Nonplanar lithography and devices formed thereby

A new method for lithographically patterning nonplanar substrates is disclosed. In accordance with this method, a nonplanar substrate surface is patterned by initially substantially conformably coating the surface with a resist. Conformality is achieved by depositing the resist either from the vapor phase or from a mist. In addition, the motions of the constituents of the vapor or mist should be sufficiently random so that the angular flux distribution at any point on the nonplanar substrate surface to be coated is substantially identical to that at any other point to be coated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention pertains to lithographic processes for producing devices such 
as semiconductor devices. 
2. Art Background 
Lithographic processes play an important role in the manufacture of devices 
such as semiconductor devices. During the manufacture of such devices, a 
substrate is coated with an energy-sensitive material called a resist. 
Selected portions of the resist are exposed to a form of energy which 
induces a change in the solubility or reactivity of the exposed portions 
in relation to a given developing agent or etchant. By applying the 
developing agent or etchant to the resist, the more soluble or reactive 
portions of the resist are removed and portions of the substrate are 
bared. The bared portions of the substrate are then treated, e.g., etched 
or metallized. 
Both organic and inorganic materials have been used as resists. The organic 
materials are typically organic polymers such as novolak resins which 
include quinone diazide sensitizers, while the inorganic materials are 
exemplified by chalcogenide glass-based materials such as 
germanium-selenium (Ge-Se) glass films supporting relatively thin layers 
of silver selenide (Ag.sub.2 Se). (Chalcogenide glass-based materials are 
materials which exhibit a noncrystalline structure and whose major 
constituent is sulfur, selenium, tellurium, or compounds thereof.) An 
organic polymer resist is typically spin-deposited onto a substrate. A 
Ge-Se glass film, on the other hand, is typically evaporated, e.g., e-beam 
evaporated or rf-sputtered, onto a substrate, and an Ag.sub.2 Se layer is 
formed on the surface of the Ge-Se glass film by dipping the film into an 
appropriate sensitizing bath. 
The two types of resists, i.e., organic and inorganic resists, are useful 
in patterning essentially flat substrate surfaces, but difficulties arise 
when patterning nonplanar substrate surfaces, such as stepped substrate 
surfaces. For example, when an organic polymer resist (having a typical 
thickness of about 1-2 .mu.m) is spin-deposited onto a stepped surface 
having, for example, step heights greater than about 5 .mu.m, the 
resulting coating thickness is either discontinuous or substantially 
nonuniform, i.e., the coating sections covering the stepped portions of 
the surface are significantly thinner (typically more than about 50 
percent thinner) than the coating sections covering the flat portions of 
the surface. Energy sufficient to ensure exposure of the thick resist 
sections causes overexposure of the thin sections. This overexposure 
results in a loss of linewidth control during pattern transfer into the 
substrate. 
Attempts to overcome the difficulties associated with the patterning of 
stepped surfaces have involved the use of multilevel, e.g., bilevel, 
resists. For example, with bilevel resists, a relatively thin resist 
region is formed on a relatively thick, and thus planar, layer of an 
organic polymer (which need not be energy-sensitive) overlying a stepped 
surface. (In the case of, for example, the chalcogenide resists, a thick, 
planarizing layer is almost always deposited onto the substrate surface, 
regardless of whether it is stepped or flat, prior to deposition of the 
resist.) After exposure and development, the pattern defined in the resist 
is transferred into the planarizing layer by using the former as an etch 
mask during etching of the latter. Finally, the substrate is patterned by, 
for example, etching or metallizing the substrate surface through the 
patterned planarizing layer. 
Bilevel resists (as well as other multilevel resists) are useful in 
patterning stepped surfaces where the step heights are less than about 5 
.mu.m. However, while bilevel (and other multilevel) resists are still 
useful when patterning stepped surfaces having step heights greater than 
about 5 .mu.m, difficulties do arise. For example, the thickness of the 
planarizing layer covering a 5 .mu.m-step (in a surface having steps 
interspersed between flat portions) is often significantly less (typically 
more than about 50 percent less) than that covering the flat portions. 
Thus, when transferring a pattern to the planarizing layer using, for 
example, an isotropic etchant, the thin sections of the planarizing layer 
suffer excessive lateral etching during the additional time required to 
etch through the relatively thick sections. This excessive lateral etching 
results in a loss of linewidth control during pattern transfer into the 
substrate. Alternatively, if an anisotropic etching, e.g., reactive ion 
etching, technique is used, the additional time required to etch through 
the thickest portion of the planarizing layer causes pattern erosion of 
the overlying resist with a concomitant loss of linewidth control. 
