Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2021 / Paper 1 / 3

The resist is positive tone, because the written regions are the regions removed during development. A suitable example is poly(methyl methacrylate), conventionally PMMA, used in its ordinary positive-tone electron-beam exposure regime. Electron-induced chain scission reduces its molar mass and increases its solubility in the developer.

The fact that this writing step fabricates an extreme ultraviolet mask does not make the resist an EUV-exposed resist: the exposure process being classified here is the electron-beam mask-writing step.

The deposited energy is a blurred, nonlocal version of the commanded write pattern. Forward scattering spreads energy within the resist; backscattered electrons from the substrate contribute a wider background; secondary electrons and the finite incident probe add further spreading. Development then removes regions whose accumulated dose exceeds the effective clearing requirement.

Line ends and isolated short rectangles receive less supporting exposure from neighboring written regions. Their ends can recede and round, and isolated features may shrink to narrow openings. Dense features receive more overlapping scattered dose. Thus one global dose cannot generally reproduce both isolated and dense shapes faithfully.

Use electron beam proximity correction: calibrate a scattering point-spread model for the actual resist, thickness, substrate and beam energy, then adjust local dwell time or beam electric current. Increase or reshape the write dose at underexposed isolated features and line ends, while compensating for the excess background in dense regions. Geometric prebias, such as modest line-end extension, can complement dose correction. A thinner resist and an appropriate higher beam energy may reduce forward-scattering blur, but the backscatter distribution must still be included. Correction must be based on the developed result, rather than assuming the incident beam diameter is the only resolution limit.

A transmissive mask is placed in contact with the resist, or a small distance above it for proximity printing. Chromium-patterned glass or quartz is a typical mask. Illumination is normally collimated ultraviolet light, with refractive illumination optics used to prepare the beam.

There is no projection objective imaging a reduced mask pattern onto the resist: the mask shadows the resist approximately one-to-one. The finite gap in proximity printing introduces diffraction and limits edge sharpness; contact printing reduces that gap at the cost of mask wear and contamination risk.

Use multilayer reflective optics and a reflective patterned mask. Near the usual extreme ultraviolet wavelength of 13.5 nm, ordinary lenses and thick transmissive masks absorb too strongly. Multilayer mirrors focus and project the image, while an absorber pattern on a multilayer-coated mask defines the exposed regions.

The optical path operates in vacuum to avoid absorption by air. The key distinction is reflective mask projection, rather than a transparent chromium-on-glass mask or a scanned electron probe.

Use electron lenses and a digitally scanned, blanked electron beam; no physical mask is required for direct writing. Electromagnetic lenses focus the charged-particle beam, deflectors place it, and the pattern generator controls position and dose. Electrostatic elements can also form part of electron optics, but these are not refractive glass lenses.

The extreme ultraviolet mask mentioned in the problem is the object being manufactured by this maskless write process, rather than a mask placed in the electron path.