the plasma density
Ne was higher than N2/H2 plasma under the same power
condition because the cross-section for ionization of NH3 was
larger than that of N2 and H2. Similarly, when the ratio of H2
and N2 was varied in the N2/H2 plasma, the plasma density was
lower in the H2 rich condition. Specifically, the electron density
is very low whereas the electron temperature is high in pure H2
plasma [53].
3.1.2. Control of plasma–wall interaction
As shown in Eq. (3.1.1), the density of each radical is
determined by the balance between the generation and the loss
of each species. To stabilize N–H plasma in a dry etching
Fig. 24. Plasma density as function of RF source power of Helicon plasma
(using NH3, the plasma density was higher than of H2/N2 because of the larger
ionization cross-section).
system, the loss terms of each radical by recombination on the
metal walls or consumption by a C–F polymer (formed during
previous step in multi-step etching of the stacked structure)
must be considered. Specifically, H radicals can react on metal
walls easily, changing the density of H by the contamination of
Cu on the walls. An example of the H radical density in the
plasma chamber measured by optical emission spectroscopy is
shown in Fig. 25. We put three different types of wafers (Si,
polymer, and Cu). Under the same etching conditions using H2/
Ar discharge, we measured the relative intensity of [H]/[Ar].
The relative amount of H radicals was reduced by polymer or
Cu on the wafers. A wafer covered with 1% of its area with Cu
induced a change in the H radical density of 5% [54]. During
over etching of the stopper material (such as SiC), the bottom
Cu of the under layer was exposed to plasma and sputtered out
from the via holes. This Cu can stick on the surface of Si or
dielectric parts on chamber walls of dry etching systems. This
Cu can cause fluctuations of etching plasmas for SiC or organic
materials because H radicals can easily disappear through
recombination on the conductive surface [55].
3.2. Surface reaction of organic polymer
3.2.1. Reaction between H, N radicals and organic polymer
The typical etch rate of organic polymer as a function of the
H2/N2 gas ratio in a high-density (Helicon) plasma source is
shown in Fig. 26a. Fukasawa found that the etch rate has a peak
at the ‘‘optimum ratio’’ of the H2/N2 gas mixture [52]. This
suggests that both H and N radicals are needed to have an
effective reaction on the surface. The etching of organic
material is related to the chemical reaction of carbon with
hydrogen or nitrogen radicals. The main desorbed molecules
during NH3 plasma beam exposure to organic low-k material
were found to be HCN and C2N2 [56]:
C þ N þ H ! HCN; 2C þ 2N ! C2N2
The effective generation of these volatile reaction products
is the key to increasing the etch rate of organic polymer.
Fig. 25. H radical density (a) changed by (b) consumption by polymer or (c)
recombination on conductive surface on wall (the change in O and N radical
density was smaller than that of H).
Fig. 26. (a) Etch rate of organic low-k (PAE) as function of H2/N2 gas ratio in
helicon plasma system. (b) Etch yield of PAr as function of ratio of absolute
densities of H and N radicals (the Ar ion energy was fixed to 500 eV. The peaks
of etch rate indicates that H and N radical densities should be controlled to an
optimal ratio).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376728
A change in gas composition also changes the ion current or
ion energy in a dry etching system, as mentioned in Section 3.1.
To separate the effects of ions (supply of energy) and radicals
(supply of etchant), the surface reaction on a PAr surface was
analyzed using a radical injection beam experimental set-up
(Fig. 26b). We introduced an Ar ion beam with H and N radicals
generated by an electron cyclotron resonance (ECR) type
radical source. The ion energy was varied from 50 to 500 eV.