The difficulties associated with patterning nonplanar substrates have 
produced constraints on the fabrication of devices, e.g., semiconductor 
integrated circuit devices, having three-dimensional geometries, such as 
devices having components on the top, the bottom, or even on the sidewall, 
of a deep (deeper than about 5 .mu.m) V-groove or high (higher than about 
5 .mu.m) step. Three-dimensional devices offer the possibility of 
increased packing densities using nominal design rules but, as discussed, 
techniques for patterning nonplanar substrates to achieve such devices 
have proven to be an elusive goal. 
SUMMARY OF THE INVENTION 
The invention involves a lithographic technique for fabricating devices on 
nonplanar substrates, as well as the resulting devices. In accordance with 
this technique, a nonplanar substrate is patterned by initially forming a 
substantially conformal resist region, i.e., a region having a 
substantially uniform thickness, over the portion (or portions) of the 
nonplanar substrate surface to be patterned (which encompasses, for 
example, coplanar, noncoplanar, and inclined portions of the surface). It 
has been found that such a region is achieved by depositing the resist 
onto the substrate surface from the vapor (gaseous) phase, or from a mist 
which includes agglomerates or droplets containing resist material. The 
dimensions of the agglomerates or droplets should be smaller than about 
three-fourths the smallest height or depth of any substantial step or 
depression in the surface. In addition, the motions of the atoms or 
molecules of the vapor, or of the agglomerates or droplets of the mist, 
should be sufficiently random so that the angular flux distribution at any 
point on the nonplanar surface to be patterned is substantially identical 
to that at any other point on the surface to be patterned. With the 
conformal mask thus produced, pattern transfer into the underlying 
substrate, with significantly improved (as compared to previous 
techniques) linewidth control, is readily achieved.

DETAILED DESCRIPTION 
The invention involves a method for fabricating devices, as well as the 
devices formed by this method. 
The inventive fabrication method includes a technique for lithographically 
patterning nonplanar substrate surfaces. For purposes of the invention, 
and as illustrated in FIG. 1, a nonplanar surface 10 includes at least one 
depression 20 and/or at least one step 30 (exemplary of possible 
depressions or steps) whose depth, d, or height, h, is greater than or 
equal to about 5 .mu.m. (The depth of a depression or the height of a step 
is defined with reference to the plane which is the least-square-fit 
planar approximation to the original substrate surface, e.g., the original 
wafer surface, which existed prior to the formation of any depression or 
step in the surface. For purposes of the invention, the depth of a 
depression is greater than or equal to about 5 .mu.m provided two criteria 
are met. First, the length of a perpendicular to the reference plane, 
extending from the lowest (relative to the reference plane) point of the 
depression (closest to the depression sidewall if there is more than one 
such lowest point) to the height of one of the nearby relative maxima in 
the surface 10 is greater than or equal to about 5 .mu.m. Second, the 
length of the shortest line from the relative maximum to the perpendicular 
should be less than or equal to the length of the perpendicular. 
Similarly, the height of a step is greater than or equal to about 5 .mu.m 
provided the length of a perpendicular to the reference plane, extending 
from the highest (relative to the reference plane) point of the step 
(closest to the step sidewall if there is more than one such highest 
point) to the height of one of the nearby relative minima in the surface 
10 is greater than or equal to about 5 .mu.m. Again, the length of the 
shortest line from the relative minimum to the perpendicular should be 
less than or equal to the length of the perpendicular.) In addition, when 
viewed in cross-section, i.e., when intersected by any plane perpendicular 
to the above least-squares-fit plane, the portion of the sidewall of the 
depression or step to be patterned should have a specific angular 
configuration to substantially conformably coat the portion with a resist. 
That is, the acute angle .theta. (see FIG. 1) between the line which is 
the least-squares-fit approximation to the left side or right side of the 
sidewall portion to be patterned (as viewed in cross-section) and a 
perpendicular projecting from the top of the least-squares-fit plane, at 
the intersection of the plane with the least-squares-fit line 
approximating the left or right side of the sidewall portion, should be 
greater than or equal to 0 degrees but less than or equal to about +45 
degrees. The acute angle .theta. is positive for the left side (as viewed 
in FIG. 1) of a depression sidewall or the right side of a step sidewall 
provided the angle extends in the counterclockwise direction from the 
perpendicular to the least-squares-fit line approximating the left or 
right side of the sidewall. On the other hand, the acute angle .theta. is 
positive for the right side of a depression sidewall or the left side of a 
step sidewall provided the angle extends in the clockwise direction from 
the perpendicular to the least-squares-fit line approximating the right or 
left side of the sidewall. 
