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BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to coating processes and miscellaneous products,
and, more particularly, to utilizing a masking coating. Coating is
provided in a vacuum by sputtering or vacuum coating. This invention also
relates to masks for deposition of thin films both by vacuum coating and
sputtering on various substrates. The masks can be formed by
electrodeposition, electroless deposition, or otherwise.
SUMMARY OF THE INVENTION
In accordance with this invention, a method is provided for producing a
thin film structure by depositing a material in a vacuum. A matrix is
applied to a substrate. A pattern of openings is formed in the matrix.
Then a lift-off material is deposited in the openings in the matrix. The
deposits have larger overhanging width at the top than at the bottom.
Then, the remainder of the matrix is removed. Subsequently, a material is
deposited in a vacuum upon the product of the previous step to a thickness
substantially less than the first layer, so as to leave the sides of the
lift-off material exposed.
Finally, the lift-off material and the coating material deposited upon it
are removed by treating the product of the last step with a chemical
solution effective to remove the lift-off material by etching or the like.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-LG show a first set of steps of producing a film by means of a
lift-off process in accordance with this invention, with overhang provided
by plating to a greater depth than resist thickness.
FIGS. 2A-2I show a second process in accordance with this invention similar
to that of FIGS. 1A-1G.
FIGS. 3A-3C show a third process in accordance with this invention, with
tapered openings in resist forming overhang in the lift-off material.
FIGS. 4A-4C show a third process in accordance with this invention, with
tapered openings in resist forming overhang in the lift-off material.
FIGS. 5A-5E show a process for producing improved overhang by heating a
resist of the form used in FIGS. 3A, 4A and 5A to obtain curvature of the
resist at edges, yielding a greater overhang.
FIGS. 6A-6D show a two-resist technique of achieving overhang.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A substrate 10 in FIGS. 1A-G is coated with a metal layer 11 of a material
or layers of materials which can provide adhesion to substrate 10 and also
serve as a plating base for electroplating.
When a metal such as Fe, Ni, Cr, or Cd is applied at an elevated
temperature upon a substrate 10, both adhesion and a plating base are
obtained without any requirement for an intermediate adhesion layer.
However, if Au, Cu, Pd, or Pt is to be applied at any temperature or if Co
or NiFe is to be applied at room temperature, as a plating base, then an
intermediate adhesion layer of a reactive metal such as Ti, Ta, Al, or Cr
is required to be applied to the substrate prior to application of the
plating base.
Upon the layer 11 is deposited a layer of radiation sensitive organic
resist 12 which has been exposed using U.V., E-beam or X-ray radiation and
developed previously to provide opening slots 14 and 15 which comprise
lengthwise slots extending back towards the back of the substrate 10.
Slots 14 and 15 are intended to form borders for a pattern such as an
electrical lead or like thin straight edge pattern. In FIG. 1B, the slots
14 and 15 have been filled with electroplated layer 25 of metal forming
elements 16 and 17 in the shape of stripes which have overhanging edges
and caps that fill and extend beyond the slots 14 and 15 both vertically
and horizontally. Note that slots 14 and 15 and thus elements 16 and 17
can be extremely wide or narrower if desired. For more satisfactory rates
of removal of the material to be lifted from substrate 10 and layer 11
between FIGS. 1D and 1E as described below, it is desirable that the
ratios of widths between narrow and broad areas to be lifted should be
less than about 25:1. This helps to obtain uniform removal of layers 25
and 19 as shown between FIGS. 1D and 1E.
