Globally planarized binary optical mask using buried absorbers

A globally planarized binary optical mask has absorbers embedded (buried) in the mask substrate, instead of on the surface of the mask. Light scattering at rough vertical edges of absorbers of prior art masks are reduced or eliminated. Also, due to the buried nature of the absorbers, a triple singularity point encountered in prior art masks at the interface of three environments of quartz, absorber and air, no longer exists. The buried absorbers have an offset distance from the surface of the substrate so that with a minimum effective offset distance, defects and contaminants at the surface of the mask are no longer in the image plane, wherein alleviating a need for a pellicle to protect the mask surface. By reducing light scattering and distortion, the mask of the present invention allows for conventional optical lithography to be extended to ranges of shorter wavelength.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of fabricating photomasks for 
use in the manufacture of semiconductor devices and, more particularly, to 
the fabrication of binary optical photomasks for use in lithography. 
2. Related Application 
This application is related to copending application entitled "Attenuated 
Phase Shifting Mask With Buried Absorbers," Ser. No. 08/342,939, filed 
Nov. 21, 1994. 
3. Prior Art 
Conventional binary intensity masks generally contain patterned absorber 
films (such as chromium, chromium oxide or molybdenum) on a highly 
polished quartz substrate. These absorber regions on the photomask absorb 
light so as to totally absorb or significantly attenuate light passing 
through the region. Thus, dark and light image patterns are projected from 
the mask onto a target, such as a semiconductor wafer. 
The absorber regions are formed on a surface of the quartz so as to present 
topographical features on the surface. The absorber features can have 
"rough" edges, as well as other topographical defects, such as cracking or 
peeling, which can contribute to light scattering at the vertical edges of 
the absorber. The interface of quartz, absorber material and air form a 
"triple" optical singularity point due to the intersection of three 
different indices of refraction. 
Additionally, pellicles are used on prior art masks to protect the surface 
of the mask from being contaminated. This covering is necessary since the 
image plane is at the surface of the quartz. The attachment of a pellicle 
in a defect free manner is difficult and involves an expensive 
manufacturing process. Often, pellicles degrade under exposure to 
ultraviolet radiation, thereby limiting the useful life of the mask. 
Because of these shortcomings, conventional binary optical masks have 
disadvantages affecting the cost of manufacture and useful life. However, 
these disadvantages are minimal compared to the "blurring" of the image 
experienced due to edge diffraction effects and the imperfect edge 
definition associated with the absorber regions. Without a sharp 
transition from dark to light, clear distinctive features are difficult to 
obtain and such distortions are amplified as device features shrink in 
size. At submicron levels, these edge distortions severely impact image 
contrast and resolution. 
The present invention describes a photomask which addresses the 
disadvantageous qualities of the conventional binary intensity mask noted 
above by providing for a photomask having buried absorbers with a built-in 
offset that significantly reduces edge diffraction effects and eliminates 
the need for a pellicle. 
SUMMARY OF THE INVENTION 
A globally planarized binary optical mask using buried absorbers and 
methods for manufacturing such masks are described. The mask of the 
present invention utilizes buried absorber regions offset from the surface 
by a known distance, wherein absorbers for the mask are embedded in the 
mask substrate and are not formed on the surface of the mask. Light 
scattering at rough vertical edges of absorbers of prior art masks are 
reduced or eliminated with the mask of the present invention since much of 
the scattering is reflected back in to the substrate and are not projected 
onto the image. 
Also, due to the buried nature of the absorbers, a triple singularity point 
encountered in prior art masks at the interface of three environments of 
quartz, absorber and air, no longer exists. The buried absorbers have an 
offset distance from the surface of the substrate greater than the maximum 
depth of focus (DOF) of the exposure system. With such an effective offset 
distance, defects and contaminants at the surface of the mask are no 
longer in the image plane, wherein alleviating a need for a pellicle to 
protect the mask surface. 
By reducing light scattering and distortion, the mask of the present 
invention allows for conventional optical lithography to be extended to 
ranges of shorter wavelength without the necessity of developing special 
pellicles for each wavelength. The reduction of edge scattering also 
provides for sharper edge definition which translates into improved image 
patterns. 
Methods are described for fabricating masks of the present invention. In 
one method, trenches are formed in a quartz substrate and filled with an 
absorber material. After planarization, a dielectric layer is formed over 
the surface of the mask and the filled trenches or another quartz layer is 
bonded over it and then etched back. The thickness of the trench 
determines the thickness of the absorber region and the thickness of the 
overlying layer determines the offset distance of the absorber region from 
the surface of the mask. 
In an alternative method, a portion of a substrate surface is exposed by an 
overlying patterned oxide layer. Subsequently, ions are implanted into the 
exposed regions to form absorber regions within the substrate at a 
specified depth. The dosage of the ions determines the thickness of the 
absorber regions and the implantation energy determines the depth, which 
corresponds to the offset distance of the absorber region from the 
surface. 
Economic Advantage: By enhancing photomasks to permit the use of optical 
lithography at shorter wavelengths, new lithography techniques and tools 
need not be utilized for producing semiconductor devices having 
smaller-dimensioned features.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A globally planarized binary optical mask, having buried absorbers offset 
from the surface of the mask by a fixed distance, for use in submicron 
optical lithography to fabricate semiconductor devices is described. In 
the following description, numerous specific details are set forth, such 
as specific structures, processes, chemical compositions, etc., in order 
to provide a thorough understanding of the present invention. However, it 
will be obvious to one skilled in the art that the present invention may 
be practiced without these specific details. In other instances, well 
known processes and structures have not been described in detail in order 
not to unnecessarily obscure the present invention. 
PRIOR ART 
Referring to FIG. 1, a typical conventional binary mask 10 is shown having 
a quartz substrate (mask "blank") 11 containing a patterned feature 12 of 
metallic absorber film formed on its surface 13. The purpose of the 
patterned absorber 12 is to absorb light in order for the mask 10 to 
project a pattern on a target specimen, such as a semiconductor substrate. 
Although not limited to these, examples of absorber materials are 
chromium, chromium oxide and molybdenum. The quartz substrate 11 is 
usually a highly polished quartz having an approximate thickness of 90-250 
mils. The absorber 12 areas are typically formed by conventional electron 
beam (e-beam) lithography and subsequent wet etching. 
Vertical edges 14 of the absorber 12 are typically non-specular and present 
a "rough" surface, resulting from the limitations imposed by the wet etch 
process. The non-specular surface of the edge 14 contributes to a 
scattering of projected light, which is an undesirable property for a 
mask. (see for example, "Chrome dry etching for photomask fabrication;" W. 
W. Flack et al., SPIE Vol. 1809, 12th Annual BACUS Symposium 1992, pp. 
85-96.) Also, the limitations of the wet etch process usually will form a 
sloped edge surface (deviation from the 90 degree wall angle) which limits 
the minimum useful feature size being imaged. Furthermore, since chromium 
has a weak absorption property and requires relatively thick films for 
sufficient absorption, films in the order of .lambda./4 to .lambda./2 are 
required for chromium absorbers. Such thick films can place topographic 
constraints on the size of the absorbers disposed on the mask surface and 
can lead to more severe edge effects. 
Another shortcoming of the prior art binary mask 10 is illustrated in FIG. 
1. With the conventional binary intensity mask 10, a pellicle 15 is 
generally utilized to protect the surface of the mask 10. Since the image 
plane of the mask is set at the position of the quartz surface and the 
absorbers 12, defects and contaminants at the surface of the mask will be 
imaged. Thus, a pellicle is essential in order to maintain the surface 13 
of the mask 10 free of contaminants. The pellicle 15 is typically 
constructed from a thin polymeric membrane and is disposed above the mask 
image plane by quartz spacers 16. The pellicle 15 prevents surface defects 
and particulates from being imaged onto the wafer. The attachment of the 
pellicle 15 in a defect free manner is generally a complex and expensive 
process requiring specialized equipment and must be performed without 
introducing defects. In addition, the pellicle 15 material, which is 
specific for each wavelength used, often undergoes degradation under 
prolonged exposure to ultraviolet or deep ultraviolet (DUV) wavelengths, 
thereby reducing the useful life of the mask 10. Ultimately, manufacturing 
yield losses can be (and generally are) experienced due to the use of 
pellicle 15. 
Referring to FIG. 2, a portion of a single absorber 12 is shown 
highlighting edge effects experienced along the edge 14. Specifically, 
effects of edge scattering are noted at edge 14. Light rays 17 which 
transition through the quartz substrate 11 are not interrupted along open 
(non-absorber) areas 19 of the mask 10. Light rays 17 impinging on the 
absorbers 12 are absorbed or at least attenuated. The absorbers either 
completely absorbs the light rays 17 or severely attenuates them, so that 
very little light (if any) passes through them to reach the target. These 
light and dark areas form the pattern for the mask. 