In accordance with the invention, a nonplanar substrate surface is 
patterned by initially forming a resist on the substrate surface which 
substantially conformably coats the portions of the substrate surface 
which it is desired to coat. As used here, a substantially conformal 
resist coating is one which satisfies two criteria: (1) the coating 
thickness, at any point on the nonplanar substrate surface to be coated, 
differs from the average coating thickness (averaged over the portions of 
the surface which it is desired to coat) by less than or equal to about 50 
percent; and (2) the average thickness of the coating is less than about 
the larger of the depth of the deepest depression or the height of the 
highest step on the portion or portions of the nonplanar substrate surface 
to be coated. (The thickness of the resist, as measured from any point on 
the substrate surface to be coated, is defined as the length of the 
shortest line extending from the point to the upper surface of the 
resist.) In general, the thickness limit imposed by criterion (2) leads to 
highly desirable results. However, in some specific situations, the 
average resist thickness is advantageously less than this limit. For 
example, to achieve a particular feature size, it is known that the 
average resist thickness should be less than about three times the desired 
feature size. Moreover, it is desirable that the resist be sufficiently 
thin so that during the exposure procedure, the resist is exposed through 
its entire thickness, i.e., sufficient exposure energy penetrates to the 
bottom of the resist to produce a detectable change in solubility or 
reactivity. For certain exposure tools with limited intensity, the resist 
thickness is advantageously less than the thickness limit imposed by 
criterion (2). 
It has been found that a substantially conformal resist coating is formed 
on a nonplanar substrate surface by depositing the resist onto the 
substrate from the vapor (gaseous) phase. Alternatively, the resist is 
deposited from a mist which includes agglomerates or droplets containing 
resist material. The largest dimensions, e.g., diameters or major axes, of 
these agglomerates or droplets should be equal to or less than about 
three-fourths the smallest depth or height of any nonplanar feature in the 
surface to be coated, i.e., any depression or step having a depth or 
height greater than or equal to about 5 .mu.m and a sidewall angle, 
.theta., equal to or greater than 0 degrees but less than or equal to 
about +45 degrees. In addition, the motions of (1) the atoms or molecules 
of the vapor, or of (2) the agglomerates or droplets of the mist, should 
be sufficiently random so that the angular flux distribution (the number 
of impinging particles, e.g., atoms, molecules, agglomerates, and/or 
droplets, per unit solid angle and per unit time) at any point on the 
surface to be coated is substantially identical to that at any other point 
of the surface to be coated. For purposes of the invention, substantial 
identity in the angular flux distribution is achieved provided that, at 
any point on the nonplanar surface to be coated, the integrated (over the 
solid angle enclosing the surface at that point) angular flux distribution 
(IAFD) differs from the average (averaged over the portions of the 
nonplanar surface to be coated) IAFD by less than or equal to about 50 
percent. Because the IAFD at a point corresponds to the rate of increase 
of thickness of deposited material at that point, the above condition 
corresponds to the condition that the rate of increase of thickness of 
deposited material (readily measured by measuring the rate of increase of 
thickness of deposited material on substrate control samples) at any point 
on the nonplanar surface to be coated differs from the average rate of 
increase of thickness across the portions of the nonplanar surface to be 
coated by less than or equal to about 50 percent. 
A wide variety of resist materials are readily vapor phase deposited using 
a variety of deposition techniques. For example, the chalcogenide 
glass-based materials, such as Ge-Se, are readily vapor-phase deposited 
using conventional e-beam evaporation or rf-sputtering techniques. In 
addition, many resist materials, particularly organic polymer resists such 
as the resist sold under the trade name HPR-204 by the Hunt Chemical 
Company of Palisades Park. N.J., are readily deposited from a mist. This 
is achieved, for example, by flowing the resist solution (organic polymer 
resists are typically purchased from commercial suppliers in solution 
form) through a vibrating nozzle, e.g., an ultrasonic nozzle (such as the 
one described below), which converts the flowing liquid into droplets. 
A number of techniques are also available for producing the degree of 
randomization (described above) needed to achieve conformality. For 
example, in the case of a vapor, it has been found that the necessary 
degree of randomization is automatically achieved provided the mean free 
path (the average distance traveled between collisions) of the atoms or 
molecules of the vapor is less than or equal to a specific value related 
to the topography of the portion or portions of the nonplanar surface to 
be coated. The specific value is readily determined by first 
(theoretically or empirically) constructing the thickest imaginary layer 
which substantially conforms to the portion or portions of the nonplanar 
surface to be patterned. Such an imaginary layer is constructed, for 
example, by drawing spheres of equal radius about, and centered upon, each 
point of the nonplanar surface to be patterned. The surface which is 
tangent to the portions of the spheres extending above the nonplanar 
surface constitutes the upper surface of the imaginary layer. (This 
procedure is similar to the procedure used in defining wavefronts with 
Huygens' wavelets. In this regard, see, e.g., Borne and Wolfe, Principles 
of Optics, 3rd edition (Pergamon Press, Oxford, 1965).) The specific value 
is just the thickness of the thinnest portion of the imaginary layer. 
