Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Patent
1997-11-25
1999-03-02
Rosasco, S.
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
430308, G03F 900
Patent
active
058768807
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for producing a structured mask for the purpose of imaging structures of this mask onto an object by means of electromagnetic radiation or particle radiation, especially for use in ion beam lithography.
Masks for use in lithography are large surface-area, generally disc shaped structures consisting of a membrane of a thickness ranging from approx. 1 .mu.m to 20 .mu.m which are held by an outer frame encompassing the membrane. In the case of so-called stencil masks, holes are created in the membrane within a central mask design field and these holes form the structures of the mask. Upon radiating a stencil mask with electromagnetic radiation or particle radiation the beams pass unhindered through the holes of the membrane, whereas the beams are absorbed in all other regions of the membrane.
Such masks are mainly produced by etch thinning a substrate, e.g. a silicon wafer, by means of a suitable etch-stop method, to the desired thickness, wherein an outer edge remains untreated, so that a portative solid edge is formed, by means of which solid edge the mask can later be clamped into a frame. The structures of mask are applied lithographically, e.g. by means of electron beam lithography, onto the upper side of the membrane and holes are produced in the membrane by means of etching.
It follows from this, that in order to produce a stencil mask a series of processing steps are necessary in which the membrane is subjected to a mechanical loading. It is therefore necessary that the membrane is under a certain amount of tensile stress. Such tensile stress can be stress which is caused by a thermic reaction or even stress caused by the doping process. Tensile stress caused by doping occurs if the substrate or the membrane is doped with a doping substance whose atom diameter is less than that of the substrate atoms, e.g. if silicon is doped with boron or phosphorus. This type of doping procedure is necessary in many cases in order to create an effective (electrochemical or wet-chemical) etching stop for the thin etching of the substrate since a membrane of uniform thickness can be produced in this manner. Methods of thinning silicon substrates by means of etching are described in detail inter alia in Extended Abstracts, vol 82-1 (1992) in abstract no. 122 entitled "Studies of Boron and Boron-Germanium doped Epitaxial Silicon Films for Ultrathin Silicon Diaphragms" (Black et. al). and in The Bell System Technical Journal, March 1970 on pages 473 ff entitled "Electrochemically Controlled Thinning of Silicon".
A method for producing a thin membrane for use as a stencil mask is also evident from EP-A-367 750 held by the applicant. In the case of the method disclosed therein, the resulting stress in the mask is taken into consideration at the time of manufacturing the membrane, in that for the process of doping the substrate in order to produce an etching stop a predetermined doping material is used in a predetermined doping concentration. In dependence upon the density of holes the localized stress in the structured mask is changed with respect to the stress in the membrane. The density of holes in conventional masks ranges from a few percent to approx. 50%.
All masks produced in the manner described above thus comprise owing to the manufacturing process a certain amount of tensile stress which causes the holes in the stencil mask to be offset with respect to their desired position. As a result, displacements can be great if one area with a high density of holes is immediately adjacent to an area with a low density of holes. Furthermore, the deviation of the structures from the desired position generally increases towards the outside. For these reasons efforts have been made for some time to reduce the stress in the membranes manufactured in this manner to a lowest possible value in order to minimize the displacements of the structures. More precise tests regarding distortions of masks can be found inter alia under the title "Pattern Distortions in EBP Stencil Masks" (Keyser and
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J. Olschimke, et al., "Periodic Si-HOLE-Maska in the .mu.m AND sub-.mu.m RANGE for Electron-Multibeamwriting," Microelectronic Engineering 5 (1986), pp. 405-412.
Waggener, "Electrochemically Controlled Thinning of Silicon," B.S.T.J. Brief, Mar. 1970, pp. 473-475.
Keyser, J. et al., "Pattern Distortions in EBP Stencil Masks," Microelectronic Engineering, 11 (1990), pp. 363-366.
Black, et al., "Studies of Boron and Boron-Germanium doped Epitaxial Silicon Films for Silicon Diaphrams," Extended Abstracts, vol. 82-1, (1982) Abstract No. 122.
Liddle, J. Alexander, et al., "Stress-Induced Pattern-Placement Errors in Thin Membrane Masks," J. Vac. Sci. Technol. B 12 (6), Nov/Dec 1994, pp. 3528-3532.
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Chalupka Alfred
Loschner Hans
Vonach Herbert
IMS Ionen Mikrofabrikations Systeme Gellschaft M.B.H.
Rosasco S.
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