CA1191477A

CA1191477A - X-ray mask substrate and method of fabrication thereof

Info

Publication number: CA1191477A
Application number: CA000426785A
Authority: CA
Inventors: Michael J. Brady; Bernard S. Meyerson; John M. Warlaumont
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1982-06-30
Filing date: 1983-04-27
Publication date: 1985-08-06
Also published as: EP0097764B1; US4436797A; JPS599921A; DE3365521D1; EP0097764A1

Abstract

X-RAY MASK SUBSTRATE AND METHOD OF FABRICATION THEREOF An improved X-ray lithography mask has been fabricated by forming an X ray absorbing lithography pattern on a supporting foil of hydrogenated amorphous carbon. The substrate foil is formed by depositing a carbon film in the presence of hydrogen onto a surface having a temperature below 375.degree.C. The hydrogen concentration is maintained sufficiently high that the resulting film has at least one atom percent of hydrogen. A film having about 20 atom percent of hydrogen is preferred. While impurities are permitted, impurities must be maintained at a level such that the optical bandgap of the resulting film is at least one electron volt. A film with an optical bandgap of about 2 electron volts is preferred.

Description

'~0981-106 ~ yt7 X-RAY MASK SUBSTRATE AND
METHOD OF FABRICATION THEREOE' DESCRIPTION

Technical Field This invention relateq to lithographic masks for fabricating microcixcuits and more particularly it relates to a composition or and a method of fabricating an X-ray txansparent substrate for supporting an X-ray absorbing pattern.

Prior Art An X-ray lithographic mask typically ~omprises a thin foil substrate carrying an X-ray absorbing pattern. Since the thin foil substrate is usually only a few micrometers thick, it usually requires additional peripheral supporting structure of some kind.

Selection of a suitable X-ray absorbing material for the absorbing pattern is not too difficult. Gold is usually selected because of its widespread use alre~dy in microcircuit technology and because other choices do not have much better X-ray absorbance and cause processing problems.

Selection of a suitable material or the X-ray mask substrate is not straight-~orward because an ideal substrate ~or an X-ray mask has many requirements.

~r;~

For example, an X-ray substrate oil should have a useable arPa as larye as possible (ideally as large as a silicon wa~er), a thickness in the micron (~m) range, virtually absolute flatness, high strength, dimensional and mechanical stability against radiation, humidity, heat, etc., compatibility with fabrication of the absorber pattern and with VLSI
technology in general, sufficient transparency to X-rays that adequate contrast can be achieved, and pre~erably transparency to visible light for alignment purposes.

Each of the currently known substrate materials meets some of these requirements well but only marginally satisfies one or more of the other require-ments. Each of the currently known substrate mater-ials does not satisfy at least one o~ these require-ments as well as other availa~le substrate materials.
~mong the materials already known for use as an X-ray mask sub~trate is silicon (Si), boron doped silicon (Si:B), boron nitride (BN), silicon nitride (Si3N4), silicon dioxide (SiO~), beryllium (Be), silicon carbide (SiC), tungsten carbide (WC), silicon oxyni-tride (SixoyNz)l alumina (A12O3), Mylar, polymers such as polyimide (Kapton), and some mixtures and laminates of these materials.

A review of %-ray mask substrate technology may be ound in A. Heuberger et al., "Mask Technology,"
par~ 3 of 1I Present Status and Problems of X-ray Lithography," in ADVANCES IN SOLID STA~E PHYSICS, 30 edited by J. ~reusch, pages 265-274 (Friedr. Vieweg &
Sohn, Wiesbaden, W.Ger., 1980)o X-ray mask substrate materials are also described, for example, in U.S.
patents 3,742,230 (Si and Si:B) t 3,925,677 (glass);
4,037,111 (polyimide); 4`,198,263 (Si:B); and 35 4,260,670 (BN, WC, Si3N4, SiO2, polymers).

~l~9~ 7~7 Currently, the requirements for an X-ray mask sub-strate are best met by boron doped silicon, organic materials, and silicon nitride/silicon dioxide laminates. The search continues ~or a material which better satisfies the requirements for an X-ray mask substrate foil.