As is known, the mean free path, .lambda., of an atom or molecule in a 
particular ambient is related to the collision cross-section, .sigma., of 
that atom or molecule, the ambient pressure, p, and ambient temperature, 
T, through the relation 
##EQU1## 
where k denotes Boltzmann's constant. (Regarding the above relation see, 
e.g., Handbook of Thin Film Technology, L. I. Maissel and R. Glang, 
editors (McGraw-Hill, New York, 1970), pages 1-21.) The collision 
cross-section, .sigma., of an atom or molecule (if not tabulated) is 
readily measured using conventional techniques (in this regard see, e.g., 
S. Dushman, Scientific Foundations of Vacuum Technique (Wiley & Sons, New 
York, 1962), page 39). Thus, knowing .sigma., a desired mean free path 
(needed to achieve conformality) is readily produced by using appropriate 
values of p and T (in accordance with the above relation). In addition, 
the mean free path, .lambda., as a function of pressure p, and 
temperature, T, has been tabulated for a wide variety of atoms and 
molecules (see, e.g., Handbook of Thin Film Technology, supra), or is 
readily measured (as described, for example, by Berne and Pecora in 
Dynamic Light Scattering (Wiley Interscience, New York, 1976)). 
Another technique for producing, or increasing the degree of, randomization 
in a resist vapor formed, for example, by the rf-sputtering method, 
involves applying a DC bias to the electrode of the sputtering machine 
supporting the substrate to be conformally coated. That is, the resist 
material (in solid form) is mounted on a first electrode (what is usually 
the power electrode) of the sputtering machine, the substrate to be 
conformally coated is mounted on a second electrode (what is typically the 
grounded electrode), and an inert gas (or gases), e.g., argon or krypton 
or xenon, is introduced into the chamber housing the electrodes. If only 
an rf-signal, e.g., a 13.56 MHz signal, is applied to the first electrode 
while the second electrode is grounded (as is the usual procedure during 
rf-sputtering), then a plasma discharge is initiated in the gas and a DC 
bias produced on the first electrode. Under the influence of this DC bias, 
ions in the plasma are accelerated toward the solid resist target, 
removing resist material by collisional impact, thus producing the desired 
resist vapor. However (unless the mean free path in the resist vapor is 
less than or equal to the specific value described above), there is often 
insufficient randomization in the motions of the atoms or molecules of the 
resist vapor to produce a conformal resist coating on the nonplanar 
surface, e.g., insufficient randomization to produce a substantially 
conformal coating on the sidewalls of nonplanar features. But 
randomization is readily increased, in accordance with the inventive 
randomization technique, by applying a DC bias (relative to ground, and 
different from that on the first electrode) to the second 
(substrate-supporting) electrode. Such a DC bias is produced, for example, 
by applying an rf-signal (whose frequency and/or peak amplitude is 
different from that of the rf-signal applied to the first electrode) to 
the second electrode. Under the influence of the DC bias on the second 
(substrate-supporting) electrode, ions within the plasma are also 
accelerated toward the second electrode, impacting, and removing, the 
deposited resist material from the substrate surface and redepositing the 
resist material onto, for example, the sidewalls of nonplanar features. 
The DC bias on the second electrode needed to produce the desired 
randomization is generally determined empirically by, for example, 
measuring the thickness increase rate at points on control samples of the 
nonplanar substrate surface. 
Randomization is also achieved, or increased, in a vapor or a mist by 
inducing turbulence or swirling motions in the vapor or mist (using 
conventional techniques), or by heating the nonplanar substrate surface 
onto which the resist is being deposited. The degree of turbulence, swirl, 
or substrate heating needed to achieve a desired degree of randomization 
is easily determined empirically. 
Once a substantially conformal resist coating has been formed on a 
nonplanar substrate surface, as described above, the surface is patterned 
using conventional techniques. That is, the resist is exposed and 
developed (using conventional techniques), and the pattern defined in the 
resist is transferred, with little loss of linewidth control, into the 
underlying substrate by, for example, etching or metallizing the substrate 
through the patterned resist. Conventional processing steps are then 
employed to complete the fabrication of the desired device. 
As a pedagogic aid to an even more complete understanding of the invention, 
the steps involved in lithographically patterning a nonplanar substrate 
surface using a Ge-Se glass film supporting a silver compound-containing 
layer, e.g., an Ag.sub.2 Se layer (an inorganic resist), and HPR-204 (an 
organic polymer resist), are described below. 
