Baseline correction is an adjustment to the nominal offset of the X and Y stages from the optical axis. It is also used to determine the angle between the stage X-axis motions and the alignment microscope objectives (microscope theta error).

Equipment and Materials

• Three inch silicon wafer, coated with HMDS and positive photoresist, pre-baked as required.
• Baseline Correction Reticle.
• Photoresist developer, glassware.
• Baseline Correction Data Sheet.

Use a copy of the BASLIN file located in the $!!$DX0 directory.

Test Procedure
Note: Using a previously exposed wafer to do the theta correction steps (marked * below) first may avoid some x-y error, which may result in fewer procedure repetitions necessary to bring the correction to the desired tolerance.

• Load and align baseline correction reticle.
• Load prepared wafer.
• EXEC BASLIN PASS 1, using full exposure time.
• Expose the photoresist.
• Do not post-bake photoresist.
• Develop wafer for one-half of the usual time.
• Reload wafer into stepper and EXEC BASLIN PASS 2, using full exposure time.
• *Align wafer in X-Y-Theta BUT DO NOT YET EXPOSE.
• *Press Control and T keys at the same time. Push the X, Y joystick to the left. This will "jog" the wafer so that the right key will now appear in the left side of the alignment monitor (i.e. stage moves so that the right key is now under the left microscope objective). If there is no theta error, the key will be aligned in Y (not necessarily in X though). If the key is not well aligned in Y, adjust the theta micrometer located on the left-rear side of the lens column to bring the key into alignment.
• *Push the X, Y joystick to the right to bring both keys back into view on the alignment monitor. The keys should still be well aligned; if not, press the Control and E keys to disable the jog mode and realign keys. Repeat as necessary.
• Expose wafer for full exposure time.
• Develop wafer using full development time.
• Do not post-bake the photoresist.
• Locate and read the right-side X-axis vernier Record the readings on the Baseline Correction Data Sheet.
• Locate and read the right-side Y-axis vernier. Record the readings on the Baseline Correction Data Sheet.
• Locate and read the left-side Y-axis vernier. Record the readings on the Baseline Correction Data Sheet.
• Using the MODE command, read and record the current Baseline Correction values.
• Using the vernier readings from the right-side vernier, calculate and record the new Baseline Correction values as follows:

New X value = Old X value - (right-side X vernier(µm)/1000)
New Y value = Old Y value - (right-side Y vernier(µm)/1000)
(Algebraic subtraction: [(-1) - (-1) = 0] and [(-1) - (+1) = -2)

• Enter the new Baseline Correction values into MODE.
• Calculate and record the alignment microscope theta error as follows:

theta error = (Y vernier left-side) - (Y vernier right-side).
(Algebraic subtraction: [(-1) - (-1) = 0] and [(-1) - (+1) = -2)

If the theta error is more than +/- 0.1 micron, then the theta micrometer needs further adjustment.
• Perform another baseline correction to verify that the new baseline correction values are correct.

Vernier Reticle

The vernier reticle has X and Y axis arrays, each composed of a female array and a male array, which in turn are each composed of a fine array and a coarse array. The fine array (larger structures) measures -1 to +1 µm in 0.1 µm increments. The coarse array (smaller structures) measures -10 to +10 µm in 1.0 µm increments. The fine and coarse verniers are not cyclic, which is to say that either the fine array is read or the coarse array is read, but not both. Therefore, a reading such as 1.1 µm is impossible. If a reading of more than 1 µm is made, it is necessary to make a baseline correction adjustment in MODE for that reading to reduce the error to less than 1 µm, which will bring the error into the range of the fine verniers. Then another baseline correction procedure needs to be performed in order to reduce the error in the sub-micron range.

File BASLIN in $!!$DX0 directory:

IF DISPLAY (Y) IS CHOSEN:

Pass #0 Array 24 Exposures Total Exposures=24
  1 2 3 4 5 6
1     X X
2   X X X X
3 X X X X X X
4 X X X X X X
5   X X X X
6     X X
Alignment Dice are Standard
R: 4
C: 6
R: 4
C: 1

IF LAYOUT? (Y) IS CHOSEN:

Array Has:              Centered:
6 Rows                  3,4
6 Cols                  3,4
1st Row, Col:
x = -31.75000
Y = -31.75000

IF "V" IS ENTERED:
R: 4     +0.00000
C: 6     +0.00000
R: 4     +0.00000
C: 1     +0.00000

IF "D" IS ENTERED:
PASS 1   1 PLUG   2 EXPOSURES     TOTAL EXPOSURES=2
  1 2 3 4 5 6
1
2
3
4 X         X
5
6
ALIGNMENT DICE ARE STANDARD
R: 4
C: 6
R: 4
C: 1

IF "V" IS ENTERED:
[SAME PARAMETERS DISPLAYED AS FOR PASS 1]

IF "D" IS ENTERED:
[SAME AS PASS 1 WITH THE FOLLOWING EXCEPTION:]
PASS 2   2 PLUG   2 EXPOSURES     TOTAL EXPOSURES=2

File MODE in $!!$DX0 directory: