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Exposure technology

Aligner: Maskless 03 is not a direct writer. In the maskless aligner, the exposure light is passed through a spatial light modulator, much like in a video projector, and projected onto the substrate, thus exposing an area of the design at a time. The substrate is exposed by scanning the exposure field across the substrate in a succession of stripes.

The light source is a laser diode (array) with a wavelength of 405nm (8W). The spacial light modulator is a digital micro-mirror device. The individual mirrors of the DMD are switched in order to represent the design, and the laser is flashed in order to yield the desired exposure dose. This image is projected onto the substrate through a lens(system). The projected image yields a pixel size of 500nm X 500nm at wafer scale. The image is scanned across the substrate, in order to expose the entire design, each stripe overlapping (2 or 5 times depending on exposure mode) in order to minimize uniformity effects and stitching errors. The address grid size is 250nm or 100nm for Fast or Quality exposure mode, respectively.

The writing head of the Aligner: Maskless 03 moves only in the z-direction. Using a pneumatic focusing system, the maskless aligner is able to do real-time autofocus. The defocus process parameter is used to compensate offsets in the focusing mechanism, and to optimize printing quality in different resists and varying thicknesses. The stage of the Aligner: Maskless 03 moves only in x and y. It has no theta-axis. All rotation during alignment is thus accomplished by transformation of the input design.

Process Parameters

The lithographic result of exposure on Aligner: Maskless 03 depends on a lot of factors, including the dose and defocus parameters, and the exposure mode used. The optimal dose and defocus depend on the type and thickness of the resist, and the optical properties of the substrate (e.g. reflective/absorbing/transparent). All of these factors influence the obtainable resolution, as well as the writing speed.

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.

Exposure mode

Aligner: Maskless 03 offers two exposure modes. The exposure mode is selected during design conversion.

Quality mode is used for optimal resolution, best overlay accuracy, and minimum stripe stitching effects.

In the high quality mode, an area of the pattern is exposed by 5 stripes, each 500µm wide and exposing a fifth of the dose. At the same time, sub-pixel interpolation is applied, yielding an address grid size of 100nm.

Fast mode is used for maximum exposure speed.

In the fast mode, each area of the pattern is exposed by 2 stripes only. This reduces the exposure time by 60%, but also increases the size of the address grid in the X-direction to 250nm. This means that alignment will be less accurate in fast mode. Due to less averaging of non-uniformities, stitching effects will be more prominent in this mode.

Writing speed

According to specs, the writing speed of Aligner: Maskless 03 is 1100mm²/min in fast mode. Using the high quality exposure mode cuts this speed in half, to approximately 550mm²/min. The writing speed for a 100x100mm² area measured during installation of the machine (acceptance test) was ~??mm²/min.

Acceptance test

Acceptance criteria on a 100 X 100 mm² area: Width of smallest resolved line 1000±100 nm (quality), alignment error 500 nm (quality), writing speed 1100 mm²/min (fast).

The acceptance test also included a verification of back side alignment (better than 1 µm), as well as a test of the conversion speed (less than 15 minutes for a dense design of circles).

Exposed on mask blank, transferred via wet chrome etch, and measured at Heidelberg (FAT)

SAT Feb 2019		Width of smallest resolved line [nm]	Alignment error (TSA) [nm]	Exposure speed [mm²/min]
405 nm	X	532±64	251	333
405 nm	Y	599±58	382	333
375 nm	X	556±44	214	347
375 nm	Y	579±63	224	347

@@ Line 35: / Line 35: @@
 Acceptance criteria on a 100 X 100 mm<sup>2</sup> area: Width of smallest resolved line 1000±100 nm (quality), alignment error 500 nm (quality), writing speed 1100 mm<sup>2</sup>/min (fast).
-The acceptance test also included a verification of back side alignment (better than 1 µm), as well as ??
+The acceptance test also included a verification of back side alignment (better than 1 µm), as well as a test of the conversion speed (less than 15 minutes for a dense design of circles).
 '''Exposed on mask blank, transferred via wet chrome etch, and measured at Heidelberg (FAT)'''