The absolute densities of H and N radicals were monitored
using vacuum ultra violet (VUV) absorption spectroscopy, and
the ratio of H/(H + N) was varied from 0 to 100%. The etch
yield (etch rate divided by ion current) as a function of the H/N
radical ratio is shown in Fig. 26b. The peak is at around H/
(H + N) = 30%. We also analyzed the surface of PAr using in
situ X-ray photo-electron spectroscopy (XPS). On the surface
of PAr exposed to Ar+ with N-rich conditions (H/
(N + H) < 30%), an increase of C–N bonding was observed
(Fig. 27a) while a remarkable decrease in benzene rings was
observed under the H-rich condition (Fig. 27b) [57]. We
determined the surface reaction on organic polymer to be as
follows; the formation of HCN determined the etch rate. H
radicals generate active sites in the network on CHx polymers
under ion injection, and consequently the reactivity of N
radicals on the surface increases. When we used a N-rich
condition, the surface of PAr was covered with stable bonding
of C–N or C N, and the formation of HCN was suppressed. By
contrast, under an H-rich condition, the surface became very
active, and an insufficient supply of N radicals induced a lower
etch rate because the formation of volatile products, such as
HCN, was limited by supplying N radicals. This is the reason
the ‘‘optimum ratio’’ appeared in the experiment varying the
H2/N2 ratio. An image of the surface reaction is illustrated in
Fig. 28. The etch rate is related to the thickness of the reactive
layer where surface bonding has been broken by physical
bombardment with ions (or chemical reaction with H), and N
radicals have been sufficiently supplied. The ratio of the
absolute densities of H and N radicals must be quantitatively
controlled to find an effective etching reaction.
3.2.2. Reducing mask erosion
Variation of CD must be suppressed to within several
nanometers. Therefore, faceting of mask material should be
minimized. SiO2 or SiOCH have been used as mask materials in
the etching of organic material. An example of an etched profile
of a SiO2/SiOCH (hardmask)/PAr structure is shown in Fig. 29.
Fig. 27. Results of surface analysis on PAr surface: (a) nitridation on PAr under
N-rich condition; (b) benzene-ring opened by H-rich plasma.
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6729
When we used the low-frequency bias power (high energy peak
observed at 720 eV), severe erosion can be seen on the corner of
the hardmask (Fig. 29c) [58]. To suppress the erosion of the
hardmask, the ion energy must be decreased below the
threshold energy (eth) of etch yield of the mask material. For
example, the threshold energies of PAr, SiOCH, and SiO2 were
80, 120, and 160 eV, respectively (see Fig. 29a) [57]. The
etching of SiO2 or SiOCH using N-H plasma is a sputtering
reaction, and the etch yield depends on the root energy. As
shown in Fig. 29b, we estimated the position of the high energy
peak of the ion energy distribution function (IEDF), which
depends on the bias power, frequency, sheath thickness, and ion
Fig. 28. Model for controlling surface reaction on organic low-k (the etch rate depe
reaction probability of N depends on H radicals, the ion energy, and the temperat
mass [59,60]. We successfully suppressed the mask faceting by
using a high frequency bias power and controlling the high
energy ion to be below the threshold energy of SiO2 sputtering
(Fig. 29e). When we used a porous material as a hardmask, the
range of the ion energy under which highly selective etching
can occur became narrower (several tens eV). Quantitative
control of the ion energy distribution as well as the ratio of H
and N radicals is necessary.
4. Control of interfaces
We described the etching of low-k materials in the previous
section, and now we can control the profiles of dual damascene
structures. However, these formations of vias and trench
profiles are only the beginning of the process integration to
create highly reliable interconnects. After the etching of low-k
material, the photo resist must be removed, and the stopper
material must be broken. These plasma treatments change the
surface of low-k materials or Cu. The existence of a thin
modified layer (only several atomic layers thick) strongly
changes the electrical properties. Next, we will show some
example of controlling interfaces between metal and low-k
materials.
4.1. Control of Cu surface
After the etching of low-k film to make via and trench
patterns, SiN or SiC, which is a stopper material to suppress Cu
diffusion into the low-k material, was etched by fluorocarbon
plasma (Fig. 30). During the over etching of this process, the
surface of Cu was exposed to the plasma. For example, when
we used CF4/CHF3/O2 plasma for the etching of SiC, we
observed high resistance of the via chain under high power
conditions. During etching, the partial pressure and dissociation
degree of each gas were quantitatively monitored (Fig. 31).
When the RF source power was increased, the dissociation
degree of the CF4 increased markedly, while those of O2 and
CH2F2 did not increase as much. The resistance also increased
when the O/(CF4 + CHF3) flow rate ratio was lower. These
results indicate that the relative densities of CFx radicals with
respect to O can be related to the resistance of the via chain.
Next, we calculated the total incident fluxes of F, C, O, and H
atoms from the absolute density, the thermal velocity, and the
nds on the reaction between C and N (formation of HCN or C2N2). The surface
ure).
Fig. 29. (a) Etch yield of PAr and SiO2 as function of ion energy; (b) high energy peak of IEDF; when (b) below threshold energy (eth) of SiO2 etch yield, faceting of
mask shoulder successfully minimized (c–e).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376730
reaction probability of each reactive species for each molecule.
