Undercut membrane mask for high energy photon patterning

A mask for use with high energy radiation sources in precision projection processing by excimer lasers, for example, is described. The mask comprises a suitable substrate, such as silicon, upon which a multilayer dielectric stack is formed which acts as a reflective coating for the impinging excimer laser radiation, minimizing energy absorption by the mask substrate. The mask transparent areas are defined by the through-holes in the mask. The through-holes are formed with a conically undercut edge profile to define a thin object plane for the mask and minimize scattering of the radiation from the through-hole sidewalls. A method for fabricating the mask is also described.

BACKGROUND OF THE INVENTION 
The present invention relates generally to transmission masks of the type 
used for laser processing of surfaces, and, more particularly, to 
non-contact shadow masks utilized in laser ablation processes fabricated 
from a thick membrane utilizing conically undercut through holes to 
provide the desired ablation pattern and laser energy reflective coatings 
in the non-patterned areas. 
In the field of microelectronic materials processing the use of high power 
lasers to provide selective etching or micromachining of substrate 
materials, metal films and layers of other materials, such as polyimide, 
is becoming increasingly important. Of key importance to this technology 
is a shadow mask or projection mask to project the pattern of the 
microstructures onto the material to be machined. It is well-known in the 
art to provide masks for ion etching, ion implantation and in optical, 
x-ray, ion and electron beam lithography. Masks of these types are used in 
relatively low power applications and typically are not suitable for high 
power laser applications, such as excimer laser ablation. 
Prior art masks presently utilized in laser etching applications typically 
comprise a suitable substrate having clear or transparent areas and opaque 
areas to define the desired etch pattern to be projected. As increasingly 
higher power lasers are utilized and more exact and smaller element sizes 
are realized, a few micrometers (um) in size, for example, mask 
requirements such as reduction of defects in both the clear and opaque 
areas become critical. 
The following criteria have been identified for ideal masks to provide 
optimal performance during high energy laser etching, micromachining, 
material deposition or other surface treatment of materials. 
1. The mask object plane must be extremely thin to provide a well-defined 
image plane together with a full depth of focus at the image plane to 
provide, for example, the ability to etch deep structures in the surface 
of a material. 
2. The mask object plane must be stable in all directions; that is, no 
bending or warpage of the mask object plane resulting from, for example, 
thermal expansion or mechanical stress or vibration of the mask. 
3. The mask clear or transparent areas must be defect-free. 
4. The mask opaque areas must be defect-free. 
5. The mask must have a versatile alignment capability; for example, mask 
alignment targets should be detectable at the process laser wavelength and 
at a convenient alignment laser wavelength, a helium-neon laser, for 
example. 
6. The mask should be compatible with a viable inspection scheme; that is, 
the mask pattern should be clearly detectable with visible radiation to 
allow for inspection utilizing an optical microscope, for example. 
7. The mask materials, both the substrate and overlaying layers, must be 
stable under the high pulse rate, high power radiation typically 
encountered during excimer laser ablation or other high-power laser 
processes. 
Prior art "chrome on glass" masks, shown in FIG. 1a, as used, for example, 
for optical lithography in IC-chip fabrication consist of a thin (several 
hundred Angstroms) layer of chrome defining an opaque pattern on an 
unstructured clear glass or quartz substrate. Typically, masks of these 
type will exhibit defects resulting from particulates in the transparent 
areas or microcracks in the substrate and pinholes in the opaque chrome 
layer. 
U.S. Pat. Nos. 4,490,210; 4,490,211 and 4,478,677, issued to Chen et al, 
all assigned to the instant assignee, disclose laser etching of metalized 
substrates and glass materials involving an intermediate step in which the 
material to be etched forms a reaction product resulting from exposure to 
a selected gas, which is then vaporized by a beam of radiation of a 
suitable wavelength. Many materials can be etched directly by laser energy 
without the need for an intermediate step creating a reaction product 
material. The Chen et al patents describe non-contact masks as having a 
transparent substrate/body of UV grade quartz with a pattern chromium film 
thereon. It has been found, however, that such chromium masks cannot 
withstand laser energy densities of the order encountered when working 
with excimer or other lasers having the required intensity to etch or 
ablate many target materials directly. While satisfying several of the 
above-defined mask requirements, chromium may absorb as much as half of 
the incident laser energy at selected wavelengths. Thus, a single excimer 
laser pulse may easily ablate the chromium and destroy the opaque pattern. 
U.S. Pat. No. 4,923,772 issued to Kirch et al, assigned to the instant 
assignee and incorporated by reference as if fully set forth herein, 
discloses a high-energy laser mask comprised of a transparent substrate 
having a patterned laser-reflective metal or dielectric coating deposited 
thereon. Typically referred to as a "dielectric mask", a mask, shown in 
FIG. 1b, comprising a transparent substrate/body having the opaque pattern 
formed of several layers of a highly reflective, abrasive-resistant 
dielectric coating provides a mask able to withstand the full range of 
laser intensities encountered in laser etching processes. For example, 
Kirch et al discloses a dielectric mask comprising many layers of such 
dielectric coatings deposited on a substrate of UV grade synthetic fused 
silica which achieves greater than 99.9% reflectivity of the incident 
laser energy. Such a dielectric patterned mask can withstand incident 
energy densities up to approximately 6J/cm.sup.2. However, defects 
resulting from particulates in the substrate material or microcracks and 
pinholes in the dielectric layers may be present. As discussed by Kirch et 
al, relatively pure substrate material such as UV grade synthetic fused 
silica is required to avoid laser absorption by impurities or inclusions 
in the mask clear areas. However, long-term irradiation with high-power UV 
radiation may induce absorptions in the mask clear areas due to 
solarization effects. Additionally, since the dielectric materials 
typically reflect radiation only in a small spectral region, near the 
wavelength of the processing laser, it is typically transparent in the 
visible range, thus presenting difficulties for a conventional optical 
system alignment, or inspection, utilizing, for example, an optical 
microscope. 
Thin metal sheets fabricated from materials such as molybdenum or steel, 
having physical through holes formed transversely through the metal sheet 
representing the transparent or clear areas, referred to as metal stencil 
masks, as shown in FIG. 1c, have also been widely used as excimer laser 
masks. However, at high energy densities and high pulse repetition rates, 
energy absorption results in excessive heating of the mask materials, 
resulting in distortions in the mask object plane and rapid deterioration 
of the mask. Additionally, since the metal stencil masks are relatively 
thick, on the order of 50 um or more, scattering of the laser beam from 
the vertical sidewalls of the through holes degrades beam focus and image 
quality. 
U.S. Pat. No. 4,417,946, issued to Bohlen et al, assigned to the instant 
assignee, discloses a mask suitable for ion etching, ion implantation and 
x-ray, ion and electron beam lithography. Such a mask comprises one or 
more metal layers deposited on a highly doped semi-conductor substrate 
with through going apertures defining the mask pattern. In the area of the 
mask pattern apertures, the substrate material is relatively thin, thus 
minimizing scattering effects of the incident beam. The highly doped 
substrate material provides mechanical stability. However, as discussed 
herein above, the metallic layers cannot withstand the high energy 
densities commonly encountered in excimer laser ablation processes. 
In "Mask for Excimer Laser Ablation and Method of Producing Same," IBM 
Technical Disclosure Bulletin, Vol. 33 No. 1A June 1990, pp. 388-390, A.C. 
Tam et al considers a silicon thin membrane stencil mask having 
transparent areas realized by suitably dimensioned apertures through the 
membrane. The thermal stability of the mask is increased by coating the 
surface of the mask facing the laser source with a multilayer reflection 
system and/or coating the surface of the mask facing the material to be 
ablated with a metallic gold layer. However, in the high power, high 
repetition rate environment experienced during excimer laser ablation 
processes, thin membranes exhibit undesirable and intolerable bending and 
warping resulting from heating induced by the small amount of laser 
radiation transmitted through the multilayer reflection system and from 
mechanical stress induced in the membrane material by residual stress in 
the multilayer reflection system. Additionally, the silicon thin membrane 
mask exhibits a tendency to vibrate when exposed to high pulse repetition 
rates. The non-stable object plane resulting from the combined effects of 
mask bending and vibration significantly reduces the depth of focus and 
may cause scattering effects, distorting the transmitted pattern and 
resulting in poor image quality. 
SUMMARY OF THE INVENTION 
A primary objective of the present invention is to provide a transmission 
mask for use in laser processing of material surfaces, excimer laser 
ablation processes, for example, having sufficient mechanical stability to 
provide high precision, high quality imaging. The thin membrane stencil 
mask, described above, effectively satisfies all requirements for an ideal 
mask with the exception of mechanical and thermal stability of the object 
plane. Thermal stability of the thin membrane mask may be greatly improved 
by coating the mask surface exposed to high energy laser radiation with a 
multilayer dielectric coating. A suitable multilayer dielectric coating 
can achieve greater than 99% reflectivity of incident radiation thus 
providing a stencil mask suitable for many laser ablation processes. 
