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Description  |
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TECHNICAL FIELD
This invention relates to a method of repairing defective photomasks. More
particularly, the present invention relates to a method for repairing a
pin hole defect in a metal photomask.
BACKGROUND OF THE INVENTION
Photomasks are particularly useful in the manufacture of circuit patterns,
such as, for use in the manufacture of integrated circuits in the
electronic arts. Typically, a photomask comprises a patterned metal film
such as chromium, nickel or aluminum in a thickness in the order of 1,000
A. deposited on a transparent base, such as glass or quartz. It is
generally manufactured by depositing a thin film of the metal on the
surface of the transparent substrate, coating the film of metal with a
photoresist coating, exposing a patterned region on the photoresist
coating, developing the photoresist coating, and removing the metal from
the unprotected areas of the film by etching, leaving a patterned metal
film on the substrate.
In the manufacture of metal photomasks, defects such as pin holes or
missing portions of metal film may occur. These defects in turn cause
defective integrated circuits or other devices produced from these masks.
Since the manufacture of photomasks is generally a time consuming and
relatively expensive operation, it is often desirable to correct a
defective photomask rather than to discard it. One method for repairing a
defective photomask has been described in U.S. Pat. No. 3,748,975. This
method is similar to the method of manufacturing the original photomask in
that it involves depositing a photoresist coating over the surface of the
photomask and exposing the photoresist coating to light in the region
overlying the defect to be corrected, developing the photoresist coating
and then, depending upon the kind of photoresist coating used, the defect
can be etched away where the defect is one consisting of a protrusion of
excess metal or can be rendered opaque where the defect consists of holes
or missing areas. When this method is employed for repairing pin hole
defects or the like, one must use essentially all of the same steps
employed in making the initial photomask. In the procedure of the present
invention, fewer steps are required and materials other than photoresists
can be employed when economically desirable.
SUMMARY OF THE INVENTION
A method of repairing a defect in a photomask which comprises a patterned
metal film on a transparent substrate comprises: (1) coating the surface
of the defective photomask with a solvent soluble film; (2) creating a
window in the film and photomask by removing a small portion of said
solvent soluble film and underlying metal film in the area of and around
the defect; (3) forming a second coating over said first coating and
window with a non-solvent soluble opaque material; and (4) treating the
mask with a solvent for dissolving the solvent soluble film thereby
causing both the solvent soluble film and non-solvent soluble material
thereon to be stripped from the mask leaving only the non-solvent soluble
opaque material remaining in the regions where the window was created.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a top cross-sectional elevational view of a typical photomask
having a pin hole defect therein;
FIGS. 2(a)-2(e) are side cross-sectional elevational views of the photomask
of FIGS. 1 through 2--2 showing the defect repair process steps;
FIG. 3 is an electron photomicrograph of a window formed through the
solvent soluble film and underlying mask material by means of a laser; and
FIG. 4 is an enlargement of a portion of FIG. 1 showing the tiered effect
produced by the laser.
DETAILED DESCRIPTION
The present invention will be discussed primarily in terms of repairing pin
hole defects in a photomask. It will be readily appreciated that the novel
method of this invention can be used for correcting voids or defects on
films on a substrate other than damaged photomasks.
In the manufacture of integrated circuits using photoresists which are
developed in accordance with predetermined patterns, photomasks are used
for determining the pattern to be developed in the photoresist. Generally,
several photomasks are used for the processing of an integrated circuit.
If the photomask that is employed contains pin holes, missing areas or
other voids in regions which were to have been opaque, the resulting
pattern developed onto the photoresist during the manufacturing process of
the integrated circuit will reflect these unwanted defects in the
photomask and will result in unsuitable integrated circuits which must be
discarded. Consequently it is important to have photomasks which are
essentially free of defects. Often, however, such defects do arise in the
manufacture of the photomask and rather than having to discard the entire
photomask it is often more economical to repair the defects if possible.
Referring to FIG. 1, there is shown a photomask 10 comprising a transparent
substrate 12 such as a sheet of glass, quartz or sapphire, for example,
having a patterned film 14, such as a layer of chromium, nickel or
aluminum formed on one surface 16 of the photomask 10. Typically, the
metallic film is from 500 A. to 1,000 A. thick. The pattern may be formed
by photolithographic means well known in the integrated circuit electronic
art. The photomask 10 may have one or more defects, particularly, pin hole
type defects, in the patterned film 14 as represented in exaggerated form
by pin hole defect 18, as shown. Typically, these defects are from 5 to 10
micrometers in diameter. However, defects as small as 2 to 3 micrometers
as well as larger defects are not unusual.