It is noted that a significant problem is encountered at the edges 14 of 
absorbers 12. The "rough" sidewall, a result of a wet etch process, and 
the non-vertical profile of the edge 14 cause a scattering (diffraction) 
of light, which is illustrated by rays 18 in FIG. 2. This scattering 
results in a "blurring" of the edge sharpness that can be projected onto 
the image and can impact the definition of the imaged pattern. Therefore, 
image contrast and resolution are affected by the scattering experienced 
at the edge 14. 
The scattering is also enhanced due to the intersection of three 
environments at an intersection point 20. At the point where edge 14 
contacts substrate 11, three different environments intersect, each having 
its own index of refraction. The three different environments are quartz 
(index of refraction "n" of 1.5), air (n=1.0) and the absorber 
(n.apprxeq.2.0 for chromium). The "triple point" optical singularity at 
point 20 diffracts light and adds to the scattering, thereby degrading the 
imaging signal-to-noise ratio at the dark-light transition areas of the 
pattern being projected. 
The "blurring" effects are better illustrated in FIG. 3, wherein light 
intensity projected from the mask 10 onto a target is mapped across the 
portion of the mask surface shown in FIG. 2. Dotted line 26 illustrates an 
ideal condition wherein a substantially instantaneous shift (vertical 
step) from dark to light is preferred at the edge of the absorber. 
However, in reality solid line 27 exemplifies a typical condition 
experienced at edge 14 of absorber 12. Due to the scattering and 
diffraction of light at the edge 14 and point 20, the dark-to-light 
transition is not as sharp. The "blurring" effect experienced is noted by 
a sloped profile 28 at the transition from dark to light. 
Therefore, it is appreciated that a reduction of the edge scattering would 
be beneficial for improving the image contrast and resolution. 
Essentially, it is preferred to increase the slope 28 to make it approach 
the vertical step noted in the ideal condition exemplified by line 26. 
Further, it would also be beneficial for improved utilization of the mask, 
if it did not require the use of a pellicle. 
PRESENT INVENTION 
Referring to FIG. 4, a mask 30 of the present invention is shown. Mask 30 
is a globally planarized structure having light absorbers 32 fully 
embedded (buried) within a highly polished quartz substrate 31. The quartz 
substrate 31 is equivalent to that of substrate 11 described earlier, 
however, now the absorbers 32 are disposed completely within substrate 31. 
Therefore, unlike the prior art mask 10, the absorbers 32 are fully 
embedded within the quartz 31. Because the absorbers are no longer on the 
surface of the quartz, the surface 33 of the substrate 31 is essentially 
flat, thereby allowing for the mask 30 to have a globally planarized 
surface 33 with no topographical features that can induce diffraction and 
scattering effects. The pattern formed by the absorbers 32 is equivalent 
to that of the prior art mask 10, wherein light rays 37 transition through 
the substrate and are absorbed completely or attenuated by absorbers 32. 
The absorber material is specifically chosen to have a high extinction 
coefficient or absorbtivity at relatively small thicknesses. Since the 
process of forming the absorbers 32 need not involve any wet etch of the 
absorber material, material other than chrome can be used to form 
absorbers 32. Materials having much higher absorption properties, such as 
gold and silicon, are preferred and can be used without manufacturing 
difficulties. 
As noted in FIG. 4, the optical absorbers 32 are offset by a distance "d" 
from the surface 33. Thickness "t" of the absorbers 32 are also denoted in 
the Figure. Thickness t and offset distance d are design parameters that 
are based on the specific material selected for the absorber 32 and the 
exposure wavelength of the light source. Thus t and d are design 
parameters and the exact values will depend on the specific application. 
For the offset distance d, it will become smaller as the exposure 
wavelength shortens. For the current generation optical lithography 
systems, the exposure wavelength resides at visible light (for example, 
365 nanometers (nm)) or DUV at 248 and 193 nm. For this range of exposure 
wavelength, d varies approximately in the range between 0.5 microns (at 
.lambda.=193 nm) to 2 microns (at .lambda.=365 nm). Thus, for DUV systems 
operating at 248 nm, d will generally fall within the range of 1-2 
microns. This distance is greater than the depth of focus of the exposure 
system. The distance d is calculated from the optical requirement that the 
maximum scattered beam has an angle of incidence greater than the critical 
angle .theta.c (shown in FIG. 5) for total internal reflection in the 
quartz. In addition the distance d must be greater than the maximum depth 
of focus (DOF) of the exposure system employed. 
For thickness t, the material selected for the absorber 32 and the exposure 
wavelength of the system will determine its value. The absorber thickness 
t is chosen based on the relationship: 
EQU I=(Io)(e.sup.-.alpha.t) 
where Io is the incident intensity, l is the transmitted intensity, .alpha. 
is the absorption coefficient for a given material and t is the thickness. 
The thickness t is chosen so that the transmitted light intensity l is a 
small fraction (such as, less than 1%) of the incident light intensity Io. 
This light absorption results in an "optically" dark region on the mask 
30. 
An important distinction regarding mask 30 is that a pellicle is not 
needed. The mask 30 does not require a pellicle since the surface defects 
and contaminants at surface 33 are beyond the maximum depth of focus of 
the exposure system and, hence, are automatically placed outside of the 
image plane. Since the image plane is situated at the absorbers 32, the 
offset distance d functions to isolate the absorbers from any defects or 
contaminants at the surface 33. 
Another important distinction of the buried absorbers 32 is that the 
surface 33 of the mask 10 can be made globally planar. Since absorbers are 
not constructed on the outer surface of the mask 10, the outer surface 33 
is made planar. Lack of surface topography on a globally planarized 
surface ensures that topographical effects at the surface do not add to 
any distortion of the image plane. Thus, there are no scattering or 
diffraction effects that occur due to topographical effects. 
Referring to FIG. 5, a portion of a single absorber 32 within substrate 31 
is shown. With the mask 30 of the present invention, significantly less 
light scattering is experienced at the edge 35 of buried absorber 32. As 
shown in FIG. 5, light scattering is noted by arrows 38. The reduction in 
the scattering is attributable to the buried disposition of the absorber 
32. Since the absorber 32 resides totally within substrate 31, much of the 
scattering 38 due to the edge effect will be trapped within the substrate 
31. Total internal reflection occurs at the quartz-air interface 39, 
wherein scattered light rays 38 are reflected back into the quartz 31 and 
are prevented from reaching the target. Additionally, a triple point 
singularity is not encountered since the absorbers do not come in contact 
with the air environment. Thus, diffraction and scattering previously 
encountered at the triple point optical singularity is minimized. The 
absorber thickness t is minimized by selecting a material with a high 
absorbtivity (.alpha.). The mask manufacturing process is made more 
independent of the material choice. 
Referring to FIG. 6, an intensity profile for a projected image is shown. 
Again, an ideal situation for imaging a pattern onto a target is shown by 
dotted line 40, wherein an ideal transition at edge 35 distinguishes an 
instantaneous dark-light transition. A solid line 41 shows the intensity 
pattern when a buried absorber mask 10 of the present invention is used. 
The vertical transition 42 of the intensity profile 41 approaches the 
ideal condition of line 40 and is significantly sharper than slope 28 of 
the prior art mask shown in FIG. 3. The sharpness is due to the reduction 
of the "blurring" at the edge, which reduction is the result of having the 
absorber layer buried within quartz 31. 
As noted with the prior art mask, materials which are currently utilized 
for absorbers films can be readily used with the mask of the present 
invention. Chromium, chromium oxide and molybdenum are examples, but other 
light absorbing films can be used as well. Generally, metal films are 
preferred, but other films, such as silicon films can be used as well. The 
thickness t is calculated equivalently to that of the prior art absorber. 
The thickness will depend on the material and the exposure wavelength. The 
offset distance d is calculated as follows. If .theta.c (shown in FIG. 5) 
is the critical angle for total internal reflection (for quartz) and w is 
the size (width) of the absorber feature, then d=w/(2 tan .nu.c). The 
critical angle .theta.c is dependent on the exposure wavelength and the 
index of refraction of quartz. 
Furthermore, the planar surface of the mask offers a number of advantages. 
The surface of the mask is free from topographic features and, therefore, 
reduces or eliminates diffraction and scattering at the surface. The 
completed mask can be cleaned and used in manufacturing with greater ease 
as compared to masks with pellicles. The absorber material will have less 
cracking and peeling due to edge roughness. The same mask can be used at a 
variety of wavelengths, since the pellicle is not utilized. 