We estimated how many atoms were supplied to a unit area of
the reactive surface on the Cu during etching. The total atom
fluxes of C, F, and O to the Cu surface are shown in Fig. 32a.
Next, double-level Cu interconnects were fabricated for
electrical testing of the via chain resistance (Fig. 32b). We
varied the ratio of the incident atom fluxes of C and O during
stopper etching with CH2F2/CF4/O2 plasma. Under condition
(B) in Fig. 32a, in which the total C flux was larger than the O
flux, some via holes had a higher resistance due to degradation
of the Cu; similarly, we also observed degradation under high
power conditions. After stopper etching, we found that excess
C–F polymer formed on the Cu surface and was the reason for
the degradation. By contrast, when the O flux was overly
supplied, severe oxidation of both the Cu and the low-k surfaces
Fig. 30. Control of Cu surface during stopper etching (Cu is not etched by fluo
was observed. The dimensions of etched features becomes
smaller as the degree of degradation becomes larger in porous
materials. Consequently, oxidation of only a few layers on a
low-k surface (or a Cu surface) induced CD variation or failure
during reliability testing. Therefore, the fluxes must be
carefully adjusted to their optimal amounts to suppress excess
polymer formation and excess surface oxidation. Under
condition (A), in which the C and O fluxes were equal, we
suppressed both excess polymer formation and excess
oxidation, and the via chain yield improved, as shown in
Fig. 32b [61,62].
The remaining C–F polymer or F on the surface of the Cu
must be minimized to suppress the fluctuation of resistance of
interconnects. Hence, the ratio of incident fluxes must be
optimized during stopper etching. The remaining F on Cu
rocarbon plasma but can be degraded by un-optimized plasma treatment).
Fig. 31. Example of Cu degradation at bottom of via hole (under high power
conditions in CF4/CH2F2/O2 plasma, the relative amount of CFx species was
larger with respect to oxygen).
Fig. 32. (a) Total atom fluxes of C, O, and F as function of gas ratio (the total
flow rate of CF4/CH2F2/Ar was kept constant); (b) total C much larger than O,
part of via chain resistance was very high (condition B) (we optimized the
plasma to condition A) where all C can be removed by chemical reaction with
oxygen efficiently. An excess supply of O induces the oxidation of Cu and the
SiOCH surface).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6731
surfaces can react with moisture in the atmosphere and
sometimes causes surface roughness or degradation of Cu as
well as low-k surfaces. The remaining F on Cu surfaces can be
removed by using additional surface treatment, such as N2 or O2
plasma [63,64]. In the actual integration of interconnects, many
kinds of countermeasures are used together to reduce the risk of
degradation on Cu surfaces.
4.2. Control of low-k surfaces
Next, we will show examples of the effects of damage of
low-k surfaces on interconnect reliability. After optimizing both
of the profile (via hole and trench) and the surface on Cu, the
surface reaction on the SiOCH sidewall must be optimized
during plasma treatment, such as ashing. SiOCH film has Si–
CHx bonding (such as methyl groups –CH3) in film networks to
reduce the film density. The Si–CH3 bond, however, is weaker
than a Si–O bond and easily oxidized during O-containing
plasma treatment (Fig. 33). This damage increases the k-value
and/or the side etching of the via profile after wet treatment. To
suppress this damage, new ashing processes were developed to
reduce the strength of side wall oxidation. Actually, the damage
could be decreased by changing the ashing plasma from O2
plasma to H2O plasma [8,65]. A thin damaged layer, however,
still exists on the surface of SiOCH sidewalls even when using
H2O ashing, and this very thin layer decreased the interconnect
reliability. For further improvement of this process, the
mechanism of damage must be understood. In this section,
we show an example of the effects of SiOCH damage on
electrical properties.
