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Forming Resist Images by Portable Conformable Masking Technique

IP.com Disclosure Number: IPCOM000045718D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 4 page(s) / 33K

Publishing Venue

IBM

Related People

Bassous, E: AUTHOR [+3]

Abstract

This article describes a process to produce high aspect ratio resist images using the portable conformable masking (PCM) technique. It is well known that the vertical dimension of semiconductor devices in integrated circuits does not scale linearly with the lateral dimensions, therefore when the lateral dimension approaches 1 Mum, it is often necessary to produce 2 Mum high resist images for the vertical structure. This type of aspect ratio is difficult to achieve using a single layer of resist with optical projection printing or E-beam writing. For optical projection printing, a 0.32 N.A. lens using a wavelength of 405 nm can resolve 1 Mum features within a depth of focus of 3 Mum. However, the 3 Mum range should accommodate focussing errors, wafer flatness variation and resist thickness.

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Forming Resist Images by Portable Conformable Masking Technique

This article describes a process to produce high aspect ratio resist images using the portable conformable masking (PCM) technique. It is well known that the vertical dimension of semiconductor devices in integrated circuits does not scale linearly with the lateral dimensions, therefore when the lateral dimension approaches 1 Mum, it is often necessary to produce 2 Mum high resist images for the vertical structure. This type of aspect ratio is difficult to achieve using a single layer of resist with optical projection printing or E-beam writing. For optical projection printing, a 0.32 N.A. lens using a wavelength of 405 nm can resolve 1 Mum features within a depth of focus of 3 Mum. However, the 3 Mum range should accommodate focussing errors, wafer flatness variation and resist thickness. It is necessary to limit the thickness of the resist to a small fraction of the depth of focus, typically 0.2 to 0.5 Mum. For E-beam writing, the scanning electrons have a very large depth of focus, but resist thickness is limited by backscattered electrons which create the so-called "proximity effects", lowering the contrast of micrometer-sized lines. In both cases, a 0.4 Mum resist thickness is preferable than 2 Mum. The PCM technique using a thin resist on a thick resist amplifies the aspect ratio by deep-UV imaging the bottom resist through the patterned thin resist layer.

Besides the advantage of aspect ratio amplification, the thick underlying resist layer separates the thin resist from the wafer surface. Backscattered electrons from the wafer surface distribute mostly near the resist-wafer interface making the top layer less susceptible to proximity effects. Another advantage of the PCM system is that a faster resist film that is normally too thin for single-layer wafer imaging can be used. An inherent sensitivity gain is achieved just by thickness reduction alone.

This invention concerns specific processing steps required to optimize the previously disclosed PCM system using novolac-based resists on deep-UV resist. Examples of novolac-based resists are Shipley AZ1350, AZ1450, AZ2400, HPR204 and HPR206. Examples of deep-UV resists are PMMA and PMIPK.

Because the bottom resist can be slowly dissolved by the novolac casting solvents, an interfacial layer is formed between the top and bottom layers. For the uncapped process which removes the top resist layer during development of the bottom resist, a plasma ashing step, ultrasonic agitation, or a combination of both is used to break through the interfacial layer. A typical plasma ashing procedure uses 0 plasma at 2 torr, with 200 watt RF power for 5 to 10 minutes in a barrel type reactor. Forming gas, CF and other typical plasma ashing gas for organic materials can also be used, Directional etching techniques, such as reactive ion etching or ion milling, are also applicable. Ultrasonic treatment is limited to the beginning of de...