In FIG. 1C, the resist 12 has been removed to leave only the electroplated
mask elements 16 and 17 which have overhanging edges 18. The resist is
removed using conventional means such as organic solvents or exposure
followed by development, plasma ashing or an organic stripping solution,
etc. The electroplated metal must be deposited uniformly and the bath must
have a good macro and micro throwing power. Throwing power measures the
degree of uniformity of plating. Alternatively, electroless plating can be
used. The electroplated metal must withstand primary pattern deposition
temperature of from 50.degree.-700.degree. C. The electroplated pattern of
elements 16 and 17 should have a uniform (about 0.1 .mu.m to 5 .mu.m)
overhang (mushroom cap). The size of the overhang depends primarily upon
the lateral and thickness dimensions of the final pattern designed to be
obtained by this lift-off process. Note that the thickness of resist layer
12 is selected to be a minimum of about 20% thicker (or at least about
0.25 .mu.m thicker when using thick films) than the desired thickness of
the material to be deposited in the next step as shown by FIG. 1D. For
very small geometries, the overhang (O) and the projection (P) shown in
FIG. 1C of the plated material above the top plane of resist 12 will be
about 5-10% of resist thickness, and the O and P should be substantially
equal. After the resist 12 is removed, layer 11 can be removed where
elements 16 and 17, etc. are absent by sputter etching, chemical etching,
reactive plasma etching or reactive plasma sputter etching, or the like.
In FIG. 1D, the elements 16 and 17 are employed as vacuum coating masks for
evaporation or sputtering of a layer of the ultimate coating material 19
such as a metal or dielectric which is deposited upon layer 11 and
elements 16 and 17. The overhangs 18 leave the sides 20 of the elements 16
and 17 exposed without any substantial coating of the material 19. The
electroplated metal forming elements 16 and 17 is selectively etched as
shown in FIG. 1E chemically, electrochemically, or removed with reactive
ion plasma, or using other means after deposition of the material 19 to
form the primary pattern desired, with minimal attack upon the primary
pattern. Thus, the undesired metal of elements 16 and 17 is "lifted off"
the layer 11.
In cases in which narrow strips are to be formed, or where some of the
material 19 is to be removed, there is deposited a layer 20' of resist
which has been exposed as shown in FIG. 1F to leave portion 21 of material
19 exposed so that it can be removed as shown in FIG. 1G by a subtractive
technique such as chemical etching or by any dry means such as sputter
etching, reactive sputter etching, ion milling, plasma etching or any
other suitable means.
By the way of example, the electroplated metals can be (a) Au, Pt, Pd, Ag,
or (b) Cu, Zn, Ni, Co, Fe, Cr, Sn, W or any binary, ternary, or even
quaternary alloy of any of the above elements or any of the above elements
with any other element. The metals in group (a) are used primarily in case
of lift-off in connection with the metallic or non-metallic patterns where
the pattern formed is made of a more noble metal or a metal which can be
selectively etched or destroyed by other means such as heating, chemical
reaction, etc. Group (b) metals are more universal because they are less
noble. They can be employed for lift-off masks for dielectrics and a
variety of more noble metals.
Although only a few selected examples of structures are shown here, the
technique can be used in fabricating all types of magnetic structures, all
types of semiconducting structures such as integrated circuits, second
level packaging, optical and electro-optical structures, and many others.
If it is a dielectric which forms the primary pattern 19, it may be
desirable to introduce a metallization removal step (sputter etching, ion
milling, plasma or chemical etching) prior to deposition of the primary
pattern. Although the technique is very general and has a very broad range
of applications, it is particularly useful in forming Schott glass,
SiO.sub.2, Si, Ti, or Cr laminated permalloy structures with very high
frequency permeability for the use in inductive heads, MR head shields,
and shields for other purposes, etc. In this situation, as shown in FIGS.
2A-2I, an electroplated structure can be a very narrow frame which is
electroformed using a copper bath. In this case, although Cu can be
selectively etched in the presence of Fe, NiFe, NiFeCr, NiFePd, etc. using
ammonium persulfate, the final layer is Schott glass which provides
additional protection of the primary structure.
If desired, the primary structure is deposited in such a way that the edges
of the primary structure are sloping (tapered) thus being easy to insulate
for step-over connections of leads in subsequent fabrication. This is
accomplished by using sputter deposition rather than evaporating a
relatively thick layer or evaporating while rocking the sample in an
appropriate fashion.
An alternative form of the process of this invention is shown in FIGS.