In general, the grain or vacancy of Cu in interconnects can be
moved by either electrical or thermal stresses. If the adhesion of
Cu to a barrier metal (Ta/TaN) is insufficient, the Cu
interconnect can be broken at an early stage of a reliability
test. The adhesion strength of Cu is affected by the degradation
at the interface between Cu and a barrier metal (or metal and
low-k materials). A typical Cu void observed in the early steps
in an electro migration (EM) test is shown in Fig. 34a. We
obtained a good etching profile of the via and trench patterns
and the optimized surface of the Cu as described in previous
sections. Therefore, the cross-sectional view and the resistance
Fig. 33. Control of SiOCH surface during ashing (methyl (–CH3) groups in SiOCH can be replaced with hydroxide (–OH) groups easily. An increase in the k-value
was observed, and moisture up-take induced problems in the reliability test).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376732
of the Cu interconnect was no problem. However, we did not
obtain sufficient reliability, as shown in Fig. 34c. When we used
a conventional ashing process, the same parts of the
interconnects were broken under a relatively short stress time.
This was due to the degradation of the SiOCH surface during
plasma exposure. In this case, we used H2O plasma for ashing,
Fig. 34. (a) Typical void in via hole observed during early stage of electro
migration (EM) test; (b) via hole observed when ashing plasma optimized void;
(c) cumulative failure as function of stress time.
where the thickness of the damaged layer was approximately
10 nm on the sidewalls of the SiOCH via structure. In these
weakly oxidized damaged layers, moisture can easily be
adsorbed on Si–OH bonds during wet treatment or air exposure
before metallization processes. H2O in the damaged layer was
released during the annealing of metals and oxidized the
metals. The interface between the metals and the damage on
SiOCH films was analyzed by using electron energy loss
spectroscopy (EELS). Much more oxygen was found around
the interface between Cu and Ta/TaN (Fig. 35).
Next, we investigated the correlation between the damaged
SiOCH thickness and the amount of the contained H2O by
thermal desorption spectroscopy (TDS). When SiOCH film is
exposed to plasma containing excessive oxygen radicals,
carbon atoms in the film are extracted, and the surface is
converted to SiOx, which can be etched by diluted hydrogen
fluoride (dHF), while undamaged SiOCH cannot. We defined
the etched thickness by dHF as the damaged thickness. As
shown in Fig. 36, the H2O degassing amount is proportional to
the damaged SiOCH thickness. The higher density of the
oxygen radicals in the ashing plasma induced greater damage to
the SiOCH. The generation of oxygen radicals in the ashing
plasma must be suppressed quantitatively to improve the
reliability. However, decreasing the dissociation of H2O (used
in the conventional process) reduces the ashing rate and
degrades the stripping ability, making the use of this method
problematic for mass production. To solve this problem, we
used O2 plasma again and clarified the relationship between the
oxygen radical density and the external parameters of the
ashing system by using optical emission spectroscopy (OES).
The density of the oxygen radicals can be expressed by both the
partial pressure and the dissociation degree (estimated from the
OES results) of the parent O2 molecules. The oxygen radical
density dropped when we reduced the gas pressure and/or the
plasma density, as shown in Fig. 37. By decreasing the O radical
density, the degraded SiOCH was reduced to less than 3 nm
thick (Fig. 38), although the ashing rate was higher than
Fig. 35. Diffusion barrier metals (Ta/TaN) oxidized by H2O released from
damaged SiOCH film during annealing (the adhesion between Cu and Ta/TaN
was weak when the interface was oxidized).
Fig. 36. Total amount of H2O (evaluated by TDS) as function of damage
thickness on SiOCH (the damage thickness was varied by varying the ashing
conditions. The amount of H2O increased linearly with the damage thickness).
Fig. 37. Control of oxygen radical in ashing system (the partial pressure and
dissociation degree of O2 determined the O radical density. To reduce the
damage on SiOCH sidewalls, low dissociation plasma under low pressure
should be used).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6733
500 nm/min (twice the rate of the conventional H2O process).
We were thus able to minimize the damage to the SiCOH film
without leaving any residue [66]. As shown in ashing condition
B in Fig. 34b, the early failure mode in the EM test, which was
caused by the formation of a Cu void inside a via, disappeared.
Consequently, the lifetime in the EM test was extended five
times. This suggests that controlling the degradation of SiOCH
films during plasma treatment is the key to enabling reliable Cu/
low-k interconnects in 90-nm nodes to be mass-produced [67].