Warpage and vibration of the mask may be substantially eliminated and thus 
the mechanical stability of the object plane greatly improved by providing 
a thick membrane substrate to strengthen the mask and improve heat 
removal. However, transparent mask areas defined by apertures having 
vertical sidewalls through the thick membrane do not provide the thin 
object plane required, thus causing scattering effects of the incident 
radiation and distorting the transmitted pattern. A solution to the 
problem of providing a thin object plane while utilizing a thick membrane 
to provide a stable object plane, is achieved by conically undercutting 
the edges of the vertical through holes. In this manner, a thick membrane 
mask is achieved having sufficient mechanical rigidity to minimize warpage 
while providing a thin mask object plane in the vicinity of the through 
holes to achieve high quality imaging. 
Accordingly, a transmission mask constructed in accordance with the 
principles of the present invention comprises a thick membrane having 
transmissive or transparent areas defined by conically undercut through 
holes and non-transmissive or opaque areas defined by a multilayer high 
reflective dielectric stack. The mask is fabricated from a suitable 
substrate material, such as crystalline silicon, heavily doped with a 
suitable dopant, boron, for example, to a depth of a few micrometers on 
selected areas of the mask substrate surfaces. Utilizing well-known 
techniques, a thick membrane, approximately 40 um thick, for example, 
having through holes with vertical sidewalls representing the mask 
transparent areas is formed. Utilizing the characteristics of anisotropic 
wet etching, the thick membrane is then etched from its back side to 
provide conically undercut profiles for the through hole walls 
representing the transparent areas of the mask, thus providing a 
relatively thick membrane of increased strength and mechanical rigidity, 
yet having a thin optical plane in the immediate areas surrounding the 
through holes. In a final step, the front or top side of the mask is 
coated with alternating layers of dielectric materials having different 
indices of refraction. Each pair of dielectric layers will reflect a given 
amount of incident radiation; by depositing many pairs of layers, greater 
than 99% reflectivity of incident radiation can be achieved. 
Mechanical stress to the substrate membrane induced by the dielectric stack 
is effectively compensated for by high tensile stress induced in the 
membrane by heavy doping. Additionally, the high tensile stress induced by 
the heavy doping provides "pseudocooling" of the mask membrane, minimizing 
warpage and increasing the rigidity of the mask membrane even for 
significant temperature differentials, 70.degree. C., for example, between 
the mask membrane and the mask frame. Excessive differential mask 
temperatures are minimized by the high reflectivity of the dielectric 
coating stack and the effective heat removal by the thick membrane 
surrounding the thin object plane areas. Thus, the disclosed conical 
undercut dielectric stencil mask of the present invention provides a thick 
membrane of sufficient strength and mechanical rigidity to minimize 
warpage and high vibrational amplitudes while at the same time providing 
the desired thin object plane for high quality imaging.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 2, a portion of a thin membrane stencil mask 1 
constructed in accordance with the principles of the present invention is 
shown. The mask 1, fabricated from a substrate of suitable material, 
preferably a crystalline wafer such as crystalline silicon, comprises a 
relatively thick substrate 2 having one or more tub-shaped recesses 6 
formed in the bottom or back side thereof to provide relatively thin areas 
of the wafer forming thin membranes 3 having lateral dimensions 
corresponding to the relative size of the structure or circuit to be 
fabricated utilizing such a mask or series of masks 1. The remaining 
thicker portions 9 of substrate 2 defining the edges of the substrate or 
ribs separating the mask areas on the substrate form mask frame members to 
support the thin membrane 3. The desired mask pattern is defined by a 
series of apertures or holes 5 having vertical sidewalls and extending 
through the thin membrane 3. The top or front side of the mask is coated 
with multiple layers 7 of pairs of dielectric materials to provide a 
highly reflective surface defining the opaque areas 4 of the mask pattern. 