Disclosure of the Invention An improved X ray lithography mask has been fabricated by ~orming an X-ray absorbing lithography o pattern on a supporting foil of hydrogenated arnorphous carbon. The substrate foil is formed by depositing a carbon film in the presence of hydrogen onto a surface having a temperature below 375C. The hyd-rogen concentration i5 maintained sufficiently high that the resulting film has at least one atom percent of hydrogen. A film having ahout 20 atom percent of hydrogen is preferred. While impurities are permitted, impurities must be maintained at a level such that the optical bandgap of the resulting film is at least one electron volt. A film with an optical bandgap of about

2 electron volts is preferred.

A foil of hydrogenated amorphous carbon formed in the preferred ~nn~r is substantially transparent to both X-ray radiation and visible light. ~he foil is also chemically inert, extremely hard, adheren~
to a supporting structure, resistant to scratching and stretching, and it apparently satisfies the requirements ~or an X-ray mask substrate better than previously known substrate materials.

7~7 Brief Description of the Drawings FIGS. 1.1 through 1.7 illustrate step-by-step the fabrication of an X-ray mask using a mask substrate foil having a composition in accordance with this invention.

Best Mode For Carrying Out The Invention In accordance with this invention, an X-ray mask is formed using a substrate foil co~prising hydro-genated amo~phous carbon. Such a foil may be fabricated by forming a thin film of carbon in the presence of hydrogell at a suitably low temperature.
The temperature at which the carbon film is formed is important because the lower the temperature (at lea~t over some range), the more hydrogen is in-corporated into the carbon film (all else beingidentical). The amount of hydrogen incorporated into the carbon film is important because the higher the amount of inccrporated hydrogen, the greater the apparent degree of four fold symmetry in the film structure. Four fold symmetry i.s desired in the film structure because it corresponds to the structure of diamond t a pure carbon crystalline structure which has unusually favorable properties for an X-ray mask substrate, including visual transparency, extreme hardness, extreme durability and excellent resistance to chemical attack.

A thin film of hydrogenated amorphous carbon having properties suitable for an X-ray mask substrate may be formed, for example, by conventional glow discharge deposition, or by conventional sputter deposition or YO981-1.06 '7~7 by reactive sputter deposition of a carbon film in the presence of hydrogen onto a flat surface having a temperature below 375C., and preferably below 300C.
The best film characteristics are obtained at deposition temperatures between about 200C and about room temperature with about 200C deposition being pre~erred.

Glow dlscharge deposition of hydrogenated amorphous carbon may be done by placing in the discharge chamber of a conventional glow discharge deposition apparatus a substrate upon which a film of hydro-genated amorphous carbon is desired. A suitable gas or mixture of gases containing carbon and hydrogen is then flowed through (or contained within) the discharge chamher and a suitable A~C. or D.C~
voltage is applied between two electrodes in the discharge chamber (one of which may carry the substrate) to produce a glow discharge between the electrodes. ~he glow discharge decomposes one or more gases in the discharge chamber to produce carbon and hydrogen ions, some of which deposit on the substrate. to form a:hydrogenated amorphous carbon film. The prefexred gas for glow discharge deposi-tion of hydrogenated amorphous carbon is acetylene (C2H2). Theoretically,.any gas or gaseous mixture which contains carbon and hydrogen and which can be decomposed by a glow discharge to produce carbon and hydrogen.ions could.be used instead of acetylene.
The gas or gaseous mixture in the discharge chamber also may include constituents which do not. ionize or which do ionize but which do not deposit in signifi cant quantity within the carbon film. Impurities and YO~81-106 '7~' dopants are permitted in the hydrogenated amorphous carbon film so long as a suitable optical bandgap is still produced.