A nonplanar substrate surface is lithographically patterned with a 
Ge-Se/silver compound resist by initially depositing a Ge-Se glass layer 
onto the substrate surface from the vapor phase using, for example, e-beam 
evaporation or rf-sputtering. If the former technique is used, then a 
Ge-and-Se-containing target, as well as the nonplanar substrate to be 
coated, is placed within an evacuated chamber, and the 
Ge-and-Se-containing target is subjected to an electron beam, resulting in 
the evaporation of Se.sub.n (n=1, 2, . . . , 8) atoms and molecules and 
Ge-Se molecules. The substrate is positioned relative to the target so 
that material removed from the target impinges the substrate. The 
background pressure within the evacuated chamber is preferably less than 
about 10.sup.-5 torr, while the beam power is preferably at least 100 
watts. Under these conditions, the mean free path of the Se.sub.n (n=1, 2, 
. . . , 8) atoms and molecules and Ge-Se molecules ranges from about 1 
.mu.m to about 20 .mu.m. Pressures greater than about 10.sup.-5 torr are 
undesirable because the resulting deposited films are often contaminated 
by foreign matter, while beam powers less than about 100 watts are 
undesirable (although not precluded) because the resulting deposition 
rates are undesirably low. 
If an rf-sputtering technique is used to vapor-phase deposit the Ge-Se 
glass layer, then the nonplanar substrate to be coated is mounted on, for 
example, the grounded electrode of a parallel-plate plasma sputtering 
machine, a Ge-and-Se-containing target is mounted on the power electrode 
of the machine, an inert gas is introduced into the machine, and a plasma 
glow discharge is struck in the gas by applying an rf-signal, e.g., a 
13.56 MHz signal, to the power electrode. Useful inert gases include, for 
example, argon, krypton, and xenon. The pressure of the atmosphere within 
the plasma sputtering machine ranges from about 3.times.10.sup.-3 torr to 
about 2.times.10.sup.-1 torr and preferably ranges from about 
5.times.10.sup.-3 to about 5.times.10.sup.-2, while the power density 
ranges from about 0.3 watts/cm.sup.2 to about 0.75 watts/cm.sup.2. Over 
these pressure and power density ranges, the mean free path, .lambda., of 
the Se.sub.n (n=1, 2, . . . , 8) atoms and molecules and Ge-Se molecules 
ranges from about 1 .mu.m to about 20 .mu.m. Moreover, as the pressure 
and/or power density is increased, the mean free path decreases. Pressures 
less than about 3.times.10.sup.-3 torr are undesirable because it is 
difficult, if not impossible, to strike a plasma, while pressures greater 
than about 2.times.10.sup.-1 torr are undesirable (although not precluded) 
because they produce undesirably low deposition rates. Further, power 
densities less than about 0.3 watts/cm.sup.2 are undesirable (although not 
precluded) because they, too, yield undesirably low deposition rates, 
while power densities greater than about 0.75 watts/cm.sup.2 are 
undesirable because the substrate is subjected to excessive, damaging 
heating. 
The (average) thickness of the Ge-Se glass layer (which is less than about 
the depth of the deepest depression or the height of the highest step to 
be coated) deposited onto the nonplanar substrate preferably ranges from 
about 0.1 .mu.m to about 1 .mu.m. A thickness less than about 0.1 .mu.m is 
undesirable because the resulting film has an undesirably large number of 
defects, while a thickness greater than about 1 .mu.m is undesirable 
because so thick a film requires an undesirably long developing time. 
After the deposition of the Ge-Se glass film, a substantially conformal, 
silver-compound containing layer is formed on the upper surface of the 
glass film. One procedure for forming such a layer is to immerse the 
glass-covered substrate in an appropriate sensitizing bath. For example, 
an Ag.sub.2 Se layer is formed on the glass film by immersing the 
glass-covered substrate in an [Ag(CN).sub.2 ].sup.- -containing aqueous 
solution, e.g., aqueous KAg(CN).sub.2. Alternatively, the sensitizing bath 
contains a silver complex whose ligand includes both hydrophobic and 
hydrophilic moieties. Such a ligand is, for example, ethylenediamine. The 
above baths, and their proper operation, are fully described in U.S. Pat. 
No. 4,343,887 issued to A. Heller and R. G. Vadimsky on Aug. 10, 1982 and 
copending, coassigned U.S. patent application Ser. No. 492,434, filed by 
C. H. Tzinis and R. G. Vadimsky on May 6, 1983, which are hereby 
incorporated by reference. The thickness of the Ag.sub.2 Se layer ranges 
from about 50 Angstroms to about 150 Angstroms, and is preferably about 
100 Angstroms. Thicknesses less than about 50 Angstroms or greater than 
about 150 Angstroms are undesirable because they result in an undesirably 
small change in the solubility of the exposed portions of the underlying 
Ge-Se glass (during the exposure procedure). 
