A Batch Processing Module

Thomas F. EdgarDepartment of Chemical Engineering

University of TexasAustin, TX 78712

Batch Processing – Definition, Advantages, Disadvantages

• A sequence of one more steps (recipe) usually carried out in more than one vessel and in a defined order, yielding a finished product

• Production amounts are usually smaller than for continuous processing

• Requires reduced inventories and shorter response times

• Final product quality must be satisfied with each batch (no blending)

• More emphasis on production scheduling in batch processing

Other Advantages

• Batch time can be adjusted to meet quality specs

• Repetition is conducive to continuous improvement in product

• Slow dynamics permit real-time calculations• Greater agility

Batch Processing Used in Manufacturing

• Electronic materials• Specialty chemicals• Metals• Ceramics• Polymers• Food and agricultural materials• Biochemicals• Multiphase materials/blends• Coatings• Composites

Representative processing steps in a semiconductor wafer fab(Deposition, Patterning, Etching, Doping, etc)

Unit operations in microelectronics manufacturing are characterized by:

1. Physical/chemical complexity2. Inability to measure directly many

process variables3. High sensitivity to process changes4. Multiple inputs/multiple outputs

CD Variation Effects in Pattern Transfer

WaferFlatnessReflectivityTopography

Resist

ReticleCDDefectsEdge Roughness Proximity Effects

Refractive Index ThicknessUniformityViscosityContrast

PEB

TemperatureUniformityTimeDelay

AberrationsLens HeatingFocusLevelingDose

Stepper

Develop

PowerPressureFlow rates

TimeTemperatureDispense PatternRinse

Etch

Environment

AminesHumidityPressure

Why Control Critical Dimension (CD)?• Small changes in CD distribution = Large $ values lost

290

Higher speed

Gate CD (nm)

210 230 250 270

Zero

Yie

ld(H

igh

leak

age)

Zero

Yie

ld(L

ow c

ondu

ctan

ce)

3σ = 12nm

3σ = 36nm

Control Hierarchy in Batch Processing

1. Sequential control to step the process through a recipe

2. Logic control to deal with device interlocks3. Within-the-batch control to make set point

changes and reject disturbances4. Run-to-run control to meet final quality

constraints5. Batch production control to maximize utilization

of equipment and minimize cycle time

Figure 22.19 Batch control system – a more detailed view

Within-the-Batch Control:Operational Challenges

• Time-varying process characteristics (no steady state)

• Nonlinear behavior• Model inaccuracies• On-line sensors often not available• Constrained operation• Unmeasured disturbances• Irreversible behavior

Run-to-Run (RtR) Control• Keeps batch process product on target by using

feedback to manipulate batch recipe for consecutive batches

• Required due to a lack of in situ, real-time measurements of product quality of interest

• Extremely useful where initial conditions or tool states are variable and unmeasurable

• Supervisory controller determines optimal setpoints for real-time control loops (typically PID)

Application: Resist etch process

• The incoming pattern is masked with linewidths greater than required.

• An additional step is added to the etch process which etches the resist pattern.

• The resist etch step trims the lines to the proper resist linewidth.

• The rest of the etch transfers the resulting mask pattern into the polysilicon, creating the poly gate structures.

Linewidth

Silicon Substrate

STI STI

Poly Silicon

Gate Oxide

SiONTrimmed

Resist

Results – Increased Cpk

0

10

20

30

40

-1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Normalized Deviation From Target

Freq

uenc

yPrior to APC Implementation

With Automated Run-to-Run Control

Metric Uncontrolled Controlled % ChangeMean Deviation From Target -0.201 0.045 -77%

Standard Deviation 0.254 0.188 -26%Cpk 1.05 1.7 +62%

Figure 22.17 Multiproduct batch plant

Characteristics of batch scheduling and planning problems (Pekny and Reklaitis)

GIVEN

Product requirementsHorizon, demands, starting andEnding inventories

Operational stepsPrecedence orderResource utilization

Production facilitiesTypes, capacities

Resource limitationsTypes, amounts, rates

DETERMINE

WhatProduct amounts: lot sizes, batchSizes

WhenTiming of specific operations, runlengths

WhereSites, units, equipment items

HowResource types and amounts

Multi-Product Processing Overview

Process 2

Fab Tool

Process 1

AB BBA A

Process 2

Fab Tool

Process 1

A A A A

Fab Tool

Process 1AB BBA A

Metrology Variations

0

0.5

1

1.5

2

2.5

3

3.5

4

A B C D E

Correct MetrologyOrdering

0

0.5

1

1.5

2

2.5

3

3.5

4

E D C B A

Incorrect MetrologyOrdering

0

0.5

1

1.5

2

2.5

3

3.5

4

C D A E B

Incorrect MetrologyOrdering

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

A B C D E

Process

Integration Across StandardChE Courses

• discrete logic in process operations• design for safe operation• measurement strategies (e.g., end point)• batch vs. semibatch (bioreactors)• use of fundamental (nonlinear) models• numerical methods, statistics• population balances• case studies