Apparatus and method for stamping a surface

An apparatus (100) including a support structure (104), a flexible stamp (106) having a stamping surface (110) including a predetermined pattern disposed opposite the support structure (104), a pressure controlled chamber (112) disposed above the support structure (104), and a mechanical attachment (114) affixed to the flexible stamp (106). A method is provided for stamping the surface (101) of an article (102) including the steps of i) placing the article (102) on the support structure (104) within the pressure-controlled chamber (112), ii) wetting the stamping surface (110) with a solution containing a self-assembled monolayer-forming molecular species, iii) aligning alignment patterns (118) on the flexible stamp (106) with alignment patterns (124) on the surface (101) of the article (102), iv) controllably contacting the wetted stamping surface (110) with the surface (101) of the article (102) by changing the pressure differential across the flexible stamp (106) so that contact commences at the center of the flexible stamp (106) and proceeds outwardly in a controlled manner, and v) removing the stamping surface (110) from the surface (101) of the article so that a self-assembled monolayer (134) having the predetermined pattern is formed on the surface (101) of the article (102).

FIELD OF THE INVENTION 
The present invention pertains to the areas of microelectronic devices, 
sensors, and optical elements and more particularly to an apparatus and 
method for stamping the surface of an article in a reproducible and 
uniform fashion. 
BACKGROUND OF THE INVENTION 
Prior art methods of patterning (etching or plating) surfaces with micron 
or sub micron features include irradiative lithographic methods such as 
photolithography, electron-beam lithography, and x-ray lithography. The 
equipment used in conventional irradiative lithographic methods do not 
easily form large-area devices; they are limited to the fabrication of 
small-area devices which must subsequently be stitched together if a 
large-area device is to be fabricated. Typically, the largest area field 
that can presently be fabricated by a panel printer has a maximum area of 
about 12 in.sup.2, and a typical photolithographic printer for 
semiconductor applications has a field area on the order of 1 in.sup.2. 
The stitching process is costly and time-consuming. 
Accordingly, there exists a need for an improved apparatus and method for 
patterning a surface which easily, economically and reproducibly aligns 
and prints large-area devices, thereby providing high throughput. 
Photolithographic aligners are known in the art. They are designed to align 
hard masks, which are rigid and planar. This is accomplished by aligning 
one or more alignment patterns on the hard mask with the corresponding one 
or more alignment patterns on the surface to be patterned. Thus, the 
pattern on the mask is brought into registration with the pattern on the 
surface. The alignment is accomplished by making the necessary 
displacements of the entire hard mask. Since the hard mask is not 
deformable, it does not tend to bow or otherwise mechanically distort in a 
manner which can distort the pattern of the mask. 
The alignment and contact printing process in photolithographic equipment 
includes several steps. The mask is placed in a photomask holder. The 
article to be patterned, or wafer, is placed on a vacuum chuck, which 
includes a plate having holes in it. When the article is placed on a 
surface of the vacuum chuck, it is held in place by suction through the 
holes in the plate. The hard mask is then positioned above, and parallel 
to, the wafer, within several hundred microns. A prealignment is performed 
wherein one or more alignment patterns on the hard mask are brought into 
registration with one or more corresponding alignment patterns on the 
surface of the article. Depending on the geometry of the corresponding 
patterns, one or two pairs of alignment patterns are sufficient to bring 
the stamp printing pattern into registration with the overall wafer 
pattern. One or two pairs of alignment patterns are sufficient to provide 
alignment regardless of the size of the mask because the mask is rigid. 