The optical bandgap (Eo) of hydrogenated amorphous carbon is deflned by the following expression:

(~E)1/2 = ~ E ) where a is optical absortlon, E is the enexgy of the incident light in electron volts and ~ is a constant. On a graph of (~E)l/~ versus E, the optical bandgap Eo is found at the point where the linear region of the plot crosses the E axis when extended. ~ more complete definition of optical bandgap may be found, for example, in ELECTRONIC
PROCESSES IN NON-CRYSTALLINE SOLIDS, by N. Mott and E. Davis, 2 ed., pages 287-89 (Clarendon Press, Oxford, 1979).

Reactive sputtex deposition uf hydrogenated amorphous carbon may be done by placing a substrate upon which a deposited hydrogenated amorphous carbon film is desired into a conventional sputtering apparatus.
A carbon taryet is used as the cathode. The deposi-tion chamber is filled with an inert gas such as argon to which hydrogen gas (a ew percent) is added.
By applying a suitable D.C. or A.C. voltage between the anode and target, a glow discharge is produced.
Inert element ions (i.e. argon ions) are formed and bombard the carbon target sputtering carbon atoms from the carbon target. The plasma contains hydrogen ions supplied by the hydroyen gas. Carbon atoms and hydrogen ions deposit on the substrate forming a hydrogenated amorphous carbon film.

~0981-106 For reactive sputter deposition, the voltage is higher than for glow discharge deposition so that ion bombardment with the cathode target ejects target atoms, thereby eroding the target. The ejecked target atoms deposit on surrounding surfaces (including the substrate) along with hydrogen ions from the plasma.
Since reactive sputter deposition of hydrogenated amorphous carbon includes deposition of the hydrogen from a plasma, it might be considered a form of plasma deposition, just as glow discharge deposition is a form of plasma deposition. Actually, the carbon atoms are sputtered while the hydrogen is plasma deposited simultaneously.

Target materials other than pure carbon can be used, including in theory materials which also contain hydrogen in a reactive ~orm. If both the carbon and the hydrogen is supplied by the target, it would be called a conventional sputter disposition process.
~ A target of polystyrene might be used, or example.
j 20 In reactive sputter deposition and in sputter deposi-tion, just as in the case of glow discharge deposi-tion, additional ions, molecules or atoms which do not deposit significantly within the film may be present within the plasma or sputtered from the cathode. Some contamination and doping constituents are permittad within the film so long as the optical bandgap is suficiently high, just as in a conventional glow discharge deposition of the film.

It has been found that a hydrogenated amorphous carbon film clings tenaciously to the substrate upon which it is deposited. Therefore, it is pre-ferred that the substrate be etched or dissolved away after deposition of the film to form a ~oil.
Portions o the substrat`e may be retained to provide ~9~4'77 support for the foil. For example, a central window region of the substrate may be dissolved away leaving a peripheral frame member supporting the Eoil, as is commonly done in this field. Obviously, in order ~o selectively dissolve the substrate or portion of the substrate, it must be composed of a material which can be selectively dissolved or etched with respect to a hydrogenated amorphous carbon film.
Fortunately, hydrogenated amorphous carbon is nearly chemically inert making a wide variety of substrate materials available. Glass and silicon are the preferred materials for the substrate upon which the film is deposited. Glass and silicon can be isotropically etched away, for ex.~mple, with hydroflouric acid (HF) while ~100> silicon can be preferentially ekched away along crystal planes, for example, with an aqueous mixture of ethylene diamine and pyrocatecol.

~xample I

A glass substxate is ultrasonically cleaned, rinsed, dried and placed into a conventional D.C. glow discharge deposition appara us. Acetylene gas is flowed through the discharge region at a flow rate of about l standard cubic centimeter per minute (SCC~).
Pressure within the discharge ch.~mber is maintained ak about 0~5 Torr. 'rhe subqtrate is not heated.
A D.C. voltage of about 300 volts is~applied to produce a glow discharge. A film deposits on the substrate surface at about 50-100~ per minute. When the coating is about l~m thick, the glow discharge i5 stopped.

Y

The deposited coating is amorphous and composed of about 60 atom percent carbon and about 40 atom per-cent hydrogen. The coating is extremely hard and adheres to the glass. The optic:al energy gap of the coating i5 about 2.1 electron volts. The coating appears light tan and is highly transparent to visible light. X-ray transparency of the coating also is excellent and equals or exceeds all other known materials for X-ray mask substrates.