Thus, a globally planarized binary optical mask using buried absorbers is 
described. 
METHODS OF FABRICATION 
A variety of prior art processes can be adapted to fabricate the buried 
absorber mask of the present invention. However, novel and preferred 
methods for manufacturing such masks are described below. 
A method for fabricating a mask 31 of the present invention is shown in 
FIGS. 7-12. Actually there are two methods illustrated in FIGS. 7-12, but 
the front end of the process as shown in FIGS. 7-11 are similar for both 
methods. FIG. 12A shows one method of completing the mask from the step 
described in reference to FIG. 11, while FIG. 12B shows another method of 
completing the mask from FIG. 11. 
Referring to FIG. 7, a quartz substrate 51 or mask "blank" is shown having 
a photoresistive layer 52 formed thereon. The quartz substrate 51 
typically has a thickness in the approximate range of 90-250 mils. Quartz 
substrates currently used as mask blanks for fabricating photomasks can be 
readily used as substrate 51. It is important to note that the quartz 
blank used should not contain any chromium or other absorber coating. The 
photoresistive layer 52 is formed on the substrate 51 by using any number 
of prior art techniques. The photoresistive layer 52 is then patterned 
using a conventional optical or electron beam (e-beam) lithography that 
exposes portions of the photoresistive layer 52. It should be noted that 
if e-beam lithography is utilized, an anti-static layer may be necessary 
to prevent charging. The anti-static layer (if used) is placed on the 
surface of the quartz below the photoresistive layer 52. The result of 
performing lithographic exposure and development to pattern the 
photoresistive layer 52 is shown in FIG. 8, in which patterned openings 
expose portions of quartz 51. 
The exposed areas of quartz 51 are then etched to form trenches using a dry 
etch process, such as a plasma etch process using a fluorine based 
chemistry, to a depth t which corresponds to the earlier described 
thickness for the buried absorber. The etch depth will depend on the 
absorber material that will be used and the wavelength of the exposure 
tool in which the mask will be used. The etch rate of the quartz is well 
characterized and, therefore, t can be obtained having a uniformity of 5% 
or better. Following the quartz etch, the photoresistive layer 52 is 
removed, leaving the patterned quartz shown in FIG. 9. 
Referring to FIG. 10, the patterned quartz substrate is then blanket coated 
with a selected absorber material 53 utilizing a technique, such as 
sputtering, evaporation or chemical vapor deposition (CVD). A variety of 
prior art materials that are known as light absorbers can be utilized for 
absorber layer 53. More specifically, a number of absorber films are used 
in the prior art for mask fabrication and these materials can be readily 
used for layer 53. Although not limited to these, examples of absorber 
films are chromium, molybdenum, tantalum and silicon. As noted earlier, 
materials having higher absorbtivity than Cr are preferred in order to 
reduce the thickness of the absorber layer. A metallic film of high 
absorbtivity is preferred for the absorber material of the present 
invention, although silicon can be used if so desired. 
Subsequently, as shown in FIG. 11, the absorber layer 53 is etched back 
using a selective etch process. Chemical-mechanical polishing (CMP) is a 
preferred technique since excellent polish selectivity can be achieved. 
Since the absorber layer 53 is generally metallic in most instances, high 
selectivity can be obtained between the metal film 53 and quartz 51. The 
CMP process can be used to stop on the quartz with good precision. 
Furthermore, the CMP is also effective in removing particles and other 
defects, thereby producing a flat (planarized) defect-free surface 54. As 
shown in FIG. 11, the quartz openings are filled with the absorber 
material. Next, the quartz substrate is cleaned and processed by one of 
the following two alternative steps. 
Referring to FIG. 12A, a dielectric layer 55, such as a SiO.sub.2 layer, is 
deposited over the quartz substrate of FIG. 11. Since layer 55 is 
deposited on a globally planar surface, it will generally have acceptable 
thickness uniformity. Layer 55 is deposited to a depth d, which 
corresponds with the offset distance d of the absorber from the surface of 
the substrate 51. Any of a well-known prior art techniques, such as a CVD 
process, can be used to deposit layer 55. The absorber material 53 is now 
fully encapsulated and has the offset distance d determined by the 
thickness of the dielectric layer 55. 
However, since the CVD deposited dielectric material, even SiO.sub.2, will 
have an index of refraction different than that of quartz, precise 
matching of layer 55 to quartz may present a problem in some instances. 