The Ge-Se glass film is patterned by first exposing selected portions of 
the glass to a form of energy which causes silver ions from the Ag.sub.2 
Se layer to migrate into the exposed regions of the Ge-Se glass film, 
decreasing the solubility of these regions to specific developers. Useful 
exposing energies are described in, for example, the above-referenced 
Tzinis-Vadimsky patent application. Then, the Se and Ag.sub.2 Se remaining 
on the surface of the Ge-Se glass film is removed by, for example, 
immersion in a KI-I.sub.2 solution which dissolves, i.e., oxidizes, the 
Ag.sub.2 Se and Se to form SeC.sub.3.sup.2- and AgI.sub.4.sup.3-. 
Patterning is achieved by either dry-developing or wet-developing the 
Ge-Se film. Dry development is achieved through contact with a plasma 
struck in an atmosphere of, for example, CF.sub.4. Useful wet developers 
include hydroxide bases such as tetramethyl ammonium hydroxide and sodium 
sulfide. Finally, the pattern defined in the Ge-Se resist is transferred 
into the underlying nonplanar substrate by a series of conventional steps. 
Rather than directly depositing the Ge-Se glass film onto the substrate, 
the imaging and masking functions of the Ge-Se film are separated, and the 
latter function dispensed with, by initially forming a masking layer on 
the nonplanar surface to be patterned, prior to the deposition of the 
Ge-Se film. This masking layer is, for example, an etch mask, a 
metallization mask, or ion implantation mask. Preferably, the thickness of 
the masking layer is substantially uniform, i.e., the thickness at any 
point differs from the average thickness by less than or equal to about 50 
percent. Such a masking layer includes, for example, a layer of silicon 
dioxide or silicon nitride which is, for example, deposited onto the 
substrate using conventional chemical vapor deposition techniques. The 
(average) thickness of the masking layer, which depends upon its 
subsequent use, is conventional. 
After the formation of the masking layer, the Ge-Se film is deposited, 
exposed and developed, as described above. The pattern defined in the 
Ge-Se film is then transferred, i.e., etched, into the underlying masking 
layer using, for example, a wet etchant such as buffered HF (for silicon 
dioxide) or phosphoric acid (for silicon nitride). The patterned Ge-Se 
film is removed using, for example, sodium hypochlorite and sodium 
thiosulfite, and the pattern defined in the masking layer is then, for 
example, etched into the underlying substrate, e.g., a silicon substrate, 
using a wet etchant such as potassium hydroxide, or by contacting a plasma 
struck in an atmosphere of, for example, CFCl.sub.3 and Cl.sub.2. 
The lithographic patterning of a nonplanar substrate using HPR-204 resist 
is achieved by depositing the resist onto the substrate from a mist. Such 
a mist is formed, for example, by flowing the resist solution through an 
ultrasonic nozzle such as the ultrasonic atomizing nozzle sold by the 
Sono-Tek Corporation of Poughkeepsie, N.Y. and depicted in FIG. 2. In 
operation, the (liquid) resist solution is introduced into the liquid 
inlet pipe 40 (see FIG. 2) and flowed through the central aperture 50 to 
the surface 60 of the ultrasonic atomizing nozzle where the resist 
solution forms a thin liquid film under the influence of surface tension. 
By applying an AC voltage to the piezoelectric crystal 70, the nozzle is 
made to oscillate vertically (as viewed in FIG. 2), resulting in droplets 
being shaken off from the surface 60. 
The size of the droplets produced by the nozzle, for a particular resist 
solution and a particular nozzle-substrate separation, depends primarily 
on the flow rate of the solution through the nozzle, as well as the 
amplitude and frequency of the AC voltage signal applied to the 
piezoelectric crystal 70. To a lesser extent, droplet size also depends on 
the flight time of the droplets to the nonplanar substrate to be coated 
(increasing the flight time results in increased resist solvent 
evaporation, and thus smaller droplets). For HPR-204 resist solution (as 
purchased from the Hunt Chemical Company), for a flow rate of 10 cc/min 
and an applied AC frequency of 78 kHz, varying the amplitude of the AC 
signal from 25 volts to 27.6 volts results in average droplet sizes, at a 
substrate positioned 6 inches from the end of the nozzle, ranging from 
about 44 .mu.m to about 60 .mu.m. 