We also used porous SiOCHs, which are more sensitive to
the ashing plasma in 65-nm and beyond devices. To reduce the
damage, a H-based ashing process was also proposed because H
radicals have less reactivity with SiOCH than those in an
Fig. 38. Degradation of SiOCH surface decreased below 3 nm under ashing
condition where O radical density was minimized (using this condition,
interconnect reliability improved (Fig. 34c)).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376734
oxygen-based process (but H radicals also decreases some Si–
CH3 bonds in SiOCH film) [65,68–70]. Using the hybrid
structure, the ashing process can be eliminated because the
resist mask was etched off during the etching step of the organic
polymer for the trench pattern. This is another advantage of
using a hybrid structure in the fabrication of low-k/Cu
interconnects. Thus, SiOCH or porous SiOCH are unstable
when exposed to various plasma/wet treatments. The surface
changed gradually in each step of fabrication of dual
damascene structures (etching, ashing, wet treatment, anneal-
ing, metal formation, etc.) The final structures of the interface
between metal/metal and metal/low-k interfaces determine the
resistance and reliability of Cu interconnects. Further analysis
of the damage-formation mechanisms and methods for solving
these damage problems are needed. The plasma, including
radial, ion and UV photons [71] must be controlled more
delicately.
4.3. Damage recovery/pore sealing
In Section 4.2, we described the importance of preventing
the adsorption of H2O. Many kinds of low damage ashing
technologies have been developed. Also, to minimize the effect
of H2O desorption, new approaches called pore sealing have
been reported [72–75]. Before metallization, the surface of
Fig. 39. Suppression of H2O adsorption by forming dense CHx thin film on damaged
H2 plasma exposure; (c) after formation of CHx thin film on sample (b); (d) after
damaged SiOCH is re-modified by a wet or dry treatment.
These post-treatment processes can prevent moisture adsorp-
tion and/or recover the k-value or mechanical strings, resulting
in higher reliability of interconnects. Some processes also
recover the mechanical strength and k-values. An example of
the amount of water out-gassing from blanket porous SiCOH
films is shown in Fig. 39. A remarkable increase in H2O
degassing was observed after H2 plasma exposure. We
examined the pore sealing using CH4 plasma. When the
damaged SiOCH was covered by a thin densified CHx polymer
3 nm thick, the water out-gassing was reduced, as (c) in Fig. 39.
This is because the CHx film prevented moisture from being
adsorbed on the damaged SiOCH films. The polymer must be
densified by ion bombardment or thermal annealing. After the
formation of thin CHx films on the surface of SiOCH, the early
failures in the electro migration test improved [76]. In the next
generation, complex plasma treatment, including etching,
ashing, and surface treatment on both Cu and damaged SiOCH
surfaces will be used.
5. Requirements for next generation
In the 45 nm and beyond devices, various low-k materials
must be used while the requirement for process control
becomes more severe. Finally, we will summarize the
SiOCH surface; total amount of H2O released from (a) initial SiOCH; (b) after
formation of CHx film without ion bombardment.
Fig. 40. Image of plasma control (in the etching of SiO2, an optimal process can
be determined by going in one direction. However, high-density plasma is not
needed for low-k etching, but quantitative and stable control of various plasmas
is required for step-by-step etching of multi-stacked structures).
Fig. 41. Instantaneous stabilization of etching plasma (in the etching of multi-
stacked structures, the history of wall surfaces affects the etching plasma. This
is an example of etch rate variation caused by a previous etching process. The
remaining C–F polymer, used in previous SiO2 etching, increased the etch rate
during SiN etching. About 180 s was needed, longer than the actual etching
time, to obtain a stable etch rate. Instantaneous stabilization within several
second is necessary).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–6737 6735
requirements of dry etching technologies for the next
generation [72,73].
5.1. Quantitative control of plasma parameters
Before we started to use low-k materials, we had improved
the etching plasmas for SiO2 etching. A higher ion current and a
lower atomic F density are necessary to improve the etching
process for high aspect ratio contact holes. Hence, the etching
plasma has been changed from low density to high density, and
a long residence time to a short residence time to obtain a higher
ion current with low dissociation of fluorocarbon molecules. A
high frequency (27–100 MHz) system with a short residence
time (created using a narrow gap and a high total flow rate of the
gases) is one of the ways to create high performance SiO2
etching. As we described in Sections 2 and 3, the process
window of etching processes for low-k materials (SiOCH and
organic material) is narrower than that for SiO2. Plasma
parameters must be adjusted to the ‘‘sweet spot’’ of each
etching step for multi-stacked structures (such as hybrid
structures with multi-layer hard masks), and ‘‘sweet spots’’
(where sufficient results can be obtained) for each material are
different from each other (Fig. 40). Small fluctuations of one of
the plasma parameters can cause problems in the etched profile
or unexpected interfaces.
