Specific Process Knowledge/Lithography/Aligners/Aligner: Maskless 02 processing: Difference between revisions

[[Category: Equipment|Lithography]]

[[Category: Lithography]]

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=Exposure technology=

The light source is a laser diode (array) with a wavelength of 375 nm (2.8 W). The spatial light modulator is a digital micro-mirror device. The individual mirrors of the DMD are switched on and off in order to represent the design, and the laser is flashed on and off, in order to give the desired exposure dose. The exposure image is projected onto the substrate through a lens-system. The projected image has a pixel size of 500x500 nm on the substrate surface. The image is scanned across the substrate in stripes in order to expose the entire design. Each stripe is overlapping 2 or 5 times, depending on the chosen [[Specific_Process_Knowledge/Lithography/Aligners/Aligner:_Maskless_02_processing#Exposure_mode|exposure mode]]), in order to reduce light-source non-uniformity effects and stitching errors. The address grid size is 250 nm or 100 nm for Fast and High Quality exposure mode, respectively.

The correct way to determine the best dose-defocus settings is to generate a so-called Bossung plot (known from projection lithography), which plots the printed linewidth as a function of dose and defocus. From this, the most stable region of parameter space is chosen, i.e. the region where the linewidth changes the least when dose and defocus changes. Any deviation from the design linewidth may be corrected using the CD bias parameter. This typically involves SEM imaging of resist cross-sections, and quickly becomes time-consuming. However, in most cases, inspection of a dose-defocus matrix (easily generated using the series exposure function) in an optical microscope will get you most of the way.  

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Aligner: Maskless 02 offers two autofocus modes; optical or pneumatic. The autofocus mode is selected via the substrate template. The defocus process parameter is used to compensate for offsets between the autofocus mechanism and the focal point of the exposure light, and simultaneously optimize print quality in different resists and varying thicknesses.

'''Pneumatic:'''

==Exposure mode==

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==Acceptance test==

<span style="color:red">New specs after the new writehead has been installed - section will be updated soon.</span>

==Substrate centring and flat alignment==

During substrate detection, the sample is scanned along the X- and Y-axes, as well as diagonally. From these measurements, the size (diameter) of the substrate is calculated, as well as the stage position matching the center of the substrate. This stage position will be the default origin for the subsequent exposure.

At the end of substrate detection, the sample is scanned twice along the bottom edge (flat), in order to determine the substrate rotation. This angle will be presented in the exposure panel along with the option to expose the design rotated in order to compensate for this angle, i.e. aligned to the flat/edge of the substrate.

'''Result of using "Expose with substrate angle" ("Expose with Global Angle"):'''

*Centring:

'''Result of loading the same substrate ~10 times without removing it from the stage:'''

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|-style="background:WhiteSmoke; color:black"

| ±1.4 mRad

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| ±1.4 mRad

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=Alignment=

'''Important information regarding alignment:'''

After alignment to the specified alignment marks, the Aligner: Maskless 02 will automatically compensate for the translation (shift) and rotation detected during the alignment. However, the translation is set using only the first alignment mark. This means that if there is any run-out (gain) in the wafer, the alignment will be perfect at the first alignment position, while the second alignment position will be off by the run-out error. The central part of the print will be misaligned by half the run-out error. This is different compared to alignment in a mask aligner, where the user usually splits the run-out error between the two sides, resulting in good alignment in the center. In Aligner: Maskless 02, run-out may be compensated by activating the scaling function, which only becomes available when using 3 or more alignment positions. To help assess whether the measured scaling or shearing is significant, 10 ppm (scaling = 1.000010 or 0.999990; shearing = ±0.010 mRad) corresponds to a difference of 1µm from one edge to the other on a 4" wafer.

'''Alignment accuracy:'''

The result shows significant misalignment between the two layers, higher than what is seen in true overlay tests. This is believed to be an effect of drift during the first, long print (most likely heating of the stage), as the misalignment is seen to be biggest between parts printed furthest apart in time (i.e. on the left side of the wafer). In a normal overlay print, this error can be at least partially compensated using the scaling and shearing function available for three or more alignment marks/positions.

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==Top Side Alignment==

<span style="color:red">New specs after the new writehead has been installed - section will be updated soon.</span>

'''Camera field of view (W x H):''' <br>

{|border="1" cellspacing="0" cellpadding="3" style="text-align:center;"  

|-

!Shearing [mRad]

!Misalignment [µm]

!Run-out [ppm]

!Rotation [ppm]

|-

|-

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|rowspan="2"|2 alignment marks

(Taran Apr 2019)

|rowspan="2"|NA

| 0.25±0.15

| 1.7±1.7

| 2.2±1.3

| NA

| 0.3±0.13

| 0.8±1.7

| 0.0±1.7

|-style="background:WhiteSmoke; color:black"

|rowspan="2"|4 alignment marks

(Taran Apr 2019)

|rowspan="2" align="center"|0.001

| 0.2±0.05

| -0.6±1.7

| 0.3±1.2

| -1

| 0.4±0.05

| 0.6±1.7

| 0.6±1.2

|-style="background:WhiteSmoke; color:black"

|rowspan="2"|4 alignment marks

(Taran Apr 2019)

|rowspan="2" align="center"|0.002

| -0.05±0.13

| -1.9±1.7

| 0.8±1.3

|'''Y'''

| -2

| 0.3±0.05

| -0.6±1.7

| 1.1±1.7

|}

{|border="1" cellspacing="0" cellpadding="3" style="text-align:center;"  

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==Back Side Alignment==

'''BSA windows:''' along the X and Y axes, 10 mm wide x 46 mm long, starting 14.5 mm from the center.

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|[[Image:MLA150 BSA 2inch.JPG|300px]]

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|'''X'''

| -0.625 ±0.125

| -0.75 ±0.125

|-

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|'''Y'''

| 0.0 ±0.0

|}

==Advanced Field alignment (TSA)==

'''Overlay accuracy (spec):''' 0.25 µm (5x5 mm² area)

!Average [nm]

|-

|-

| 150 ±50

| 128 ±50

|-

| 0 ±50

| 16 ±50

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