2A-2I using a dry process to produce Permalloy laminations, i.e.,
Permalloy/Schott glass/Permalloy with an inorganic frame. In FIG. 2A upon
a glass substrate 10, a metallized film 11 carries a layer 12 of Shipley
resist which is 1.mu. thicker than the ultimate laminations of Permalloy
and glass. The resist is exposed to radiation and developed as shown in
FIG. 2B yielding slots 24 which are 0.2 to 0.4.mu. wide in 4.mu. to 6.mu.
thick resist 12, preferably.
In FIG. 2C, the result of electroplating metal elements 25 into slots 24 is
shown. The metal selected must be easily etched selectively in the
presence of Permalloy. Copper is satisfactory and it can be etched with
ammonium persulfate (high pH.perspectiveto.7-9) in which Ni-Fe is
insoluble. Other metals such as Cr, Zn, Cd, and Sn also are suitable for
plating. The metal is plated to a depth of about 3-7.mu. thick to form a
mushroom shaped cross-section with an overhang of 0.25 to 3.mu.
preferably. The resist is removed with a solvent (acetone for Shipley) to
yield the result shown in FIG. 2D.
Then, in FIG. 2E the elements 25 and exposed surfaces 11 are shown coated
with Permalloy layer 19, Schott glass layer 29, and Permalloy layer 39 to
a 2 to 3.mu. thickness overall. Preferably, an additional layer of Schott
glass (not shown) is deposited to protect the Permalloy layer 39 from
attack. Various W, Cr, Pd, or Mo alloys with Permalloy and SiFe can also
be used.
Sandwiches of laminations can be formed as shown in Table 1 below:
TABLE I
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W-Permalloy
Mo-Perm. Pd-Perm. 3-6% SiFe
SiO.sub.2 or W
SiO.sub.2 or W
SiO.sub.2 or W
SiO.sub.2
W-Perm. Mo-Perm. Pd-Perm. 3% SiFe
SiO.sub.2 or W
SiO.sub.2 or Mo
SiO.sub.2 or Mo
SiO.sub.2
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Alternatively, Sendust (Si 9.6%, Fe 85%, Al 5.4% by weight) can be
sputtered whether laminated or not.
In FIG. 2F the result of etching away the elements 25 is shown with the
Permalloy sandwich above them lifted off the substrate. When the elements
are composed of copper, they are etched as stated above. With Zn ammonium
sulfate is the etchant (pH.perspectiveto.7-9). When the elements 25 are
composed of Cr, the etchant is AlCl.sub.3 plus Zn or any other suitable Cr
etchant.
In FIG. 2G, Shipley resist 20' has been applied to cover slots 40 and 42 as
well as strip 41 between them, in order to protect the Permalloy sandwich
segment which will comprise the yoke of the head.
The sections 43 and 44 are removed by etching using FeCl.sub.3 and HF
separately or HF plus FeCl.sub.3 mixture to yield the result of FIG. 2H.
The resist is removed to yield the desired yoke structure in FIG. 2I,
which comprises the primary pattern. There are situations where dummy
structures need not be removed, since they do not exist or because they
can remain without harming the product.
Groove with Sloping Sides
Various techniques can be used to produce a pattern as in FIG. 3A with
groove 50 having sloping sides as a result of shining radiation 52 through
a mask 53 onto a positive resist. Resists which can be used include
Shipley photoresist, KTFR, and PMMA. FIG. 3A is a typical showing of
multiple steps from exposure to development.
In the case of PMMA, the pattern is obtained when radiation 52 is an
electron beam of low intensity and development time is long and/or
developer concentration is high. Metal 25 is deposited in FIG. 3B. Then,
resist 12 is removed and metal 19 is vacuum deposited in FIG. 3C. Elements
25 are etched away in the usual way although that is not shown.
In negative working electron beam resists, such sloped edge patterns can be
obtained by adjusting the electron beam intensity and the development
time.
In PMMA and PMMA copolymers, by suitable electron beam modulation
(intensity) and suitable development, an exactly opposite slope can be
obtained such as shown in FIG. 4A. As can be seen from the sketches, FIGS.
3B and 3C and FIGS. 4B and 4C, both sloping versions are useful in forming
a suitable pattern to be used for forming an inorganic, high temperature
mask used in the lift-off, producing slots 50 or 60.
