dose determination using zep520a resist as a model

Dose Determination Using ZEP520A Resist as a Model

Nicole DevlinDevin BrownAugust 2010

Experimental Conditions and Process

Spin coat ZEP520A on a silicon wafer at 4000 rpm, 2000 r/s

for 60 seconds, bake at 180°C for 2 minutes

Expose with a JEOL JBX 9300FS electron beam lithography system at 2 nA

and 100 kV

Develop sample in amyl acetate for 2 minutes, rinse in isopropanol for 30 seconds, and nitrogen blow dry

Determine the Base Dose

Method 1: expose large squares with a range of doses and plot the resist thickness left at each square verses the dose applied to that square to determine which dose will clear large features

Method 2: expose 200 nm line and space patterns and expose with a range of doses to determine which clears with the correct size

100 µm squares

70 µC/cm² 80 µC/cm² 90 µC/cm² 100 µC/cm² 110 µC/cm² 120 µC/cm² 124 µC/cm² 130 µC/cm² 140 µC/cm² 150 µC/cm²

250 µC/cm²240 µC/cm²230 µC/cm²220 µC/cm²210 µC/cm²200 µC/cm²190 µC/cm²180 µC/cm²170 µC/cm²160 µC/cm²

50 µm squares



25 µm squares



The dose curve shifts for different sized squares

The 100 µm and 50 µm squares were measured with a refractometer. The 25 µm squares were measured with a profilometer.

Dose (µC/cm²)

Thickness (Å)

100 µm 50 µm 25 µm

70 3690 3791 3446

80 3543 3716 3410

90 3295 3609 3351

100 2832 3421 3281

110 2157 3158 3199

120 1024 2749 3073

124 601 2540 2982

130 158 2153 2811

140 95 1281 2643

150 65 469 2266

160 41 94 1815

170 35 43 1116

180 9 5 165

190 0 0 0

200 0 0 0

210 0 0 0

220 0 0 0

230 0 0 0

240 0 0 0

250 0 0 0

200 nm line and space at 170 µC/cm²

Actual gap (exposed area) width:

Mean= 185.5 nm

σ= 26 nm

Method 2 base dose determination: 200 nm line and space exposures



Mean= 205.6 nm

σ= 4.1 nm



Mean= 222.7 nm

σ= 3.9 nm


Actual gap (exposed region) width:

Mean = 221.2 nm

σ

= 2.5 nm



Mean = 226.2nm

σ= 3.2 nm



Mean = 226.2 nm

σ=5.4 nm



Mean = 244.8 nm

σ=4.1 nm



Mean = 246.9 nm

σ= 5.9 nm

Plot of Method 2 for Dose Determination

The dose needed to clear the resist also makes the features larger than the specified CAD design. The solution to this problem is biasing the line widths.

Conclusions

190 µC/cm²

is the correct base dose using method one (large squares).

190 µC/cm²

is the closest to the correct base dose using method two, however the lines need to be biased

(drawn smaller than the intended size) to both clear the resist and print with the correct dimensions.

Method 1 (large square patterns) is easier to complete, so this method is recommended

Sceleton Simulation and Proximity Correction

Number of Electrons Analysis

Using more electrons significantly increases the time of the simulation (1E8 electrons took over 24 hours).

It was found that the energy values are approximately the same for 1E6, 1E7, and 1E8 electrons (see the next two slides).

Sceleton simulation for 340 nm of ZEP on a silicon substrate

There were three different simulations completed with different numbers of electrons simulated in each (1E6, 1E7, and 1E8 electrons). The data was extracted at the middle of the resist layer.

Radius (µm)

Energy (eV/µm³)

1E8 1E 7 1E6

0.00 5.34E+04 2.80E+05 3.15E+05

0.02 7.50E+02 7.41E+02 7.36E+02

0.04 9.16E+01 9.23E+01 9.45E+01

0.06 2.58E+01 2.54E+01 2.74E+01

0.08 1.09E+01 1.08E+01 1.11E+01

0.10 5.75E+00 5.80E+00 5.64E+00

0.12 3.62E+00 3.53E+00 3.29E+00

0.14 2.48E+00 2.42E+00 2.26E+00

0.16 1.83E+00 1.79E+00 1.58E+00

0.18 1.46E+00 1.45E+00 1.55E+00

Zoomed in and log-scale graphs

Select data points from the plots

Sceleton simulation at two different ZEP resist thickness, 400 nm and 340 nm thick. Both simulations used 1E6 electrons and were extracted from the center of the resist layer.

Zoomed in Sceleton Comparison with a log x-axis

Conclusion: Minor variations in resist thickness can affect the electron scattering

Comparing different mesh values in the sceleton simulation

The mesh value is the increment by which the sceleton simulation

takes energy measurements. It is automatically set to 20 nm.

To change the mesh value in sceleton, add these lines to your .sip file:

MeshSizeLateral/nm 1.0

MeshNumberLateral 60000

This will give you a 1 nm increment for a 60 µm radius

Note: having a mesh value of 1 nm will make your simulation take

approximately an order of magnitude longer to run (than having the default mesh value of 20 nm).

