Question

(a) For the soft-contact lithography process, the usual application methods of the resist of shallow structures is by using a spinner. Suggest alternative choices for resist application for deep (e.g. MEMS) features.
(b) Explain the options for a zero gap distance in contact lithography.
(c) Using schematics where necessary, describe;
(i) How diffraction effects modify the resist profile for positive and negative resist.
(ii) Methods by which an ideal resist profile for a lift-off processes may be realised.
(d) A resist is applied by spinner giving a particular thickness. Describe possible strategies to double the resist thickness for this resist.

Answer 1

a) Pattern Transfer:-

Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. A photosensitive material is a material that experiences a change in its physical properties when exposed to a radiation source. If we selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation) the pattern of the radiation on the material is transferred to the material exposed, as the properties of the exposed and unexposed regions differs (as shown in figure ).

This discussion will focus on optical lithography, which is simply lithography using a radiation source with wavelength(s) in the visible spectrum.

In lithography for micromachining, the photosensitive material used is typically a photoresist (also called resist, other photosensitive polymers are also used). When resist is exposed to a radiation source of a specific a wavelength, the chemical resistance of the resist to developer solution changes. If the resist is placed in a developer solution after selective exposure to a light source, it will etch away one of the two regions (exposed or unexposed). If the exposed material is etched away by the developer and the unexposed region is resilient, the material is considered to be a positive resist behaviour pressure.

Lithography is the principal mechanism for pattern definition in micromachining. Photosensitive compounds are primarily organic, and do not encompass the spectrum of materials properties of interest to micro-machinists. However, as the technique is capable of producing fine features in an economic fashion, a photosensitive layer is often used as a temporary mask when etching an underlying layer, so that the pattern may be transferred to the underlying layer . Photoresist may also be used as a template for patterning material deposited after lithography .The resist is subsequently etched away, and the material deposited on the resist is "lifted off".

The deposition template (lift-off) approach for transferring a pattern from resist to another layer is less common than using the resist pattern as an etch mask. The reason for this is that resist is incompatible with most MEMS deposition processes, usually because it cannot withstand high temperatures and may act as a source of contamination.

(b).

A simple and straight forward approach is contact printing. In contact printing, the mask is pressed against the resist-coated wafer during exposure, i.e., the optical parts missing, but the other components like the illuminator and mask are kept. An important advantage is that feature sizes as small as 0.1 m can be made using comparatively inexpensive equipment [18]. The mask is held chrome-side down in intimate contact with the wafer. Ideally, the gap between mask and wafer goes to zero, which minimizes diffraction effects. The resolution is only limited by scattering effects that might occur inside the resist due to its finite thickness. In reality additional limitations of contact printing result from the non-uniformity of mask and wafer. This problem can be reduced by applying pressures ranging from 0.05-0.3 atm. The major disadvantage of such hard contact methods is the extremely high defect generation. Defects are generated both on the wafer and mask during every contact cycle. Mask lifetime is severely reduced and printed defect levels are increased. Therefore, contact printing is only used in device research or other applications that tolerate high defect rates.

Proximity printing avoids defect generation because a small gap ranging from 10-50 m is introduced between mask and wafer. The separation is usually controlled by a flow of nitrogen gas. The gas flow keeps the mask away from the wafer surface. The main disadvantage of proximity printing is a severe reduction in resolution due to diffraction spreading. The achievable resolution is governed by the expression

whereby dg denotes the mask-to-wafer distance, is the exposure wavelength, and the technology parameter k ranges between 1-2 depending on the resist process. The square root behavior is a consequence of the Fresnel diffraction theory valid in the near field region just below the mask openings. For optical lithography, typical values are k = 1.6, = 0.4 m, and dg = 25 m, yielding a resolution of W = 4m. Resolution can be enhanced by either decreasing the gap at the risk of contact and defect generation or by reducing the wavelength. Using wavelengths in the DUV or even EUV range will not suffice for optical proximity printing to compete with projection printing. However, using X-rays with a wavelength of about 1 nm feature sizes below 0.2 m can be produced with proximity methods. This makes 1x proximity X-ray a promising candidate for the 0.13 m and 0.10 m technology .