Prebaking substrates

The polymers were dissolved in methylisobutylketone (MIBK) and spin-coated on oxjdized silicon wafers (1100 X thick Si02 layers) to form 5000 A thick films. After a prebaking to improve adhesion to the substrate, the resist samples were irradiated 0 through the mask (A) using the Al K 152 emission line at 8.3 A as X-ray source. The electron beam gun was operated at a 300 W power and the source to sample distance was U.9 cm. Taking into account the absorption of the aluminum foil mask,the different X-ray fluxes available on the sample were calculated from the relation given by (9) ... 

Samples for analysis were prepared by spin-coating Si substrates from cyclohexanone solutions of 2,6-dinitrobenzyl tosylate and TBMS or TBS. The samples were prebaked at 90 C for 15 min in a convection oven, and analyzed after exposure and heating at 120 C for 30 min. The films on each substrate were dissolved with 5 ml of THF and a 1 ml aliquot of 0.1 vol % benzene in THF added. Finally, the solutions were diluted to 10 ml with THF, and IOOjj.1 injected onto the GPC for analysis. 

For a positive resist, the film thickness of the irradiated region after development decreases until eventually a critical dose Dp is reached which results in complete removal of the film 8,9). The sensitivity and contrast (7p) are evaluated in a manner similar to that for a negative resist. After they have been spin-coated and prebaked, a series of pads of known area are exposed to varying doses. The substrate is developed in a solvent that does not attack the unexposed film and the thickness of the film remaining in the exposed areas measured. The film thickness is normalized to the original thickness, and this value is plotted as function of log dose, as shown in Figure 5 where Dp represents the sensitivity of the positive resist. Contrast (7p) is determined from the extrapolated slope of the linear portion of the response curve as... 

Once a satisfactory film has been obtained, the coated substrate should be transferred to a suitable storage container and moved directly to the subsequent processing step, viz., prebaking. It is important that unbaked, freshly spun films not be stored for periods of time in excess of a few hours at this stage, since many resists are particularly vulnerable to particulate contamination. Once particles contact a resist surface prior to prebaking, they are almost impossible to remove and will cause opaque spots or pinholes after exposure and development. 

Poly(methyl methacryIate-co-3-oximino-2-butanone methacrylate) (P(M-OM) and poly(methyl methacrylate-co-3-oximino-2-butanone-methacrylate-co-methacrylonitrile) (P(M-OM-CN)) were dissolved in methoxyethyl acetate (10% solution). Where appropriate, the specified amount of sensitizer was added to the solutions before coating onto a silicon substrate with a Headway Research spinner. Films were prebaked at 120 C for 60 min. 

Lithographic Evaluation. Films were spin-coated onto silicon substrates from 10% solutions in chlorobenzene and prebaked at temperatures between 90 °C and 100°C for 1 hour to ensure solvent removal. The thickness of each film was about 5000 A. Electron beam exposures were performed on the AT T Bell Laboratories electron beam exposure system (EBES-I) operating at 20 kV with a beam adress and spot size both equal to 0.25 . A minimal cure time was required since there is no post-exposure reaction (4,16). 

Frequently, adhesion promoters are used. The substrates are primed or coated with a thin layer of an adhesion promoter, typically HMDS, prior to spin coating with resist (18). Possibly, the primary action of HMDS is to negate hydrophilic sites such as SiOH and trapped moisture on the substrate surface that would otherwise repel the photoresist. HMDS is applied by dipping, vapor priming in special chambers, or spin coating. Spin coating is most often accomplished with the same equipment that is used to coat resists. No high-temperature bake other than the resist prebake is required. 

The photopolymerizable composition can be used both as liquids to coat a substrate with light sensitive films and as dry photoresist films. In the latter case, the photoresist is a trilayer pie consisting of a top layer made of poly (ethylene terephthalate), a middle layer being the photopolymerizable composition, and an underlayer made of polyethylene. The underlayer is removed before exposure and the remained bilayer composite is laminated onto a clean substrate. The article is prebake to anneal a polymer layer and to enhance adhesion to the substrate. The selective exposure by the UV light through the top layer hardens the exposed areas of the photoresist. After exposure, the top layer is removed and the photoresist layer... 

The out diffusion of dopants from highly doped wafer areas Into the epitaxial layer during epitaxial deposition or prebake Is called autodoping. SIMS depth profiles during autodoping showed a pile up of As at the SI substrate surface. Spreading resistance measurements showed that the source of the autodoping to be electrically Inactive As (41). 

Wafer Processing. Testing substrates were either 2 or 3" silicon wafers with 5000 of SiO. Following DI water scrub and rinse, wafers were dehydrated in a 200 C N -purged oven for 1-2 hours. An adhesion promoter, either of the amino or halogenated silane type, was always employed prior to resist coat and prebake—as specified by the resist vendors. 

Synthesis of P(CMS-2VN), aiming at Dg =1 p. C / cm2 sensitivity, was carried out by ordinary radical polymerization in an inert-gas-filled flask with stirring (60 °C, 5 hrs.). The polymer after fractionation was Mw = 7.8 x 10-5, and polydispersivity Mw /Mn = 1.3 (as measured by gel permeation chromatography). The mole fractions in the polymer were found to be 9 mole % of CMS and 91 mole % 2VN ( as determined by elemental analysis). These values are very close to the mole fractions in the feed. The polymer was dissolved in xylene (5 wt.%), spin-coated (2000 rpm) and prebaked (100 °C for 30 min) to get uniform 0.5 jl m-thick-films on Si substrate. Pattern delineation was made by a 3EOL 3BX-5A (20 kV acceleration) electron beam exposure system without proximity corrections. Images were developed by dipping in 1,1,2,2-... 

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