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GENERATION OF A RE-ENTRANT LIFT-OFF PHOTORESIST PROFILE BY USING A TWO-LAYER PMMA STRUCTURE

IP.com Disclosure Number: IPCOM000025701D
Original Publication Date: 1987-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Document File: 4 page(s) / 168K

Publishing Venue

Xerox Disclosure Journal

Abstract

One method for creating IC patterns is referred to as "lift-off. A pattern is defined on a wafer using a photosensitive polymer structure, usually with a high aspect ratio. A deposition ste is performed which deposits an overlying

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XEROX DISCLOSURE JOURNAL

10

GENERATION OF A RE-ENTRANT LIFT-OFF PHOTORESIST PROFILE BY USING A TWO-LAYER PMMA STRUCTURE
Robert R. Allen

4%K PMMA

Proposed Classification

U.S. C1. 156/661.1 Int. C1. B44c 1/22

10 SUBSTRATE

FIG. I

RESIST NOVOLAK 50 1 1

40

      I 30 4 I

10

I I

FIG. 2

50K PMMA/ \ 24

-I ( 1

FIG. 3

Volume 12 Number 3 May/June 1987 127

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GENERATION OF A RE-ENTRANT LIFT-OFF PHOTORESIST PROFILE BY USING A TWO-LAYER PMMA STRUCTURE (Cont'd)

One method for creating IC patterns is referred to as "lift-off. A pattern is defined on a wafer using a photosensitive polymer structure, usually with a high aspect ratio. A deposition ste is performed which deposits an overlying

solvent which will break away the photopolymer and overlying film while leaving the deposited film in the unmasked areas. It is generally accepted that in order to achieve a reliable lift-off process, one needs a photopolymer profile with a high aspect ratio and a re-entrant sidewall profile to provide a discontinuity or microcrack in the deposited film in order to allow easy solvent penetration into the photopolymer, thus aiding in the liftoff and insuring complete se aration of the waste filed and the desired feature. Due to the

been confined to defining patterns in layers for which there is no convenient etchant or where there was a need for defining very closely spaced, small geometries with no undercut.

The procedure shown in Figures 1-3 allows the production of masking films having very fine geometries, high aspect ratios and re-entrant profiles. This procedure utilizes a two layer structure of polymethylmethacrylates (PMMA) of differing molecular weights which results in differential development rates for the two layers when exposed to the same nominal dose of radiation. When the lower of the two PMMA layers has a lower molecular weight than the upper layer, a re-entrant profile is obtained after exposure and development. The two layer PMMA in this case is masked via portable conformable masking followed by flood deep UV exposure, but the two layers PMMA structure can also be masked directly using a deep UV aligner.

The first step, shown in Figure 1, involves the conventional application to substrate 10 of a low molecular weight PMMA layer 20. In the example shown, the molecular weight of layer 20 is 496,000. After baking, a second PMMA layer 30 of higher molecular weight is applied. In the example shown, the molecular weight of layer 30 is 950,000.

Intermixing which occurs at the interface between layers 20 and 30 during the step in Figure 1 is not a serious problem since the solvent systems are identical and the resist constituents differ only in molecular weight. Intermixing causes a gradual transition from one molecular weight layer to the other rather than a discrete step. If a sharp transition is desired between layers 20 and 30, buffer layers which are not soluble in...