Through the experience of process development for various
devices, our database of plasma processes has grown larger. To
find an optimal plasma condition for each material or each
structure, the models for controlling the surface reaction are
very important. To adjust the plasma parameters, the energy of
ions and the amount of total atom fluxes must be controlled
precisely. Unfortunately, most commercial dry etching systems
only show external parameters, such as power, pressure, and
flow rate, which do not show the correct value of the ion energy
and the absolute density of reactive species. Knowledge of the
relationship between external parameters and internal plasma
parameters (ne, Te, IEDF, etc.), as well as in situ quantitative
plasma monitoring technology is required.
5.2. Instantaneous stabilization of plasmas
To fabricate multi-stacked dual damascene structures,
several different types of plasmas (C–F plasma, N–H plasma,
etc.) were used in the same chamber. In the next generation,
critical dimensions (both width and depth) will be smaller. As a
result, the etching time for each material will be shortened to
only several seconds. However, the densities of reactive species
are very sensitive to changes of the wall conditions, as
discussed in Sections 2.1.2 and 3.1.2. As a result, during the first
several seconds of each etching step, the plasma can be affected
by wall conditions (such as polymer deposition), which depend
on the plasma used in previous steps. As a result the density of
reactive species can be changed gradually during etch step. For
example, the etch rate of SiN stopper material was changed by
the etching plasma used in the previous SiO2 etching, as shown
Fig. 42. Quantitative control of the reactions in several atomic layers on the etched surface (the pattern width and depth will be smaller, and the fluctuation of the
profile control must be suppressed to within several nanometers. This is comparable to ion penetration, degradation, and the thickness of surface polymers. Process
development with a microscopic point of view is necessary. The surface reaction is not unique, but gradually changes with changing depth and/or time).
T. Tatsumi / Applied Surface Science 253 (2007) 6716–67376736
in Fig. 41. In this case, a period of about 180 s was required to
stabilize the etching plasma, while the etching was finished
within 40 s (with thick polymer on the walls) [68,77].
This means a transition plasma is being used, which
gradually changes from one steady state to another steady state.
However, similar to Section 5.1, the plasma parameters must be
kept at the ‘‘sweet spot’’ for each material during etching.
Ideally, the stabilizing time of each plasma should be within 1 s
for each etching step. To improve the stability of the processes,
plasma–wall reactions must be controlled actively. After the
optimization of wall conditions, the stabilizing time can be
minimized.
5.3. Atomic layer modification
The variation of the critical dimensions must be controlled
within several nanometers. However, to optimize the etching
reaction on low-k surfaces (Sections 2.2 and 3.2) or to optimize
the surface degradation on Cu/low-k surfaces (Sections 4.1 and
4.2), the thickness of the C–F polymer, the penetration depth of
ions, and the diffusion of radicals, as well as the absorption of
VUV photons must be controlled. These parameters are also on
the order of several nanometers (Fig. 42) [78]. From a
microscopic point of view, a small change in surface layers
must be imagined, where the atomic composition is gradually
changed layer by layer at every moment. Directly monitoring
these non-static reactions on the surface layers is difficult.
Simulations for the prediction of surface reactions must be
improved.
6. Conclusion
We clarified the mechanisms of plasma and surface reactions
in processes for low-k (SiOCH and organic material)
integration. The process window for each process is quite
narrow because the requirements of process control are high
and the materials (both low-k and metal) are less stable and
more sensitive to changes in plasma parameters. To create a
higher reliability interconnect, the balance between incident
fluxes must be controlled quantitatively for each process,
including ashing or surface treatment on Cu surfaces. The
controllability and stability must be further improved.
Acknowledgements
We would like to thank to Prof. Toshiaki Makabe of Keio
University, Prof. Masaru Hori of Nagoya University, Mr.
Kazunori Nagahata, Mr. Masanaga Fukasawa, Mr. Keiji
Ohshima, and Dr. Shoji Kobayashi of Sony Corporation, Mr.
Keisuke Urata of Sony Semiconductor Kyushu Co. Ltd., Mr.
Kohich Yatsuda and Takahiro Saitoh of Tokyo Electron Inc., Dr.
Hisataka Hayashi of Toshiba Corporation, and Dr. Keizo
Kinoshita of SELETE for their useful discussions. Some part of
this work was done at ASET and was supported by NEDO.
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