Mask Requirements and Fabrication

In this section we will describe the fabrication of a typical conformable mask suitable for near field exposure. Conformable amplitude masks have been fabricated on either 2 |im thick low stress silicon nitride SixNy films grown on silicon substrates using low-pressure chemical vapour deposition (LPCVD), or on thicker (100-150 |im) glass or fused silica masks; fabrication of the silicon nitride masks will be presented in detail here. Membranes of silicon nitride were formed by wet etching of the backside of the silicon substrate in 50% by weight potassium hydroxide (KOH) solution. Typically, a membrane 5x5 mm2 square is formed in the centre of a larger sample, leaving an outer silicon supporting ring. The membrane mask-making process is illustrated in Fig .17.3.

Fig. 17.3. Mask fabrication process, showing steps taken for forming the membrane and patterning the absorber features

Pattering of the mask patterns onto the SixNy was performed using electron-beam lithography (EBL) onto membranes coated with a bilayer of polymethylmethacrylate (PMMA). The bilayer resist was used to obtain an undercut profile desirable for the lift off metallisation process that was used to form the mask patterns. Arrays of 30 nm thick NiCr metal gratings of varying duty cycle were used to define the opaque regions on the mask. The linewidths in the gratings vary from 2 |im down to 60 nm. A typical amplitude mask with 130 nm gratings, isolated lines, and dense lines of NiCr patterns on a silicon nitride membrane is shown in Fig. 17.4.

Fig. 17.4. Scanning electron microscope image of a typical membrane mask patterned with 25-nm thick NiCr gratings. The pattern on the left is isolated 130 nm lines on 390 nm period, the middle pattern contains 260 nm wide lines on 390 nm period, and the right-hand side pattern is a grating with 260 nm period (130 nm lines and spaces)

Fig. 17.4. Scanning electron microscope image of a typical membrane mask patterned with 25-nm thick NiCr gratings. The pattern on the left is isolated 130 nm lines on 390 nm period, the middle pattern contains 260 nm wide lines on 390 nm period, and the right-hand side pattern is a grating with 260 nm period (130 nm lines and spaces)

Other types of mask material, including glass or fused silica substrates have also been successfully employed, and these are generally more robust than the thin nitride membrane masks. However, obtaining conformal contact over large areas with these thicker masks in not so reliable. In addition, a subtractive process has been developed for forming the absorber patterns, using reactive-ion etching (RIE) of tungsten in a sulphur hexafluoride (SF6) plasma; EBL in PMMA is used for pattern definition in this case also. This process reduces the occurrence of unwanted protrusions (so-called 'lift-off tags') that can sometimes prevent intimate contact in certain areas, leading to poor definition of sub-wavelength patterns.

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