In accordance with the present invention, the pin hole defects 18 are
repaired by coating the surface of the photomask with a solvent soluble
film 20, locating the defect 18 or defects to be repaired, forming a
window or opening 22 through the solvent soluble film 20 and underlying
film 14 of the photomask in the region of the defect 18, said opening 22
generally being larger than the size of the original defect 18, coating
the solvent soluble film 20 and opening 22 with an opaque material 26
which is not soluble in the solvent which dissolves the solvent soluble
film 20 and which is adherent to the exposed surfaces of the photomask,
and then stripping the solvent soluble film 20 with solvent thereby
causing the opaque film 26 to be removed in the areas in which it overlies
the solvent soluble film 20 but remain in the areas in which it overlies
the window or opening 22 thereby covering the region which was originally
the pin hole defect 18.
Suitable solvent soluble films include photoresists which are commercially
available and well known in the electronics art, solvent soluble polymers
such as polyvinyl alcohol or polymethylmethacrylate, solvent soluble
resins and soluble grease. The solvent soluble film is generally applied
in a thickness in the order of 10,000 A. or 10 times the thickness of the
metal film of the photomask to at least the top surface 16 of the
photomask or alternatively may be applied to the entire photomask. Coating
of the solvent soluble film may be accomplished by well known coating
techniques such as dip coating or spray coating or, depending upon the
consistency of the film forming material, may be brushed or rubbed on or
otherwise applied to the surface of the photomask so as to uniformly coat
the photomask.
It is desirable to employ a solvent soluble film which has reasonably good
adhesion to the surface of the glass so that it will not peel or break
away during processing and will form a good uniform coating over the
photomask. It has been discovered that the adhesion of many film materials
can be enhanced by first coating the photomask with a thin transparent
film 30 of a transparent material which is not readily soluble in the
solvent used to strip the solvent soluble film but which contains moieties
which adhere well to the surface of the glass while providing other
moieties which adhere well to the solvent soluble film. Suitable adhesion
promoting films for many photoresists and other polymers are films of
silanes and siloxanes. A preferred adhesion promoting film is
hexamethyl-disiloxane. This film may be formed on the photomask, by
example, by spin coating from a 0.2 percent solution in ethanol, air
drying and heating to 150.degree. C. for 15 minutes for a thin layer of
siloxane.
Typically the opening or window 22 that is formed in the photomask in
accordance with the novel procedure is several times larger than the
diameter of the defect 18 to be repaired. It has been discovered that when
the opening 22 is formed by use of a laser a surprising and unexpected
result is achieved which facilitates the repair of the pin hole 18. More
particularly, the hole 22 produced in the solvent soluble film 20, and
adhesive layer 30, when present, by means of burning through the coatings
with a laser, is wider than the hole produced in the underlying metallic
patterned film 14. This provides a lip 24 over which the opaque repairing
film 26 lies so as to completely overlap the exposed surface of the
substrate 12. The general shape of the opening 22 formed with the laser is
that approximating concentric circles (FIG. 3), the bottom circle being
the hole in the metallic patterned film 14 and the larger other circle
formed over the overlay or lip region 24 of the metallic film representing
the hole in the solvent soluble film 20. It should be understood that
while forming the opening 22 with a laser is preferred, especially for the
reason that in one step one may obtain an opening which has an overlay or
lip region 24 for facilitating better contact and repair of the photomask,
other methods for forming openings may also be employed.
In the preferred procedure, in order to assure adhesion of the opaque
repairing film to the photomask, one should etch both the surface of the
lip of the metallic pattern and the exposed portion of the transparent
base material. This can be accomplished, for example, by dipping the
photomask, after formation of the opening, into a solution comprising
ceric ammonium nitrate so as to etch the metallic patterned surface of the
exposed lip followed by dipping the photomask into a solution comprising
hydrofluoric acid, fluoboric acid or a mixture thereof, to etch the
transparent base.
The opaque film 26 used in repairing the photomasks is preferably a
metallic film which can be formed over the surface in any one of many
ways. For example, the film may be formed by vacuum evaporation,
sputtering, vapor deposition or electroless plating techniques. It has
been found preferable to form the film utilizing electroless plating
techniques. Generally, photomask repair is accomplished by forming films
of nickel or chromium by standard electroless plating methods. These
methods are well known in the art and generally comprise the steps of
first sensitizing and activating the surface to be plated followed by
immersion or treatment with an electroless plating solution so as to form
a metallic film over the surface. Typical electroless solutions useful for
repairing photomasks are electroless nickel and electroless chromium
solutions. Such solutions are commercially available.