As described in greater detail below, in one preferred embodiment, the mask 
10 is fabricated from a crystalline silicon wafer utilizing well-known 
photolithographic etching techniques. In a first step, the top side of the 
silicon wafer 2 is doped with a suitable 5 dopant to a depth of 2-3 um 
forming a heavily-doped layer 8 which acts as an etch stop defining the 
thickness of the thin membrane 3 and inducing a high tensile stress in the 
thin membrane 3 to compensate for any mechanical stress induced by the 
dielectric coating 7. Recesses 6 are formed utilizing an anisotropic 
etching technique to wet etch the back side of silicon wafer 2. Wet 
etching of the silicon wafer is terminated at the boundary of the doped 
layer 8 to form thin membrane 3 having a thickness of approximately 2-3 
um. Utilizing a silicon dioxide mask layer (not shown) with an anisotropic 
dry etch process, the silicon thin membrane 3 is etched from the front 
side to form apertures 5 extending through the thin membrane into recess 
6. In a final sequence, the front side of thin membrane 3 is coated with 
multiple layers of pairs of dielectric materials to form a reflective 
coating having a thickness of 1-2 um covering the remaining surface area 
of the thin membrane 3 thus defining the opaque areas 4 of the desired 
mask pattern. 
The described membrane stencil mask provides a high resolution laser etch 
mask having a rigid object plane approximately 3-5 um thick. The highly 
reflective dielectric coating minimizes absorption of incident laser 
radiation by the thin membrane thus minimizing heating of the thin 
membrane. However, in high power, high repetition rate environments, even 
the small proportion of radiation incident on the mask surface transmitted 
by the dielectric coating is sufficient to produce undesirable heating of 
the thin membrane. In addition, the silicon thin membrane exhibits a 
tendency to vibrate when exposed to high pulse repetition rates. 
As noted above, while a thin membrane stencil mask having a patterned 
dielectric mask provides excellent thermal and mechanical characteristics 
suitable for many applications, in high power, high repetition rate 
environments both thermal and mechanical properties deteriorate. In the 
alternative, a preferred mask suitable for use in high power, high 
repetition rate environments comprises a thick membrane wherein the 
desired mask pattern is defined by a plurality of apertures extending 
through the thick membrane and having conical undercut sidewalls providing 
the optical characteristics of a thin membrane in the mask areas 
immediately adjacent the apertures together with the thermal and 
mechanical of a thick membrane. 
Referring now to FIG. 3, a portion of a thick membrane conical undercut 
stencil mask 10 constructed in accordance with the principles of the 
present invention is shown. The mask 10, fabricated from a substrate of 
suitable material, preferably a crystalline wafer such as crystalline 
silicon, comprises a relatively thick substrate 11 having one or more 
tub-shaped recesses 16 formed in the bottom or back side thereof to 
provide relatively thin portions of silicon forming thick membranes 13 
having lateral dimensions corresponding to the relative size of the 
structure or chip to be fabricated utilizing such a mask or series of 
masks 10. The thicker portions 15 of substrate 11 form mask frame members 
to support the thick membrane portions 13. The thick membrane 13 has a 
thickness preferably of the order from 35-40 um, while the thickness of 
the thicker supporting mask frame 15 can be of the order from 200-1,000 
um. A plurality of apertures 21 extending transversely through thick 
membrane 13 define the lateral geometry of the desired etch pattern to be 
produced utilizing the mask 10. The apertures 21 are formed in thick 
membrane 13 having vertical sidewalls 22 extending a short distance 
through the thick membrane to intersect with conical sidewalls 23 
extending the remaining distance through the thick membrane 13. Conical 
sidewalls 23 allow a thick membrane mask to be utilized while providing a 
thin optical plane defined by vertical sidewalls 22. The top or front side 
of the mask 10 is coated with multiple layers of dielectric materials to 
provide a highly reflective surface defining the opaque areas 19 of the 
etch pattern. The dielectric coating 19 can provide greater than 99 % 
reflection of incident radiation. The vertical sidewall 22, including the 
thickness of the dielectric coating 19, extends into thick membrane 13 a 
distance, preferably 3-4 um, within the depth of focus for the radiation 
wavelength in use. A highly doped layer 17 formed in the surface of the 
substrate 11 induces a high tensile stress in the thick membrane 13 to 
provide pseudo-cooling and to compensate for any mechanical stress induced 
by the dielectric coating 19 and to act as an etch stop defining vertical 
wall 22. While suitable for many types of operations, the thick membrane 
stencil mask of the present invention is preferably used in projection 
systems for excimer laser ablation processes. 