The degree of randomization (in the motions of the droplets) produced by 
the ultrasonic nozzle is sufficient to substantially conformably coat a 
variety of nonplanar substrates. However, randomization is enhanced by 
inducing a swirling motion in the droplet flow. For example, placing a 
circular cylindrical surface, 6 inches in diameter, just beneath the 
nozzle, and tangentially (to the cylindrical surface) injecting N.sub.2 
gas into the droplet-containing atmosphere beneath the nozzle, at a flow 
rate of about 30 liters/min, yields increased randomization. Unexpectedly, 
this also produces a decrease in average droplet size. For example, for 
the flow rate and frequency conditions described above, varying the 
amplitude of the AC voltage signal from 24 volts to 26.5 volts results in 
average droplet sizes (at a substrate positioned 6 inches from the end of 
the nozzle) ranging from about 26 .mu.m to about 16 .mu.m. Further varying 
the AC voltage amplitude from 26.5 volts to 28.5 volts results in average 
droplet sizes ranging from about 16 .mu.m to about 22 .mu.m. 
The thickness of the deposited HPR-204 resist preferably ranges from about 
0.3 .mu.m to about 2 .mu.m. A thickness less than about 0.3 .mu.m is 
undesirable because such a resist has an undesirably large number of 
defects, e.g., pinholes, while a thickness greater than about 2 .mu.m is 
undesirable because so thick a resist yields undesirably poor feature 
resolution. 
Once a layer of HPR-204 has been substantially conformably deposited onto a 
nonplanar substrate, the resist is exposed and developed, and the 
substrate patterned, using conventional techniques. 
EXAMPLE 1 
The following describes the patterning of the (100) surface of a 3-inch 
silicon wafer to form a nonplanar surface, as well as the e-beam 
deposition onto this nonplanar surface of a substantially conformal layer 
of Ge.sub.0.15 Se.sub.0.85. 
A 5000 Angstrom-thick layer of SiO.sub.2 was grown on the (100) surface of 
a 3-inch silicon wafer using conventional thermal oxidation techniques. 
The SiO.sub.2 layer was selectively etched in buffered HF to form 100 
.mu.m.times.200 .mu.m rectangular SiO.sub.2 islands. Each of the four 
sides of each island was aligned with a different one of the four &lt;110&gt; 
directions. Using the patterned SiO.sub.2 layer as an etch mask, the 
underlying silicon was then anisotropically, i.e., crystallographically, 
etched in a solution of KOH and isopropyl alcohol to form crisscrossing 
trenches. The depth and width of the trenches were, respectively, about 80 
.mu.m and about 100 .mu.m. Moreover, each trench sidewall formed an acute 
angle of +35.26 degrees with a perpendicular drawn to the original (100) 
surface of the silicon wafer. After the etching of the trenches, the 
SiO.sub.2 etch mask was removed with buffered HF. 
The patterned silicon wafer was mounted on the planet of a conventional 
e-beam deposition machine. A Ge-and-Se-containing (Ge.sub.0.15 
Se.sub.0.85) target was also placed within the e-beam machine. The ambient 
pressure within the machine was about 2.times.10.sup.-6 torr. The target 
was then subjected to an electron beam having a power of about 200 watts. 
The resulting rate of deposition of Ge.sub.0.15 Se.sub.0.85 onto the 
patterned surface of the silicon wafer, as determined from a conventional 
quartz crystal thickness monitor, was about 12 Angstroms per second. 
Deposition was continued until the average thickness of the deposited 
Ge.sub.0.15 Se.sub.0.85 (as also determined from the quartz crystal 
monitor) was about 2000 Angstroms. 
The mean free path of the Se.sub.n (n=1, 2, . . . , 8) atoms and molecules 
and Ge-Se molecules within the e-beam machine was estimated to be less 
than 10 .mu.m. This estimate was arrived at by noting that the Ge.sub.0.15 
Se.sub.0.85 target was melted by the e-beam. Because the melting 
temperature of Ge.sub.0.15 Se.sub.0.85 is approximately 520 degrees C. (in 
this regard see, e.g., F. A. Shunk, Constitution of Binary Alloy (2nd 
Supplement), McGraw-Hill, 1969, p. 394), it follows that the deposition 
temperature was at least about 520 degrees C. At this temperature, the 
partial pressure of Se.sub.n (n=1, 2, . . . , 8) and Ge-Se is, 
respectively, 15 torr and 0.45 torr (in this regard see, e.g., Proceedings 
of the Electrochemical Symposium on Inorganic Resist Systems, Montreal, 
1982, p. 227 and references therein). Thus, assuming a partial pressure of 
15 torr, the plot of partial pressure versus mean free path on page 1- 22 
of the Handbook of Thin Film Technology, supra, for atoms and molecules 
having effective cross-sections in the range 2-5 Angstroms, which 
encompasses the effective cross-sections of Se.sub.n (n=1, 2, . . . , 8) 
and Ge-Se, was used to arrive at the estimated mean free path. 
Scanning electron microscopic (SEM) photos were made of cross-sectional 
slices of the Ge.sub.0.15 Se.sub.0.85 -covered silicon wafer. Using a 
ruler (1 cm=1 .mu.m), the average thickness of the deposited Ge.sub.0.15 
Se.sub.0.85 layer, across the entire nonplanar surface of the silicon 
wafer, was measured to be about 2000 Angstroms, with thickness variations 
less than or equal to .+-.15 percent. 