The alignment is accomplished by detecting the relative positions of the 
alignment patterns and making the necessary adjustments in the position of 
the hard mask and/or wafer by making x-y adjustments and 
angular/rotational adjustments in position. Alignment detection is 
achieved by using an alignment microscope. One or, at most, two alignment 
microscopes are included to detect alignment of the pair(s) of alignment 
patterns. When alignment is achieved, the hard mask and article are 
brought into contact. The printing gap between the mask and wafer is about 
0-50 micrometers: hard contact is achieved by providing a high vacuum 
between the mask and wafer; soft contact is achieved by providing a low 
vacuum, about 50-500 mm Hg. It is recognized in the art that abrupt 
pressure change to vacuum conditions can trap gas between the mask and 
wafer. However, the solution is generally a step change from 
large-gap/high-pressure to soft-contact/low pressure followed by a delay 
for gas release through a valve; thereafter, hard-contact/vacuum are 
provided by dialing in the desired distance and, optionally, by flowing a 
stream of inert gas, at a given flow rate, from the underside of the wafer 
on the wafer chuck. These step changes in the distance between the wafer 
and mask, and in the pressure of the gas between them, are sufficient to 
prevent gas bubble formation between a hard mask and wafer. 
As described above, prior art hard mask aligners provide contact between 
the hard mask and wafer by providing step changes in the distance and 
pressure between the mask and wafer. If this method is utilized to contact 
a deformable, flexible stamp with a surface of an article, gas bubbles 
will form between the surface of the stamp and the surface of the article. 
A prior art aligner would fail to properly align or contact a flexible 
stamp in a stamping process, resulting in non reproducible and non uniform 
printing. However, to be of any practical use, a stamping technique needs 
to provide reproducibility and uniformity. 
Accordingly, there exists a need for an improved apparatus and method for 
aligning a flexible stamp with the surface of an article and for stamping 
the surface so that the pattern on the flexible stamp is transferred 
reproducibly and uniformly. 
Micro-contact printing of self-assembled molecular monolayers (SAMs) is 
known in the art. The SAMs are comprised of molecules which have a 
functional group that binds to certain types of solids, and the remainder 
of the molecule (usually a long-chained hydrocarbon) interacts with 
neighboring molecules to form a dense structure which is impenetrable by 
certain chemical species. Current micro-contact printing methods for 
producing a SAM on a surface cannot reliably or reproducibly print 
surfaces, particularly large-area surfaces having surface areas greater 
than about 1 in.sup.2. 
Accordingly, another purpose of the present invention is to provide a 
cost-effective, reproducible method for patterning large-area surfaces.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 there is depicted a side elevational view of an 
embodiment of an apparatus 100 for stamping a surface 101 of an article 
102 in accordance with the present invention. Apparatus 100 includes a 
flexible stamp 106 having an outer surface 108 and a stamping surface 110. 
In an embodiment of a method for stamping surface 101 of article 102 in 
accordance with the present invention, apparatus 100 forms a 
self-assembled monolayer (SAM) on surface 101, as will be described in 
greater detail below. A general description of SAMs and flexible stamps 
suitable for use within apparatus 100 are described in co-pending U.S. 
patent application entitled "Apparatus and Method for Patterning a 
Surface", filed on even date herewith, by Maracas, et al, and being 
assigned to the same assignee, said application being incorporated herein 
by reference. Embodiments of the stamp disclosed in the above application 
can be incorporated into the embodiments of the present invention, thereby 
providing, for example, flexible stamp 106. Apparatus 100 further 
includes, as illustrated in FIG. 1, a support structure 104 having a 
surface 105, a first pressure controlled chamber 112, and a mechanical 
attachment 114. Support structure 104 includes a hard platform and is 
designed so that article 102 can be positioned on surface 105 and within 
pressure-controlled chamber 112. Flexible stamp 106 is positioned above 
support structure 104 so that stamping surface 110 opposes surfaces 105 
and 101. Stamping surface 110 and surface 105 are positioned within first 
pressure-controlled chamber 112, which includes an enclosed region being 
operably connected to pressure-controlling apparati (not shown), which are 
known to those skilled in the art. The pressure within pressure-controlled 
chamber 112 can be controlled and manipulated to provide the desired 
contacting between flexible stamp 106 and surface 101, as will be 
described in greater detail below with reference to FIGS. 10-12. 