Energy at the injection point:

1 nm mesh > 5 nm mesh > 20 nm mesh

Radius (µm)

Energy (eV/µm³)

1 nm mesh5 nm mesh 20 nm mesh

0 5.34E+07 4.33E+06 3.32E+05

0.02 4.99E+03 3.52E+03 1.39E+03

0.04 3.70E+02 2.92E+02 1.75E+02

0.06 8.26E+01 6.95E+01 4.71E+01

0.08 2.63E+01 2.25E+01 1.93E+01

0.1 1.19E+01 1.24E+01 9.98E+00

0.12 6.35E+00 5.63E+00 5.78E+00

0.14 4.02E+00 4.72E+00 3.99E+00

0.16 2.62E+00 3.07E+00 2.71E+00

0.18 1.52E+00 2.52E+00 2.00E+00

0.2 1.36E+00 1.77E+00 1.36E+00

As the mesh value decreases, the energy at the injection point goes up by orders of magnitude.

Proximity Effect Correction (Layout BEAMER) and Exposure

zep_dose03.mgn exposed on 6/28/2010 to test the proximity correction software abilities (Layout BEAMER outline of .v30 files)

line: space

50 nm lines

1:5 1:10 50 µm space1:2

100 nm lines

1:5 1:10 50 µm space1:21:1

20 nm lines

1:5 1:10 50 µm space

These patterns, along with the 200 nm line and space pattern, were exposed at three different base doses: 190 µC/cm², 200 µC/cm², and 210 µC/cm²

After exposure, they were developed with the standard develop recipe (Amyl acetate for 2 minutes, rinse in isopropanol for 30 seconds) and sputter coated with gold for imaging.

Dose Correction with a 20 nm mesh and no short range correction

50 nm 100 nm 200 nm

1: 50 µm 1:10 1:5 1:50 µm 1:10 1:5 1:2 1:1

190 µC/cm² 60.0 60.3 62.9 116.0 110.6 113.8 116.4 224.3

200 µC/cm² 67.1 68.3 62.9 ---- 119.0 115.1 119.1 231.1

210 µC/cm² ----- 67.8 79.7 132.1 130.8 127.6 141.9 252.5

Actual Line width sizes using a Sceleton Simulation and Proximity Correction software (200 nm lines have no correction)

20 nm 50 nm 100 nm

1:50 µm 1:10 1:5 1: 50 µm 1:10 1:5 1:2 1:50 µm 1:10 1:5 1:2 1:1

20 nm mesh 1.74 1.56 1.43 1.74 1.55 1.42 1.21 1.74 1.57 1.42 1.19 1.01

With short range 1.81 1.62 1.47 ---- ---- ---- ---- ---- ---- ---- ---- ----

5 nm mesh 1.453 1.36 1.29 1.453 1.35 1.28 1.14 1.453 1.36 1.28 1.13 1.02

1 nm mesh 1.687 1.52 1.41 1.695 1.52 1.41 1.18 1.695 1.52 1.40 1.18 1.02Dos

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In the 5 nm and 1 nm mesh sections, the 20 nm lines have short range correction and the 50 nm and 100 nm lines do not.

100 nm lines 1:1 and 50 nm lines 1:2 collapse

100 nm lines 1:1 dose = 190 µC/cm²

50 nm lines 1:2

dose = 230 µC/cm²

These lines fell over because the aspect ratio was too high. Aspect ratio is the height of the lines divided by the width of the lines. ZEP has a maximum aspect ratio of about 4. The resist thickness is just over 400 nm so the aspect ratio is just slightly too aggressive in both of these cases.

The base dose for all of these lines is 190 µC/cm². Proximity correction was used and the actual dose applied is specified for each image

50 nm and 100 nm lines look good but are not quite the right size

50 nm lines 1:5 270 µC/cm²

width = 62. 9 nm

100 nm lines 1:2 226 µC/cm²

width = 116.4 nm


Line Biasing

Experiment Description: The exposed line widths printed too large, so to get an accurate line size, the CAD file was biased using Layout BEAMER.

(We cannot just decrease the dose because there will be residue left in the

exposed regions) A 5 nm bias means the line is 5 nm smaller on each side (10 nm smaller in total).

6.45.08.512.03.9---8.94.32.7standard deviation

185.585.690.382.680.3---32.235.234.0average10 nm

8.15.23.86.31.84.8---0.85.7standard deviation

203.4103.697.697.298.2100.3---51.949.5average5 nm

1:11:50 µm1:101:51:21:11:50 µm1:101:5Spacing

200 nm100 nm 50 nm Line Width

Line

Bia

s

Results: The 5 nm biased lines are exposed to the correct sizes

200 nm 1:1 spacing 5 nm bias width= 203.4 nm

50 nm 1:5 spacing 5 nm bias width = 49.5 nm 100 nm 1:2 spacing 5 nm bias

width = 98.2 nm


Dose = 193 µC/cm²

Dose = 278 µC/cm² Dose = 231 µC/cm²

All of the 20 nm lines came out under exposed even with proximity correction

20 nm lines 1:5

Dose correction= 1.43 (from Layout BEAMER)

Base dose=210 µC/cm²

Base dose =200 µC/cm²

Base dose =190 µC/cm²

We have been working with GenISys

to fix this problem.

Dose Needed for 20 nm Lines

1:10 spacing dose= 400 µC/cm²

No proximity correction was used on these lines

Average width: 33.5 nm The 20 nm lines are too wide and need to be biased, but at 400 µC/cm²

was the dose to clear 20 nm lines of residue.

dose determination using zep520a resist as a model

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