High resolution, steep profile resist patterns
`J. M. Moran and D. Maydan
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`Citation: Journal of Vacuum Science and Technology 16, 1620 (1979); doi: 10.1116/1.570256
`View online: http://dx.doi.org/10.1116/1.570256
`View Table of Contents: http://avs.scitation.org/toc/jvs/16/6
`Published by the American Vacuum Society
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`Page 1 of 6
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`High resolution, steep profile resist patterns
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`J. M. Moran and D. Maydan
`Bei'i' Laboratories Murray Hi”. New Jersey 07974
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`(Received 13 June 1979; accepted 25 September 1979)
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`High resolution and steep profile patterns have been generated in a 2.6 pm thick organic
`layer which conforms to the steps on a wafer surface and is planar on its tep. This thick
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`organic layer (a photoresist in the present experiments) is covered with an intermediate layer
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`of SiOz and a top, thin layer of x-ray or photoresist. After exposure and development of the
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`top resist layer,
`the intermediate layer is etched by CHF3 reactive ion etching. The thick
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`organic layer is then etched by 02 reactive ion etching. Submicron resolution with essentially
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`vertical walls in the thick organic material was achieved. The technique is also applicable to
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`photo and electron lithography.
`reduces the need for thick resist patterns for the
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`lithography step and, at the same time, ensures high resolution combined with good step
`coverage.
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`PACS numbers: 07.68. + rn. 07.85. + n, 31.60.Dq
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`I.
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`INTRODUCTION
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`With the advent of higher resolution lithographies, such as
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`x rayL2 and electron beam, as well as increased resolution
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`irom optical lithography, there is a demand for using these
`capabilities for making VLSI devices. Often, the ability to
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`achieve good Iinewidth control, high resolution, and good step
`coverage tend to be mutually exclusive. Good step coverage
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`requires thick resist while high resolution is more easily oh-
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`FIG.
`] X-ray resist thickness variation as it step covers 1.0 pm of p—giass
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`flowed over a 1.0 pm] high poly-silicon feature. Resist varies from 0.8 pm on
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`top of the feature to 1.7 pm in the valley between features [SEM profile
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`photo}.
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`1620
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`J. Vac. Sci. Technol.. 16(6), NovJDee. 1979
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`Page 2 of 6
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`tained in thin resista This is true for all resists, both positive
`and negative.
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`With any resist, the ideal conditions for obtaining high
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`resolution and good linewidth control are a flat surface and
`a thin resist (0.3—0.4 pm). The flat surface ensures that the
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`spun-on resist has very little variation in thickness and, as a
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`result, there should be little variation in resist linewidth.
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`However, resist linewidth variations will occur when lines
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`traverse a step. Figure 1 illustrates the variation in resist
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`BU22-5355ITBIDS1620-0580100
`(Q 1980 American Vacuum Society
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`FIG, 2., Result of plasma etching p-glass when the resist is too thin on top of
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`[he feature. The holes in [he p-glass (the resist has been stripped away) in-
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`dicalt‘ resist erosion on top of the feature. (SEM photo).
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`Page 2 of 6
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`1521
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`J. M. Moran and D. Maydan: High resolution. steep profile resist patterns
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`1621
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`REACTIVE ION ETCH
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`40003 PHOTO.
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`ELECTRON, 0n
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`X—RAY RESIST
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`2 - 3 pm
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`THICK
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`PHOTDRESIST
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`5:02
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`PHO T0RESiSI' VLSI SUBSTRATE
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`SCHEMATIC PRESENTATiON OF THE VARIOUS STEPS
`REQUIRED TO DEFINE A STEEP PROFILE fiESIST
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`PATTERN.
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`FIG. 4. Processing sequence for three level. high resolution patterning.
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`||. EXPERIMENT
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`A 2.6 pm thick layer of photorcsist {HI’R—204—Htint
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`Chemical Co.) serving as the thick organic layer, was spun on
`a silicon wafer. The intermediate layer of 010 pm of silicon
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`Ethan—«i
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`FIG. 3. SEM profile of a very thick resist covering the same features as shown
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`in Fig. 1. Note the flat surface due to the very thick resist spun-on the sur-
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`face.