This is particularly true at 193 nm where SiO.sub.2 absorbtivity is quite 
high. Thus, an alternative (and slightly more complicated) process is 
described in reference to FIG. 12B. In this approach, a quartz plate 58 is 
bonded on to the quartz substrate 51 of FIG. 11. This second quartz layer 
58 should be of sufficient thickness for etch-back. A thickness of 
approximately 20 mils is sufficient, although the actual thickness is a 
design choice. Although a variety of techniques could be used to bond 
quartz layer 58 onto the quartz substrate 51, the preferred step is 
performed in a rapid thermal processor (RTP) at a temperature of 
approximately 800 degrees centigrade in a nitrogen ambient. The higher 
temperature ensures an extremely strong quartz-to-quartz bond and the 
nitrogen ambient strengthens the bond so that subsequent delamination does 
not occur. 
Next the bonded layer 58 is etched back as shown by arrows 59 in FIG. 12B 
to a thickness 57 of approximately 5000 .ANG.- 5 um. A combination of CMP 
and chemical etch techniques are used. Initially, bulk of the layer 58 is 
removed using CMP. Then, a chemical etch (either wet or dry), well-known 
in the prior art, is used to carefully etch back the remaining quartz 
until thickness 57 is reached. The chemical etch process will allow for 
precise control of the etch-back to a final thickness within a tolerance 
of 5% or less. Thickness 57 is the offset distance d for the absorber 53. 
The quartz-to-quartz bond results essentially in a single quartz substrate 
so that this interface does not present a change of the refraction index 
for the light rays traversing through the quartz. 
A completely different and alternative method for fabricating a globally 
planar, buried absorber mask of the present invention is described in 
reference to FIGS. 13-16. Instead of forming absorber regions and 
encapsulating them, this alternative approach utilizes an ion implantation 
technique to implant metal ions below the surface of the substrate. 
Referring to FIGS. 13-16, a quartz substrate 61 or mask "blank", equivalent 
to that of quartz substrate 51, is coated with an oxide layer 63, such as 
a SiO.sub.2 film layer, to a thickness of approximately 1 um using a prior 
art technique, such as deposition by a CVD process. Next, the SiO.sub.2 
layer 63 is coated with a photoresistive layer 62 using known techniques 
for depositing photoresists. The photoresistive layer 62 is then patterned 
using a known lithographic technique, such as the afore-mentioned e-beam 
or optical exposure process. After forming the pattern on layer 62, which 
is shown in FIG. 14, the underlying SiO.sub.2 layer 63 is patterned using 
an etch process, preferably a dry etch process using fluorine based 
chemistry, to expose portions of quartz substrate 61. 
After patterning the SiO.sub.2 layer 63, the photoresistive layer 62 is 
stripped. Next, the substrate 61 is subjected to an implantation step in 
which metal ions are implanted into the exposed portions of the quartz 61 
by high energy implantation as shown in FIG. 15. The SiO.sub.2 functions 
as a shielding layer so that the implantation occurs only into the exposed 
quartz. Implantation dosage in the range of 3.times.10.sup.17 /cm.sup.2 
-2.times.10.sup.18 /cm.sup.2 at energy levels in the range of 1-5 MeV will 
implant metal ions to a depth ranging from 5000 .ANG. to 2 um. At these 
dose and energy levels, the metal ions will form a continuous metallic 
layer 65 in the quartz 61 matrix, wherein layer 65 will have a thickness 
in the range of 500 .ANG.-800 .ANG.. The actual depth of implantation, as 
well as the thickness of the implanted layer 65 is a design choice and 
will be determined by the application of the mask for which it is being 
fabricated. Furthermore, a variety of metal ions can be used for 
implantation. Although not limited to these ions, examples of metal ions 
are chromium (Cr.sup.+), gold (Au.sup.+), titanium (Ti.sup.+) and tantalum 
(Ta.sup.+). 
Finally, as is shown in FIG. 16, the remaining SiO.sub.2 63 is stripped and 
the quartz substrate 61 is annealed at approximately 1000 degrees 
centigrade to remove any residual implant damage in the quartz 61. The 
substrate 61 is then cleaned and inspected. The implanted layer 65 is the 
buried absorber layer of the mask. Thus, the thickness of this layer 65 
corresponds to thickness t of the buried absorber and the implant depth 
corresponds to the offset distance d of the mask. By utilizing this 
method, absorber thickness t and offset distance d can be tightly 
controlled.