Mechanical attachment 114 is attached to flexible stamp 106 at the edges 
for positioning and securing flexible stamp 106. Mechanical attachment 114 
can translate and rotate flexible stamp 106 during an alignment step, as 
will be described in greater detail below. Mechanical attachment 114 is 
also utilized to mechanically deform (stretch or compress) flexible stamp 
106 during alignment. Specific configurations and elements of mechanical 
attachment 114 will occur to one skilled in the art and include, for 
example, "micrometers", similar to those employed in hard-mask aligners. 
Referring now to FIG. 2 there is depicted an enlarged cross-sectional view 
of stamping surface 110 includes a plurality of contacting surfaces 113 
which define a predetermined pattern to be transferred, or stamped, onto 
surface 101 of article 102. Due to the elasticity and/or local strain of 
flexible stamp 106, runout or a local deformation 111 may occur. In this 
particular embodiment, the predetermined pattern includes equal spacing 
between plurality of contacting surfaces 113, and local deformation 111 
includes a larger spacing. By compressing or stretching flexible stamp 106 
at the appropriate locations, the predetermined pattern is restored. 
Mechanical attachments can be provided with the appropriate detection and 
control elements to provide this correction when required. Suitable 
elements will be known to those skilled in the art. 
Referring now to FIGS. 3 and 4 there are depicted cross-sectional and top 
plan views, respectively, of another embodiment of an apparatus 200 for 
stamping a surface of an article including a flexible stamp 206 which has 
a plurality of piezoelectric crystals defining a disc 207. Disc 207 is 
operatively coupled to a voltage source 209 to provide corrective 
deformation as described above with reference to FIG. 2. When the 
appropriate voltage is applied around disc 207, the plurality of 
piezoelectric crystals are attracted toward or repelled from the applied 
voltage thereby stretching or compressing, respectively, flexible stamp 
206. In this manner, distortions of the predetermined pattern, due to 
runout and/or local deformations of flexible stamp 206, can be corrected. 
Other means for correcting pattern distortions will occur to those skilled 
in the art. 
Referring now to FIG. 5, there is depicted a top plan view of flexible 
stamp 100 further including a plurality of local alignment fields 116 and 
a plurality of alignment patterns 118. Flexible stamp 100 is effectively 
divided into plurality of local alignment fields 116, each of which 
contain alignment pattern 118 so that each local alignment field 116 can 
be individually aligned with a corresponding alignment pattern on article 
102. This local alignment provides correction for local deformation. 
Additionally, plurality of local alignment fields 116 are included so that 
the nature of an overall deformation can be ascertained and corrected. For 
prior art rigid masks, two alignment patterns are sufficient to correct 
for x-y and angular displacements of the mask pattern with respect to the 
article pattern. The provision of only two alignment patterns is 
insufficient for aligning flexible stamp 106 with article 102. 
To demonstrate the nature of this insufficiency, FIGS. 6 and 7 depict top 
plan views of a flexible stamp 506 having two alignment patterns 518 (FIG. 
6) and three alignment patterns 518 (FIG. 7). Also illustrated are the 
corresponding alignment patterns 520 on article 102. An interpretation of 
the misalignment of patterns 518 and 520 in FIG. 7 reveals that stamp 506 
needs to be stretched in order to bring alignment patterns 118 into 
registration, or alignment, with alignment patterns 520. If only two 
alignment patterns are used, under the same circumstances, as depicted in 
FIG. 6, an interpretation of the misalignment of FIG. 6 can be that stamp 
506 requires displacement to the left in order to achieve registration 
between alignment patterns 518 and alignment patterns 520. With too few 
alignment patterns, or fields, the nature of the distortion may not be 
properly ascertained. Having too few local alignment fields, or pairs of 
alignment patterns, over the area of the stamp may not reveal the nature 
of the misalignment. It may be possible that an observed misalignment is 
attributable to either overall stamp misalignment (the correction of which 
requires changing stamp position, but not deformation of the stamp) or 
stamp deformation, such as bowing (the correction of which requires only 
stamp deformation). To provide the appropriate correction, multiple pairs 
of alignment patterns are included. The number of local alignment fields 
116 and alignment patterns 118 increases as the area and flexibility of 
stamp 106 increases. As explained previously, prior art mask aligner do 
not provide multiple alignment patterns, the number of which increases 
with mask area. This is because the alignment of the rigid mask can be 
sufficiently accomplished with one or two pairs of alignment patterns. 