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`FIG. 5. Resulting pattern. which is all thick HPR resist, after using the triIeveI
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`processing described in Fig. 4. Top pattern was done with x-ray resist. The
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`walls are 90" and there is no Iinewidth loss (SEM photo}.
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`J. Vac. Sci. Technol.. Vol. 16, No. 6, NovJDec. 1979
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`Page 3 of 6
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`PHOTOHESIST
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`LSl SUBSTRATE
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`—V
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`thickness, of the spun-on resist, as it covers a 1.0 pm thick
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`phosphosilicate glass flowed over a 1.0 pm thick polysilicon
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`step. The darker area is the resist which measures 0.8 pm thick
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`on top of the feature and 1.7 run thick in the valley between
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`features.
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`Patterning a wafer with resist thickness variations as shown
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`in Fig. 1 will result in both a difference in feature size as well
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`as a very thin resist on top of the step. In the case of a chemical
`etch, the thin resist does not present a problem. However, if
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`plasma etching is used, the resist is relied on as a mask. For a
`thin resist, the result of plasma etching the phosphosilicate
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`glass is shown in Fig. 2. The horizontal lines are p-glass which
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`has been etched. The slant line and vertical line in the center
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`of the photo are polysilicon steps. Note that there are two
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`round holes in the p-glass where it goes over the step. This is
`due to erosion of the thin resist on the step during plasma
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`etching of the p~glass The resist has been stripped completely
`awayr to reveal this problem.
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`Presented hereris a method for generating high resolution,
`steep profile resist patterns by first preparing a flatter surface
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`on the wafer. This is done hy spinning a thick organic layer
`of resist which conforms with the wafer surface and is planar
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`on its top. Figure 3 illustrates this with a 2.6 pm thick layer
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`of HPR—ZOS spun down on the same topography shown in
`Fig. l. The curved, lighter area is the p-glass and the rough
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`darker area on top is the thick resist. The thick layer is then
`patterned using an intermediate masking layer of plasma
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`deposited Si02 and a thin top layer of resist. The result is that
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`a very thick resist material can be patterned with submicrori
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`resolution and steep sidewalls comparable with those in pos-
`itive photoresist.
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`Page 3 of 6
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`1822
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`J. M. Moran and D. Maydan: ngh resolution, steep profile resist patterns
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`1622
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`TABLE I. High resolution. steep profile resist patterns.
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`A dva ntages
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`“O‘V‘PP‘E‘JF‘
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`Planar surface for resist patterning
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`Excellent step coverage
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`Good iinewidth control
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`Thinner resist can be used for better resolution
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`Eliminates standing waves and scattering in photolithogra phy
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`Reduces proximity effects in electron lithography
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`Minimal resist erosion during substrate etch by plasma or ions
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`Disadvantages
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`it In iui
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`I. Requires extra processing steps
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`dioxide was plasma deposited, at 200°C, on the photoresist
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`and then a 1.0 pm thick layer of chlorine based negative x—ray
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`resist4 was deposited on top of the oxide. A schematic pref
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`sentation of the processing sequence is shown in Fig. 4.
`The top layer of x-ray resist was exposed and developed to
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`a final thickness of 0.45 pm using an x-ray exposure tool.”
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`With the x-ray resist as a mask, the Sl02 was reactive ion
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`etched with a CHFS gas. The pattern was then transferred into
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`the thick organic (resist) layer using reactive ion etching, with
`pure 02 gas forming the plasma and the SiOg acting as the
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`mask. The rf power density was 0.30 VV/ctn2 and the time
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`required to etch the resist was 20 min. Figure 5 shows the
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`resultant pattern which is 2.5 gm high and has a trench width
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`of 1.5 ,um. The photos are taken with a scanning electron
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`microscope at a very steep angle so as to clearly Show the wall
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`structure of the resist. Note that the walls are perpendicular
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`UlllrtluF—l
`H-iEilifl
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`FIG. 6. A snhmicron window, in 2.1 pm thick resist using the trilevel tech—
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`nique and x-rsy patterning (SEM photo).