EXAMPLE 2 
The (100) surface of a 3-inch silicon wafer was patterned, to form a 
nonplanar surface, as described in Example 1. A 5000 Angstrom-thick layer 
of SiO.sub.2 was grown on the nonplanar surface, using conventional 
thermal oxidation techniques. (The thickness of the SiO.sub.2 layer was 
determined from SEM photos of cross-sectional slices of the SiO.sub.2 
-covered wafer.) A layer of Ge.sub.0.15 Se.sub.0.85 was deposited onto the 
SiO.sub.2 layer, again yielding a thickness of 2000 Angstroms .+-.15 
percent. 
EXAMPLE 3 
A 2 .mu.m-thick layer of SiO.sub.2 was grown on the (100) surface of a 
4-inch silicon wafer using conventional thermal oxidation techniques. 
Buffered HF was then used to etch an array of square, 1 cm.times.1 cm, 
windows through the thickness of the SiO.sub.2 layer. Each of the four 
sides of each window was aligned with a different one of the four &lt;110&gt; 
directions. Using the patterned SiO.sub.2 layer as an etch mask, the 
underlying silicon was then crystallographically etched in a solution of 
KOH and isopropyl alcohol to form an array of wells in the silicon. At the 
original wafer surface, the wells were square in outline with dimensions 
of 1 cm.times.1 cm. The depth of each well was about 200 .mu.m. Each 
sidewall of each well formed an acute angle of +35.26 degrees with a 
perpendicular drawn to the original (100) surface of the silicon wafer. 
After the etching of the wells, the SiO.sub.2 etch mask was removed with 
buffered HF. 
A 5000 Angstrom-thick layer of SiO.sub.2 was grown on the patterned surface 
of the silicon wafer using conventional thermal oxidation techniques. 
Successive layers of Ti, Pd, and Au, having thicknesses of, respectively, 
500 Angstroms, 5000 Angstroms, and 5000 Angstroms, were deposited onto the 
SiO.sub.2 layer using conventional rf-deposition techniques. 
The SiO.sub.2 -and-metal-covered silicon wafer was placed on the grounded 
electrode of a parallel plate, rf-sputter deposition machine. A 
Ge-and-Se-containing (Ge.sub.0.15 Se.sub.0.85) target was placed on the 
power electrode of the machine, which was spaced 2 inches from the 
grounded electrode. The machine was evacuated and argon was flowed into 
the machine to produce a pressure of about 5.times.10.sup.-3 torr. A 13.56 
MHz AC signal was applied to the power electrode, producing a power 
density of 0.5 watts/cm.sup.2, and resulting in the sputter deposition of 
Ge.sub.0.15 Se.sub.0.85 onto the silicon wafer. Under these conditions, 
and using the procedure described in Example 1, the mean free path of the 
atoms and molecules in the sputter deposition machine was estimated to be 
less than 10 .mu.m. From previous experience, it was known that the 
deposition rate was 7 Angstroms per second. Deposition was continued for a 
time sufficient to deposit a 2000 Angstrom-thick layer of Ge.sub.0.15 
Se.sub.0.85. 
SEM photos were made of cross-sectional slices of the silicon wafer. Using 
a ruler (1 cm=1 .mu.m), the average thickness, and the thickness 
variations, of the deposited Ge.sub.0.15 Se.sub.0.85 layer were measured 
to be the same as in Examples 1 and 2. 
EXAMPLE 4 
The (100) surface of a 3-inch silicon wafer was patterned, to form a 
nonplanar surface, as described in Example 1. The patterned wafer was 
placed 6 inches from the output end of an ultrasonic atomizing nozzle 
purchased from the Sono-Tek Corporation of Poughkeepsie, N.Y. and depicted 
in FIG. 2. HPR-204 resist solution, as purchased from the Hunt Chemical 
Company, was then flowed through the nozzle at 10 cc/min for 1 minute 
while an AC signal having a frequency of 78 kHz and an amplitude of 24 
volts was applied to the piezoelectric crystal of the nozzle. The 
resulting averge droplet size at the wafer surface was known (from 
previous experience) to be about 44 .mu.m. 
SEM photos were made of cross-sectional slices of the resist-covered 
silicon wafer. Using a ruler (1 cm=1 .mu.m), the thickness of the 
deposited HPR-204, at the corners defined by the intersections of the 
original (100) silicon wafer surface and the sidewalls of the trenches 
etched into the wafer, was measured to be 0.5 .mu.m. Everywhere else the 
thickness was measured to be 0.7 .mu.m.