Example II

Glass substrates are coated one at a time as described in Example I, except that each of the substrates is maintained by a substrate heater at a different and progressively higher elevated temperature between room temperature and 4~5C.

All of the coatings are composed of carbon and hydrogen with the atom percent of hydrogen decreasing with increasing deposition temperature until at a deposition temperature of about 425C virtually no hydrogen can be ound in the coating.

At a deposition temperature of 425C the coating has properties much like graphite and is unsuitable for use as an X-ray mask substrate.

The composition of the coating at a deposition temperature of 375C i5 about 1 atom percent hydrogen and the optical bandgap is about 1.0 eV. The coating appears dark in color and does not transmit visible light very well. Transmission of visible light is about the same as silicon. Transmission o X~rays equals or exceeds the X-ray transmission o~ silicon (depending upon wavelength) and the ~L9 !lfl~7~7 coating is harder, mor~ scratch resistant, and more resistant to chemical attack than silicon. Although visual alignment cannot be done very well through such a coating, it is no worse in this respect than siiicon and much better in other respects. Accordingly, this hydrogen composition (1 atom percent hydrogen) and the associated optical bandgap of l.OeV are considered the lowest acceptable :Eor an X-ray mask substrate.

At progressively lower deposition temperatures, the hydrogen perrentage quickly increases along with visual transparency. At a deposition temperature of about 300~C, ~he film is suf~iciently transparent to visible light that visual alignment can be done through the film. At a deposition temperature of about 300C, the optical bandgap is about 1.6e~. Since X-ray substrate films which are op~ically transparent are particularl~ desired, in accordance with this invention, it is preferred to deposit the hydro-genated amorphous carbon film at a temperaturebelow 300C.

At a deposition temperature of about 200C, ~he film is transparent to visible light and has an optical bandgap of about 2.OeV with a hydrogen concentra-tion of about 20 atom percent. This is the preferred de-position tempexature because no substantial increase in visible light transparency is experienced at still lower ~emperatures and it is expected that furthex increase in the percentage o hydrogen might adversely affect mechanical properties of the film.

YO9~1-106 t~

Example III

Two substrates are coated as in Example I except that while coating the first substrate, diborane (B2H6) gas is mixed with the acetylene gas (1~
S diborane) and while coating the second substrate, phosphine (PH3) gas is mixed with the acetylene gas. The first coating is hydrogenated amorphous carbon with a boron doping or impurity while the second~coating is hydrogenated amorphous carbon ~ith a phosphorus doping or impurity. Both coatings are optically transparent and have physical characteristics similar to the coating of Example I except that a slightly lower optical bandgap is obtained.

Example IV

A silicon substrate is coated as described in Example I. A central window i5 then etched through the silicon to the coating using an SiO2 mask and ethylene ~; ~m; ne pyrocatecol. The coating stretches across the etched window and has the same physical and optical charactexistics described in Example I.

Example V

A silicon substrate is placed in a conventional diode sputtering apparatus with a carbon target.
An argon atmosphere is provided in the deposition chamber with a few percent hydrogen (H2) gas added.

\

Pressure in the chamber is maintained at about 10 milliTorr. An RF voltage is applied between the anode and ~he carbon target creating a plasma and sputtering of the target. A film deposits on the substrate at about lA per second. When the film is about l~m thick, sputter deposition is stopped. A window is then back etched in the silico~
substrate using an SiO2 mask and ethylene diamine pyrocatecol, leaving the film stretching across the etched window. The film is composed of hydro~
genated amorphous carbon having physical and optical properties substantial like the film described in Example I.