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`FIG. '1'. Thick resist pattern obtained using optical patterning on a Perkin—
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`Eirner projection printer for the top layer. (SEM photo).
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`FIG. 8. Suhrnicron trench for a Ca As gate obtained usng it Kaslrr [Oil step
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`and repeat camera for the top layer. (SEM photo]
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`J. Vac. Sci. Technol.. Vol. 16, No. 6, NovJDec. 1979
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`Page 4 of 6
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`Page 4 of 6
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`1623
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`J. M. Moran and D. Maydan: High resolution, steep profile resist patterns
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`1623
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`FIG. 9 Lines and spaces obtained using the trilevel technique with a negative
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`electron resist (CMC) for the top patterning layer {SEM photo)
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`and there is very little undercut. The oxide is still on top of the
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`organic (resist) layer and its thickness loss during the reactive
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`ion etch was less than 0.02 ,urn. Table I lists the advantages and
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`disadvantages of this technique herein denoted as the tri—level
`technique.
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`Figure 6 is an SEM photograph of the high resolution ca—
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`pability of the combination of the tri-lcvcl structure and a high
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`resolution x~ray resist“ A 05 .um window has been transferred
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`to a 2.1 ,urn thick photoresisl layer with no perceptable un—
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`dercut apparent
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`Ii. DISCUSSION
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`The tri-level technique for obtaining high resolution fea—
`tures in thick resist can be applied to other lithographies be~
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`sides x-ray lithography since there are advantages for optical
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`and electron-beam lithography as well.
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`In the case of optical lithography, the thick underlying layer
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`of resist and the intermediate layer of Si02 significantly re—
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`duce reflection from the wafer surface and, as a result, reduce
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`standing wave problems. The flat surface of the thick organic
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`layer keeps scattering down and the top resist can be made
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`thin for high resolution. There is also the advantage of having
`the same surface to coat and expose on for all process levels
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`which means a fixed exposure time, developing time, and a
`consistent surface for the top resist to be coated on every time.
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`Figures 7 and 8 are SEM photos of results obtained from an
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`optically exposed top resist layer. Figure 7 is a 2.0 our wide
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`feature, 2.6 pm high, that is routinely achieved on a device
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`wafer using a Perkin—Elmer projection printer and PIPE—204
`J. Vac. Sci. Technol., Vol. 16, No. 6, NovJDec. 1979
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`Page 5 of 6
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`WWW
`ire-HM J’EIII 3.45
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`Whirl—4
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`in) to ,urn lines and spaces in thick resist, Iismg x ray tor the top
`FIG. 10.
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`layer The lines are traversing a 1.0 [all] high. periodic grating to demonstrate
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`good step cow-rage with no linewidth variation over steps (HEM photo). [bi
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`Profile- nl the wall of resist [or one of the I 0 am lines showing conformal
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`coating of the. wafer topology at the resist bottom and a flat surface at its top
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`[SEM photo).
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`Page 5 of 6
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`1624
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`.i. M. Moran and D. Maydan: High resolution, steep profile resist patterns
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`1324
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`both 1 pm and 2 pm lines and spaCes traversing a i ,um high
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`p—glass step. The light lines running vertically are the linal
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`tri-level resist lines initially patterned with a 0.45 pm of s—ray
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`resist. The horizontal lines are the p—glass—covered 1 pm high
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`polysilicon steps. Figure lOibi is also an SEM photo, at a very
`shallow angle, of the broadside of the 1 pm lines and spaces
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`demonstrating the flatness of the top surface and the con-
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`formal coating of the step.
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`Finally. we would like to indicate the usefulness of this
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`technique for patterning device wafers. This technique has
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`been used for patterning silicon nitride, polysilicon, phos-
`phosilicate. glass and aluminum. As an example, Fig. 11 is an
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`SEM photo of 0.65 pm thick polysilicon that was reactive ion
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`etched using the tri—leyel structure initially patterned with
`x-ray lithography as a mask. There is seen to be very little
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`((0.1 p.111) linewidth loss in the. etched polysilicon.