In the case of Shipley resist or other positive working resists, sloping
sides such as shown in FIG. 3A can be obtained by:
1. Defocussing or decollimating ultraviolet light.
2. Removing the mask from intimate contact with the resist.
3. Using very thick resist, i.e., thicker than about 2.mu. even when the
light beam is not defocussed or decollimated and even when the
mask-to-resist contact is good.
4. Using a disperse (non-collimated) source of light.
5. Using any one or a combination of methods (1), (2) and/or (3) and (4)
combined.
In the case of negative working resists such as KTFR, KOR (Eastman Kodak
resists) as well as others, techniques (1) through (5) or any combination
thereof can be used to form similarly sloping sides as in FIGS. 3B and 3C
and in FIGS. 4B and 4C.
After such resist patterns with sloping edges are obtained, as in FIGS.
3A-3C and 4A-4C, the openings in the resist are filled by a suitable
deposition means such as electroplating, electroless deposition,
evaporation, sputtering, etc. The deposited metal 25 or material
(dielectric) can be brought to within a thickness equal to about 2/3 of
the desired final deposition layer to be produced by lifting or about
0.25.mu. from the top in thick deposits. This metal (or dielectric), Cu,
Zn, Sn, Al, etc., forms an inverted trapezoid as shown in FIGS. 3B and 4B.
When the sloping sides are present, it is not necessary to continue to
electrodeposit the metal or the dielectric 25 until a mushroom shape is
formed over the resist. Depending on the shape of the inverted trapezoid
50 or 60, one may or may not need a mushroom-shaped pattern to accomplish
the liftoff.
After removal of resist, the desired metal layer, dielectric laminate
magnetic layer, or a dielectric layer is deposited by evaporation, ion gun
plating, and/or sputtering, etc. The lifting-off process can now be
accomplished as per FIGS. 2E-2I or FIGS. 1D-1G. The only necessary
condition is that the choice of the inorganic lift-off material be such
that it can be selectively dissolved, attacked or etched otherwise without
attacking the final pattern desired to be formed by the high temperature
lift-off process.
In the technique of FIGS. 5A-5E, the resist 12 (Shipley, KTFR,
polymethylmethacralate (PMMA), PMMA copolymer or any other thermally
deformable polymer) is shaped into patterns (as by exposure and
development or by etching (reactive plasma, chemical, etc.). The pattern
is subsequently heated to a temperature which is sufficiently high for the
polymer 12 to flow (deform due to surface tension pull) and form shapes
shown in FIG. 5B. For Shipley resist, this can be accomplished by rapid
heating and cooling the sample to about 150.degree. C. to 160.degree. C.
or by slow heating to about 100.degree. C. to 130.degree. C. and slow
cooling (taking the time temperature product factor which makes Shipley
resist still easily removable). Shipley resist is removable by use of
conventional means such as acetone or exposure to U.V. light followed by
development using conventional Shipley resist developers. It can be seen
that the heating will produce the desired shape in the resist which is
useful in producing sloping sides or even slight overhang in the inorganic
mask, FIG. 5A. The remaining steps are discussed above for FIGS. 1B-1G or
2C-2I. The rounding of edges or sloping edges can be produced by short
exposure of organic resists to reactive plasma and or sputter etching.
In the technique of FIGS. 6A-6D, two resists of two different sensitivities
are used. The higher sensitivity resist 70, on top, is a positive working
resist and on the bottom is the negative working resist 12. The bottom
resist layer 12 is about 25% thicker than the thickness of the material to
be formed by lift-off (see FIG. 6A). After the exposure to U.V. or E-beam
or X-ray source and development, a shape shown in FIG. 6B is obtained.
Upon electrodeposition or electroless deposition, an inorganic mask is
formed with a well defined overhang as shown in FIG. 6C. In FIG. 6D
material 19 is deposited after resist 12, 70 has been removed.
The remainder of the process is described above. Alternatively, negative
working resist can be used with the more sensitive resist in layer 12 at
the bottom.
It will be obvious to one skilled in the art that the above techniques can
be used in combination with each other as well as in combination.
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