Specific Process Knowledge/Lithography/Aligners/Aligner: Maskless 01 processing

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Process Parameters

Exposure dose

The exposure dose needed inn the Aligner: Maskless 01 seems to follow the dose needed to process the same substrate in KS Aligner. As doses get higher, there is a tendency for the dose needed in the Aligner: Maskless 01 to exceed the dose needed in KS Aligner.

Defocus

The optimal defocus setting is probable a function of the resist thickness, but for 1.5µm resist, a defocus of -4 seems to be optimal.

Substrate positionning

During load, the machine will focus on the surface of the sample. Then, using the pneumatic focusing mechanism, it will detect the edges of the sample (dependent on the substrate template used) in order to determine the center of the sample. The following results rapport findings using the "4 inch wafer" template on a standard 100mm Si substrate.

Substrate centering

During the substrate detection, the sample is scanned along the x- and Y-axes, as well as diagonally. From these measurements, the 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.
Unfortunately, the centering does not compensate for major or minor flats. The center position will therefor typically be displaced several mm from the center of the substrate along the Y-axis due to the major flat, and possibly also shifted due to any minor flats. These shifts have successfully been compensated during file conversion, in order to give a centering accuracy of ±200µm.

Flat alignment

At the end of the substrate detection, the sample is scanned twice along the 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.
The flat alignment accuracy has been measured to be 0±0.1° quite consistently. Out of a total of 13 exposures, only two were misaligned by more than 0.1°

Alignment

The alignment accuracy of the Aligner: Maskless 01 is a combination of the position accuracy of the stage, the accuracy of the alignment mark detection, and the accuracy of the pattern already on the wafer (first print).
By measuring the stitching accuracy between two layers printed on the same substrate (without unloading the substrate), we can assess the stage accuracy. By aligning to a pattern previously exposed by the Aligner: Maskless 01, we can assess the mark detection accuracy. And finally, by aligning to a pattern exposed on a mask aligner, we can assess the mask-less aligner's ability to compensate for any scaling and orthogonality errors between the two machines.

The results reported here use printed verniers to assess the misalignment along the two axes at different points on the wafer using an optical microscope. Two different designs were used; a ±5µm vernier and a ±1µm vernier. Both consist of a scale of 5µm lines with 10µm pitch, and a vernier scale to enable subdivision of the 5µm or 1µm scale into tenths, i.e. 0.5µm or 0.1µm. During read-out, symmetry enables the observer to measure with ±0.25µm or ±0.05µm accuracy.
The samples are 100mm Si wafers coated with a 1.5µm layer of the positive tone resist AZ 5214E.

Stitching

In the stitching test, the design consists of ±5µm and ±1µm verniers along the X and Y axis placed in a 3 by 3 matrix covering a 60mm by 60mm area centered on the wafer. The sample is loaded, and the first layer (the linear scales) is printed. Without unloading, the second layer (the vernier scales) are printed on top of the first, and then the sample is developed.

The results in the table below show that the errors are at or below the measurement uncertainty for the stitching tests using no flat alignment.
When flat alignment is used, a rotation error of ~1.5ppm appears, along with a ~0.3µm shift and a +6ppm scaling of the Y axis. This is the level of accuracy we can expect when the rotation compensation is applied, i.e. for alignment to a previously printed layer.

		Median [µm]	Error [µm]	Run-out [ppm]	Rotation [ppm]
No flat alignment (two samples)	X	-0.03±0.05	0.01±0.05	1.00±2.33	0.00±1.18
	Y	-0.05±0.05	-0.07±0.05	0.42±2.33	0.00±1.18
With flat alignment	X	0.00±0.05	0.05±0.05	-0.67±2.33	-1.67±1.18
	Y	-0.25±0.05	-0.29±0.05	6.33±2.33	-1.38±1.18

Overlay

		Scaling [ppm]	Shearing [mRad]	Median [µm]	Error [µm]	Run-out [ppm]	Rotation [ppm]
2 alignment marks (two samples)	X	NA	NA	-0.03±0.05	0.01±0.05	1.00±2.33	0.00±1.18
	Y	NA		-0.05±0.05	-0.07±0.05	0.42±2.33	0.00±1.18
4 alignment marks	X		0,0001	0.00±0.05	0.05±0.05	-0.67±2.33	-1.67±1.18
	Y			-0.25±0.05	-0.29±0.05	6.33±2.33	-1.38±1.18