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`III. CONCLUSION
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`The steep- profile three-layer pattern generation technique
`(tn-level) presents many potential advantages and applications
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`to all phases of lithography For optical lithography, it can
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`allow preparation of fine feature devices using present optical
`exposure systems. For xvray and electron lithography, sub-
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`micron resolution features with better linewidth control and
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`good step coverage are possible.
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`ACKNOWLEDGMENTS
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`The authors wish to thank D. B. Fraser. C. W, Van Hise,
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`C. J. Mogab, F. B. Alexander. B. A. Dein, J. M. Zimmer, C.
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`C. Chang, D. N. K. Wang. R. B. Marcus, S. E. Haszko, T. A.
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`Shankoli, and W. T. Willenbrocl: for their resistance in var-
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`ious phases of the project. They would also like to thank R. F.
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`W, Pease, H.
`Levinstein, and M. P. Lepselter for their
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`continuous support of this project. and to acknowledge the
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`photolithogmphic support provided by R. C. Brandes, J. H.
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`Velebir, J12, R. E. Kerwin, and
`V. DiLorenzo.
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`Pu. Maldonado. M. Moran, 5. ti, Somek h, and
`lD. Mayilan, (i. A (.quiiin,
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`C. N. Taylor, Conference on Micrnlilhography, Paris. June. 21724 (t. Tit,
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`pp.
`IQFi—IQQ.
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`‘r'. Lou, and C.
`2L). Maydan, G. A. Coquin, j. R. Maldonado, S. Somekh, I).
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`N. Taylor, iFIFIFI Trans. Electron Devices Elli—22, 429—433 [1975]
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`5As an example see Ll. Weissmanlel. M. Host, 0. Fietller, }. EsIer. H. [itc-
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`gengack and T. Horn, Proc iith International Vacuum Congress (197-1).
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`pp 439-442.
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`‘J. M. \Jloran and G. 5i. Taylor, Filter-nth Symposium on Electron [on and
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`Photon Beam Technology (May, HITS}.
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`3L. F. Thompson, E. M Doerrtes. and 1.. D. Yau. Electrochem. Son. [to he
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`published}.
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`5M. linwdell. L. F Thompson, and]. P. Ballulityne, j Var: Sci, Tet-hoot. [2.
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`1294 {1975)
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`7L. F. Thompson and M. Bow-den, Electrochan Soc. IEU, FREE [1973}.
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`n a: so
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`FIG 11 Application oi the trilr‘vel resisl to reactive ion etching of 0.65 ,am
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`of polysilicon on a rlevict= water. The thick resist is still on and is used as the
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`etch musk.
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`photoresist. The thin 0.1 pm thick SiOg layer can just be seen
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`at the top of the feature. Figure 8 is a submicron trench that
`was patterned using a Kasper 10:1 step and repeat camera.
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`For electron-beam lithography, backseattering from a
`substrate covered by 0.1 urn of SiOg on top of 2.6 pm of
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`polymer is less than that from an Si or SiOg/Si substrate be-
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`cause of the lower atomic number material. Thus, proximity
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`effects for thick resists (>05 um) are reduced and better
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`linewidth control has been observed with both negative and
`positive resists. in addition, high speed electron resists, such
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`as CMC5 or P8515"? can be used for patterning of waters since
`the top layer of resist need not be thick. Figure 9 shows an
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`SEM photo of a tri—level pattern in which 0.5 pm thick CMC
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`was used to generate the initial pattern. The lines are 0.75 urn
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`wide and the space is 1.5 pm.
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`In the case of x-ray lithography, the resist currently being
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`used ior direct water exposure is a negative, high speed resist.
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`However,
`1 pm resolution is only possible4 when the final
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`resist thickness is 504 pm. This precludes using it for direct
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`wafer exposure since it does not have good plasma resistance.
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`Howewer, the tri-level technique permits the use of thin resists,
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`since there are no steps. Figure lOia) shows an SEM photo of
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`J. Vac. Sci. Technol.. Vol. 16. No. 6. NovJDec. 1979
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`Page 6 of 6
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`Page 6 of 6
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