Specific Process Knowledge/Lithography/Aligners/Aligner: Maskless 01 processing: Difference between revisions

=Exposure technology=

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The light source is a 10W 365nm LED with a FWHM of 8nm. The spacial light modulator is an 800 X 600 pixel digital micro-mirror device. The individual mirrors of the DMD are switched in order to represent the design, and are timed in order to yield the desired exposure dose, while taking into account illumination uniformity, soft-stitching, and possibly also sub-pixel features. This image is projected onto the substrate through a lens(system). The projected image yields a writing field of 400µm X 300µm, and thus a pixel size of 0.5µm X 0.5µm at wafer scale. This writing field is stepped across the substrate, in order to expose the entire design, each field overlapping slightly in order to minimize stitching errors.  

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The site acceptance test performed during the installation of the Aligner: Maskless 01 showed an exposure time/speed of 0.025 min/mm², which is slightly lower than the 0.02 min/mm² given in the specifications. At such speeds, a full 4" design would take 2:30-3:15 hours to expose. In practice, we observe exposure times in excess of 2 hours for a full 4" design.

The exposure time increases linearly with exposure dose and writing area. However, due to the stepped nature of the exposure, the exposure time as a function of fill factor is highly nonlinear. It takes the same time to expose a single pixel as an entire 300µm X 400µm writing field, so the exposure time depends on the number of addressed writing fields, rather than on the fill factor of the design. In practice, there will probably not be much variation in exposure time with fill factor. Exposure tests using a 50mm² design have shown that the exposure time increases linearly from 40s at 10mJ/cm², to 257s at 1000mJ/cm². The fill factor of the design is 39%, but ~80% of the area is addressed by the writing fields. Scaled to a full 4" wafer, the exposure time is estimated to 2:37 hours at a dose of 100mJ/cm².

==Resolution==

The table below shows the result of a resolution test using 1.5µm and 0.5µm positive resist. For 1.5µm resist the resolution is 2µm, maybe even a little lower, while it is 1µm, or at least close to, for 0.5µm resist. The optimal dose depends on the designed structures; dots require a lower dose in order to print to size than lines. In the case of a dark field design, trenches would probably require a lower dose in order to print to size than lines, while holes would require a higher dose to print than trenches.

During load, the machine will focus on the surface of the sample. Then, using the pneumatic focusing system, it will detect the edges of the sample (this function dependes 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.

During (4") 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.  

<br/>Unfortunately, the centering does not compensate for major or minor flats. The center position will therefore typically be displaced several hundred µm 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, which yielded a centering accuracy of ±200µm. The table below lists the corrections needed during file conversion in order to compensate for shifts due to major and minor flat, dependent on the position of the minor flat. Keep in mind that the tolerances of substrate diameter and flat lengths in the SEMI wafer standard introduce uncertainties to these numbers in the order of 0.4-0.9mm. Accurate positioning of the design relative to the substrate center would require measurement of diameter and flat lengths of the individual substrate, and subsequent recalculation of the correction values.  

<br/>The flat alignment accuracy has been measured to be 0±0.1° (1.7mRad) quite consistently. Out of a total of 15 exposures, 13 showed misalignment of 0.1° or better, despite initial sample rotations exceeding 5°.

=Alignment=

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 4µm 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 inspection, observation of the symmetry of neighboring lines enables the observer to read the shifts with ±0.25µm or ±0.05µm accuracy.

<br/>The deviations (±) given for the results here are calculated as half the range of measurements. If the range is small, the measurement uncertainty is used in stead.

<br/>The samples used for these tests are 100mm Si wafers coated with a 1.5µm layer of the positive tone resist AZ 5214E.

==Stitching==

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The easiest approach to mix-and-match is to use a (simple) positive tone resist, such as AZ 5214E, and develop the pattern after the first exposure. The remaining resist can be exposed using the same parameters as normal, and there is plenty of contrast for the alignment process. This process can also be used if the maskless aligner is used first.