Referring now to FIGS. 4a-4g, a preferred method for fabricating the 
stencil mask according to the present invention will be described. Various 
steps utilized in the fabrication method of stencil mask 10 will not be 
described in extensive detail as they are well known in the 
photolithographic masking art. Alternative methods readily adaptable for 
fabrication of the instant stencil mask are provided in detail from U.S. 
Pat. No. 4,256,532, assigned to the instant assignee and the 
aforereferenced U.S. Pat. No. 4,417,946. 
As shown in FIG. 4a, an initial silicon wafer 31 cut in the (100) 
crystallographic plane having a thickness of 200-1,000 um is heavily boron 
doped to a depth of approximately 1-3 um at both the front and back sides 
37, 39, respectively. The boron doping is by well- known methods, such as 
blanket boron capsule diffusion to provide a doping level of for example 
1.times.10.sup.20 atoms per cm.sup.3 at the interface 35. The wafer front 
and back sides 37 and 39, respectively, typically are highly polished. 
Next, shown in FIG. 4b, a layer of silicon dioxide 41, approximately 2 um 
in thickness is formed over both the front and back sides, 37, 39, 
respectively, of the silicon wafer 31. The silicon dioxide layer 41 is 
preferably deposited by a non-thermal growth technique such as 
conventional sputter deposition or chemical vapor deposition to minimize 
distortion and stress induced in the silicon wafer 31. Next, the layer of 
silicon dioxide coating the back side 39 of the wafer is coated with 
photoresist. Utilizing conventional photolithographic etching techniques, 
the photoresist layer 43 is developed and wet etch of the silicon dioxide 
layer 41 utilizing a single-side etch tool is accomplished to form a mask 
on the back side 39 of the substrate 31. Apertures in the silicon dioxide 
mask thus formed serve to define the pattern of tub-shaped recesses 16, 
which are to be formed in the back side of the substrate 31. 
Recesses 16 are formed in substrate 31 utilizing an anisotropic etching 
technique to wet etch the back side of the silicon wafer. Wet etching of 
the silicon wafer is terminated at, for example, approximately 40 um from 
the front surface 39 of the silicon wafer to form the thick membrane 13 
and mask frame sections 15. The anisotropic etching minimizes undesirable 
lateral etching of the silicon while the recesses 16 are formed. 
As shown in FIGS. 4c and 4d, the silicon dioxide layer 41 over the front 
side 37 of the silicon wafer is coated with a photoresist layer 45. In a 
similar manner, as described above, the photoresist layer 45 is developed 
and the silicon dioxide layer 41 is wet etched in a single-side etch tool 
to form a mask in silicon dioxide layer 41. The apertures 47 in the 
silicon dioxide mask serve to define the desired mask pattern to be formed 
in thick membrane 13. Utilizing anisotropic dry etch techniques, such as 
reactive ion etching (RIE), the silicon is etched from the front side 37 
to form apertures 49 extending through thick membrane 13 into recess 16 at 
the pattern areas 47. The RIE dry etch process results in through-holes 49 
having vertical sidewall profiles. 
As shown in FIGS. 4e, 4f and 4g, anisotropic wet etch techniques are then 
utilized to form a conically undercut sidewall profile for through-holes 
49. The anisotropic characteristics of crystalline silicon provide etch 
rates (ER) normal to the various indicated crystalline planes, for 
example, in 10-molar KOH solution at 62.degree. C., of 
EQU ER(110)=2.times.ER(100)=120.times.ER(111) 
with ER(110) being approximately 0.6 um per minute. When immersed in the 
etch solution, the vertical wall 52 (as shown in FIG. 4d), the (110) 
plane, will etch fastest whereas the thick membrane back plane 51, the 
(100) plane, will etch only half as fast, thus preventing substantial 
additional thinning of the thick membrane 13. The etch rates of the (110) 
and (100) planes, in combination with the essentially zero etch rate of 
the (111) plane, provide for the formation of the desired conical sidewall 
55 parallel to the (111) plane. Wet etching from the silicon wafer front 
surface 37 is prevented by the silicon dioxide layer 41. Since the heavily 
doped silicon is not etched by the wet etch solution, the through-hole 49 
retains a vertical lip 53 at the front surface 37 of the thick membrane, 
thus preserving the desired mask pattern as defined during the 
lithographic and RIE process steps described hereinabove. The heavily 
doped front surface 37 represents the mask object plane and has a 
thickness approximately equal to the depth to the boron doping in the 
silicon substrate. 