MASK PREPARATION

FIGS. 1.1 through 1.7 illustrate step-by~step the preparation of an X-ray mask in accordance with this invention. First, a film 10 of hydrogen~
ated amorphous carbon (about 0.1 to 1.0~m thick) is deposited on a substrate 12 (FIG. 1.1~ by plasma deposition or sputter deposition of carbon and hydrogen onto a substrate at a temperature less than 375C
and preferably less than 300C. Preferably substrate 12 is silicon with a thickness of about 250-700~m.
Then an etch resistant mask 14 is applied to the back side of the substrate to define one or more openings 16 (FIG. 1.2). The back side is etched through the mask until a window 18 is formed defined by a supporting frame 20 (FIG. 1.3). A window as large as 100mm i~
possible. Film 10 stretches across and covers window 18 orming a foil substrate suppor~ed by frame 20.

~0981-106 ~:~9~'7~7 Next, an adhesion layer 22 (FIC. 1.4) ls applled over the hydrogenated amorphous carbon foil.
~bout 50~ of chrome or titanium is suitable. On top of the adhesion layer, a plating base 24 is deposited. About 300A of gold or nickel is suitable.
A resist layer 26 is deposited on top of the plating base 24. A resist layer thickness of abou~
8000~ would be suitable.

A mask pattern 28 is then defined in resist layer 26 and developed (FIG. 1.5). For convenience, etch resistant mask 14 is shown to have been removed at this point during the developing step. It could be removed instead either earlier or later or not at all, if desired. An X-ray absorbing pattern 30 is then electroplated up through the mask pattern 28 to a thickness less than the thickness of resist layer 26 (FIG. 1.6). ~inally, the resist layer 26 is removed, if desired or necessary, to produce an X-ray mask (FIG. 1.7).

Certain variations and modifications of this invention would be apparent to those of ordinary skill in this field and may be made without departing from the spirit and scope of this invention for-example, while it is preferred that the window bP formed before formation of the X-ray absorbing pattern, it should be apparent that the window could be formed instead after formation of the X-ray absorption pattern.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An X-ray mask comprising an X-ray absorbing pattern supported by a film of hydrogenated amorphous carbon having an optical bandgap of at least one electron volt.

2. An X-ray mask substrate as defined in claim 1 wherein said film has at least 1 atom percent of hydrogen.

3. An X-ray mask substrate as defined in claim 1 wherein said film has an optical bandgap of at least 1.6 electron volts.

4. An X-ray mask substrate as defined in claim 1 wherein said film has about 20 atom percent of hydrogen.

5. An X-ray mask substrate as defined in claim 1 wherein said film has an optical bandgap of about 2 electron volts.

6. An X-ray mask as defined in claim 1 wherein said film has been formed by depositing carbon and hydrogen onto a surface having a temperature less than 375°C.

7. An X-ray mask as defined in claim 1 wherein said film has been formed by depositing carbon and hydrogen onto a surface having a temperature less than 300°C.

8. An X-ray mask as defined in claim 1 wherein said film has been formed by depositing carbon and hydrogen onto a surface having a temperature of about 200°C.

9. An X-ray mask as defined in claim 8 wherein said depositing of carbon and hydrogen has been done by glow discharge of a gaseous medium containing carbon and hydrogen.

10. An X-ray mask as defined in claim 9 wherein said gaseous medium comprises acetylene gas.

11. An X-ray mask as defined in claim 8 wherein said depositing of carbon and hydrogen has been done by sputtering a carbon target in an inert gaseous medium containing hydrogen gas.

12. An X-ray mask comprising a substantially X-ray absorbing patterned layer supported by a substantially X-ray transparent substrate film, said substrate film comprising hydrogenated amorphous carbon and having an optical bandgap of at least one electron volt.

13. An X-ray mask as defined in claim 12 and further comprising a frame supporting said X-ray transparent substrate film, said frame comprising silicon.

14. A method of forming an X-ray mask substrate film comprising the step of depositing carbon in the presence of hydrogen onto a surface having a temperature less than 375°C, while maintaining the concentration of hydrogen sufficiently high that the resulting film has at least one atom percent of hydrogen, and; maintaining any impurities in the film at a level such that the optical bandgap of the resulting film is at least one electron volt.

15. A method as defined in claim 14 wherein said depositing is done by creating a glow discharge in acetylene.

16. A method as defined in claim 14 wherein said depositing is done by sputtering a carbon target in the presence of hydrogen.