Chapter 3.2

GCAWS2 Wafer Alignment Key Design Guide

(gcaws2)

1.0 Title

GCAWS2 Wafer Alignment Key Design Guide

2.0 Purpose

GCA Steppers employs a unique off-axis wafer alignment system that separately aligns a mask to the optical column and a wafer to the optical column. Alignment of the wafer to the optical column is referred to as wafer alignment. This is done manually by utilizing wafer alignment keys printed and etched into the wafers at previous layers. Alignment key specification is outlined in this manual.

3.0 Scope

This document describes the design guidelines for wafer alignment keys for the GCA 4800/6200 DSW Wafer Stepper System. It details requirements for dimensions and placement of manual wafer alignment keys.

4.0 Applicable Documents

Revision History

The following documents are published by GCA and are kept in the Microlab office.

4800 DSW Wafer Stepper System Instrument Instruction Manual (Document Part No. 011819, Rev. 1 Printed 12/15/82). This manual details the hardware.

DSW WAFER STEPPER SYSTEM - System Control (Document Part No. 011817, Rev. 4CPrinted 10/25/82). This manual is the software manual that details how to specify, edit, and run a job.

See also Chapter 3.1 (Mask Generation using CAD software).

5.0 Definitions & Process Terminology

N/A

6.0 Safety

N/A

7.0 Statistical & Process Data

N/A

8.0 Process Notes

8.1 Readily Available Wafer Alignment Keys (gds Format)

You can find a .gds copy of both dark field and clear field version of the alignment marks in the .gds format on silicon.

At silicon prompt, simply type: cd /mercury4/cad/gcaws2

Following files are available for lab members to download:

clear_field_gcaws2_key.gds

dark_field_gcaws2_key.gds

These marks can be incorporated into your chip layout design by importing them first into Cadence or L-Edit or similar CAD software that you are using for your design, as long as .gds format is accepted. One can also generate these keys in KIC layout design tool, as per instruction provided in the Appendix section of this manual.

9.0 Procedure for Generating the Alignment Key and Proper Stepping Distance for GCAWS2

The standard wafer alignment keys are used by an operator to align the wafer manually.

Figure1 shows schematics of a clear field (light field) alignment key at the wafer level.

Figure2 shows schematics of a dark field alignment key at the wafer level.

9.1 Alignment Key Placement Guideline

The GCA Stepper at Berkeley is installed with a 100-millimeter wafer chuck and objective lenses spaced 76.2 millimeter (3.0 inches) apart. Wafer alignment keys must be forward of the wafer center in the Y direction by 3.2 millimeters (0.125 inch). You can consult page 6-3 of the 4800 DSW Wafer Stepper System Instrument Instruction Manual. The shaded region of the illustration shows the allowable region for placing wafer alignment keys.

Wafer alignment requires two alignment keys: a right-hand wafer alignment key (X and Y, or primary, alignment key), and a left-hand wafer alignment key (theta, or secondary, alignment key).There are two ways to place the keys so that they are physically separated by 76.2 millimeters (3 inches), the objective separation, on the wafer.

9.1.1 Placement Method 1

This method is preferred when you don't want to worry about the step size, and it is the simpler of the two. This first method involves using a reticle that has only one wafer alignment key and specifying a step size in X, so that there will be two keys spaced exactly 76.2 millimeters in your wafer layout. A typical example of a wafer layout is shown below. The die size is defined as the size of dies on a wafer, and step size in X is defined, as the horizontal distance between centers of successive exposures in the array. Step size in X determines the stepping distance between the adjacent columns.

When specifying a job on the GCA Stepper, you should indicate that standard keys are used and enter an appropriate step size in X, as per table1 below. Your step size should be greater than the die size in order to generate scribe lanes or streets between your dies. This means you need to size your dies appropriately. This extra area between your printed dies will help you see the die border, and later can be used for wafer dicing at the end of your process (diamond saw will run through these streets not impacting your product die). A typical scribe lane size is in the order of 100 µm – 200 µm at wafer level.

Column Number

Step Size in X (mm)

76.2 / (n – 1)

5.86154

6.35000

6.92727

7.62000

8.46667

9.52500

10.88571

12.70000

15.24000

19.05000

Table1 - Die Column and Step Size Chart

9.1.1.1 Example

This example has 10 columns and step size of 8.46667 millimeters.

Each die will repeat the alignment mark, once on the wafer. Spacing between the first alignment key to n_thalignment key (n - 1 die over) for this particular die size/stepper job, and at the wafer level can be calculated by the following formula:

Spacing = (n - 1) x 8.4667 mm

Spacing between the first alignment key and the 9^th die = 9 x 8.46667 = 76.2 mm.

This will provide proper mark spacing for the two microscopes objectives at a distance of exactly 76.2 mm apart for GCAWS2 stepper.

9.1.2 Placement Method 2

The second method involves using a reticle that has two wafer alignment keys, and specifying a step size in X, so that two keys will become the left- and right-hand wafer alignment keys. This method essentially provides an extra key in the die to correct for proper alignment marks spacing between the first and n_th die, needed to be spaced specifically 76.2 mm apart at the wafer layout. This method is used when the step size in X and the die size are predetermined. The offset used for the placement of the extra key in the dies will make the secondary key show up at correct spacing under the objective lenses of the stepper Microscopes.

9.1.2.1 Algorithm

Follow the algorithm below to calculate the separation of the two wafer alignment keys on the reticle, and the number of columns.

Variables Definitions:

Input: OS = objective spacing = 76.2 (mm)

DS = die size (mm) = user's input

SSX = step size in X (mm) = user's input

Output: WAKRS = The separation of the wafer alignment keys on the reticle, to be used when designing the reticle.

NCOL = The number of columns of dies, to be entered when specifying the job on the GCA Stepper.

Algorithm: # this is a line of comments.

# I assume the following functions:

# intdiv(a, b) = integer divide

# = integer(a / b)

# mod(a, b) = the remaining function

# = a - intdiv(a, b) * b

if mod(OS, SSX) < DS then

# The right-hand key on the reticle is the left-hand wafer.

# alignment key, and the left-hand key on the reticle is

# the right-hand wafer alignment key.

# Key offset is associated with the left-hand key on the

# reticle.

NCOL = intdiv(OS, SSX) + 1

WAKRS = mod(OS, SSX)

else

# The right-hand key on the reticle is the right-hand wafer

# alignment key, and the left-hand key on the reticle is

# the left-hand wafer alignment key.

# Key offset is associated with the right-hand key on the

# reticle.

NCOL = intdiv(OS, SSX) + 2

WAKRS = SSX - mod(OS, SSX)

9.1.2.2 Example

Assuming die size (DS) equals to 10.0 millimeters and objective spacing (OS) equals to 76.2 millimeters. Following table was compiled:

SSX (mm) 9 10 11 12

WAKRS (mm) 4.2 6.2 0.8 4.2

NCOL 9 8 8 7

Which clause was followed:

then then else then

Which key on the reticle is chosen as the right-hand wafer alignment key (the X and Y, or primary, alignment key):

L L R L

An example of the left-hand key on the reticle is chosen as the right-hand wafer alignment key.

SSX = 12

Column	1	2	3	4	5	6	7
Reticle	L R	L R	L R	L R	L R	L R	L R
Wafer	R L	R L	R L	R L	R L	R L	R L
Alignment Key spacing	0	12	24	36	48	60	72 76.2

An example of the right-hand key on the reticle is chosen as the right-hand wafer alignment key.

SSX = 11

Column	1	2	3	4	5	6	7
Reticle	L R	L R	L R	L