The pin hole defects to be repaired in the photomask are generally located
using a scanning optical comparator device employing collimated light
which compares the photomask to be repaired with a photomask free of
defects such that when a defect is reached during the scan a signal
indicates the presence of a defect. Other techniques such as microscopic
examination are, of course, also available for determining the location of
the defects. The laser used for forming the opening can be directly
associated with the defect locating means. For example, the laser can be
part of the scanning optical comparator used to detect the defect or can
be coupled to the microscope used for detecting the defects so as to emit
its beam through the microscope lens directly onto the defect.
It should be understood that in the case of photomasks, where endurance and
abrasion resistance is desired, it is preferable to use metallic films for
repair. However, films of other opaque materials can be used.
EXAMPLE I
About a 10 micron thick coating of Shipley AZ 1350Z photoresist is spin
coated on a photomask to be repaired. The coating is dried at 25.degree.
C. and the coated photomask is placed in a laser equipped comparator for
detecting the pin holes. When a pin hole is detected, holes are burned
through the photoresist coating and underlying metal coating of the
photomask in the region of the defect using an yttrium aluminum garnet
laser having approximately a 5 watt laser output. FIGS. 3 and 4, which are
photomicrographs of the opening formed by the laser, clearly show the
tiered effect obtained when forming the holes with the laser. The center
portion of the hole represents the transparent base, the first ring or lip
region represents the metallic patterned film of the photomask while the
top layer represents the photoresist film. As can be seen from the
photomicrographs the holes formed in the metallic film are approximately
25 to 30 micrometers in diameter while the hole formed in the photoresist
is approximately 20 micrometers larger in diameter thereby forming an
approximate 10 micrometer lip onto which the hole repairing plating
material can be formed. The photomask is then etched by dipping in chromic
acid followed by dipping in the solution of hydrofluoric or fluoboric acid
at room temperature. The photomask is then rinsed and treated with a tin
chloride electroless metal sensitizing solution followed by treatment with
a palladium chloride catalyzing solution so as to sensitize and catalyze
the surface for electroless nickel plating. The catalyzed surface of the
photomask is then dipped in an electroless nickel plate solution such as
MacDermic Incorporated J-67 or J-28R electroless nickel solutions which
are operated at 70.degree. C. After formation of the electroless nickel
deposit over the surface of the photomask, the photomask is treated with
methanol which is a solvent for the Shipley photoresist used. By this
treatment, the photoresist is stripped from the photomask together with
the nickel deposit thereover leaving only the metal deposited in the
opening formed by the laser.
EXAMPLE II
A silane film in the order of about 1,000 A. is spin coated onto the mask
to be repaired in order to promote adhesion of the subsequent coating. The
silane coating is air dried and the mask is then heated to 150.degree. C.
for 15 minutes. A 10.mu. thick photoresist layer of AZ 1350Z Shipley
photoresist is then spin coated onto the mask. The pin hole areas are then
registered in the comparator and an yttrium aluminum garnet laser having a
5 watt output is used to burn through the photomask in the region of the
pin holes. The photomask is then etched with a solution consisting of 91
grams ceric ammonium nitrate, 24 milliliters of acetic acid diluted to 500
milliliters with distilled water. Etching is preferably accomplished in an
ultrasonic bath environment. The photomask is rinsed and then placed in a
fluoboric acid etch solution to etch the underlying glass substrate of the
photomask. After rinsing the photomask to remove any excess etch solution
thereon the photomask is placed in a tin solution containing 1.5% stannic
chloride and from 1.5 to 2% stannous chloride so as to sensitize the
surface of the photomask for electroless plating. The sensitized surface
is rinsed and placed in a 5% palladium chloride solution to form a
catalytic palladium deposit over the surface of the photomask for
electroless nickel deposition. The photomask is then rinsed and placed in
a commercial electroless nickel solution. After forming an electroless
nickel deposit over the surface of the photomask, the photomask is then
dipped in a solvent so as to remove the photoresist and nickel coating
thereon leaving nickel deposited in the area of the hole formed by the
laser.
It should be understood that any of the sensitizing solutions known in the
art are suitable whether they are one-step or two-step sensitizers or
activators and whether or not they employ noble metals or not. Sensitizing
procedures may be found, in part, in Metallic Coating of Plastics, William
Goldie, Electrochemical Publications, 1968, or with reference to U.S. Pat.
Nos. 3,011,920; 3,532,518; 3,772,056; 3,772,078; 3,907,621; 3,925,578; and
3,930,963.
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