The lateral underetch or undercut resulting from the very slow etching of 
the diagonal or (111) plane results in a relatively small undercut 
distance 57 to provide a thin object plane and the desired conical 
undercut etch profile for each through-hole 49. As shown in FIG. 4g, the 
anisotropic wet etch process produces an underetch pattern determined by 
the orientation of the (111) planes. In particular, for crystalline 
silicon a generally rectangular underetch pattern results about the mask 
apertures, including circular apertures. In this example, the undercut 57 
is approximately 0.4 um. For mask pattern structures having a horizontal 
distance L between adjacent through-holes 49, where 
L&lt;2.times.D.times.TAN(.phi.), where D is the thickness of thick membrane 
13 and .phi. is the angle of conical sidewall 55 from the vertical, the 
thickness of the membrane separating adjacent through-holes will be less 
than D. Such local variations in membrane thickness typically do not 
impose a problem since their relative area is extremely small. The silicon 
substrate 13 will etch such that the conical wall 55 will form an angle 
.phi. of 35.26.degree. with the vertical when the silicon has a (100) 
crystallographic orientation. 
In a final sequence, as shown in FIG. 4h, the back side 51 of the thick 
membrane 13 including the conical walls 55 are boron doped to the same 
concentration as the front surface 37 to provide symmetric doping on both 
the front and back sides of the thick membrane 13. The silicon dioxide 
layers 41 are then stripped from the front and back sides 37, 39 in a 
conventional manner and the front side 37 of the thick membrane 13 is 
coated with a dielectric stack 61 to form a reflective coating covering 
the surface area of the thick membrane 13 thus defining the opaque areas 
of the mask pattern. 
The dielectric stack 61 is a highly reflective, abrasion-resistant 
dielectric coating formed of adjacent layers of materials having different 
indices of refraction. If the index of refraction of a quarter wavelength 
of film is higher than that of the underlying layer, a substantial amount 
of light incident on these layers will be reflected. Thus, each pair of 
dielectric layers will reflect a given amount of incident radiation. Thus, 
by depositing many layers, a desired reflectivity may be obtained. The 
number of layers deposited is dependent on the intended usage. For 
example, a dielectric stack of alternating layers of silicon dioxide and 
hafnium oxide of approximately 20 layers having a total thickness on the 
order of 2 um provides greater than 99% reflectivity of incident radiation 
at a wavelength of 248 nm. High index materials may be hafnium oxide, 
scandium oxide, aluminum oxide or thallium fluoride. Low index materials 
may be silicon dioxide, and magnesium fluoride. These materials are cited 
as examples and the lists are not intended to be exhaustive. 
Referring now to FIG. 5, a conically undercut stencil mask 63, as described 
above, is shown utilized in a projection system for direct etching of the 
surface of a substrate 65. Incident radiation 69 is able to pass through 
aperture 71 to lens system 67 and is thereby imaged on the surface of the 
substrate 65 in the desired pattern defined by the apertures 71. Radiation 
69 incident on the mask opaque areas 73 is reflected by the dielectric 
stack coating to minimize energy absorption and consequent heating of the 
thick membrane 75. The undercut etch profile of aperture 71 effectively 
provides a thin object plane in the vicinity of aperture 71 while the 
angled, conical sidewalls 77 minimize scattering of radiation transmitted 
through aperture 71. 
Dielectric coated masks are capable of withstanding incident energy 
densities of the order of 6J/cm.sup.2 thus providing ideal masks for laser 
projection purposes. In such a laser projection system, the mask of the 
present invention may provide structures having dimensions as small as 1 
um or less. For example, in a five-to-one reduction projection system, an 
aperture 71 of 5 um at its greatest dimension produces a structure in the 
surface of substrate 65 of 1 um at its greatest dimension. Being durable 
and largely impervious to chemical damage, dielectric coated masks can be 
utilized for other applications such as deposition and etching induced by 
laser irradiation. Furthermore, the composition and arrangement of the 
dielectric layers may be chosen to provide a desired level of reflectivity 
and chemical stability for the intended purposes. 
Although the present invention has been described in its preferred form 
with a certain degree of particularity, it is understood that the present 
disclosure of the preferred embodiments has been by way of example and 
that the teachings of the invention are not limited to the particular 
materials or processing steps described, but that numerous changes in the 
details of construction and the combination and arrangements of elements 
may be resorted to without departing from the spirit and scope of the 
invention as hereinafter claimed.