Especially when printing large-area surfaces having surface areas greater 
than about one square inch, a plurality of pairs of alignment patterns are 
required. The area of individual local alignment fields 116 will depend on 
the mechanical properties of the stamp. 
Referring now to FIG. 8, there is depicted a side-elevational view of 
apparatus 100 further including a plurality of microscopes 120 in 
accordance with the present invention. In this particular embodiment, 
flexible stamp 106 is optically transparent so that a plurality of 
alignment marks 124 located on article 102 can be viewed through flexible 
stamp 106 with microscopes 120. Microscopes 120 are provided one per local 
alignment field 116 are employed so that the simultaneous alignment of 
plurality of local alignment fields 116 can be attained and verified, such 
as on a video monitor 122. Other types of alignment detectors may occur to 
one skilled in the art in accordance with the present invention. 
Referring now to FIGS. 9-12 there are depicted side-elevational views of 
apparatus 100 as used in an embodiment of a method for stamping a surface 
of an article in accordance with the present invention. In this particular 
embodiment, stamping surface 110 is first wetted with a fluid including a 
solution of a SAM-forming molecular species. This is done by providing a 
sponge-like substrate 126 having a surface 128 which is saturated with the 
fluid. Sponge-like substrate 126 is positioned outside first 
pressure-controlled chamber 112. Surface 128 has an area at least equal to 
the area of stamping surface 110 so that surface 128 can receive stamping 
surface 110. Stamping surface 110 of flexible stamp 106 is then contacted 
with surface 128 thereby wetting stamping surface 110 with the solution. 
Then, flexible stamp 106 is positioned over article 102 so that surface 
101 opposes stamping surface 110. FIGS. 10-12 depict the step of 
controllably contacting stamping surface 110, now wetted, with surface 101 
of article 102 so that a SAM 134 (FIG. 12), having the predetermined 
pattern of stamping surface 110, is formed on surface 101. Prior to the 
step of controllably contacting stamping surface 110, plurality of 
alignment patterns 118 on flexible stamp 106 are aligned with plurality of 
alignment patterns 124 on article 102. Alignment indicated that flexible 
stamp 106 is positioned so that the predetermined pattern of flexible 
stamp 106 can be printed onto surface 101 in a predetermined orientation 
relative to surface 101. This alignment step can include deforming 
flexible stamp 106, as described in greater detail with reference to FIGS. 
1-7. Flexible stamp 106 is positioned so that stamping surface 110 is in 
close proximity to surface 101 thereby forming a printing gap having a 
height, G, between stamping surface 110 and surface 101. The printing gap, 
G, is about 100 micrometers. Flexible stamp 106 is immovably secured at 
its edges 107. An inert gas 130 is provided within pressure-controlled 
chamber 112 so that, initially, the pressure within pressure controlled 
chamber 112 is sufficient to maintain the printing gap. Then, the pressure 
within pressure-controlled chamber 112 is reduced by removing inert gas 
130 in a controlled manner, which is schematically represented by the 
arrow in FIG. 10 showing inert gas 130 exiting pressure-controlled chamber 
112. The pressure is reduced at a predetermined rate so that contact 
between stamping surface 110 and surface 101 of article 102 commences 
substantially at the center of flexible stamp 106 and proceeds outwardly 
away from the center (as depicted by the arrows above flexible stamp 106 
in FIG. 10) in a controlled fashion thereby preventing undesired 
entrapment of inert gas 130 between stamping surface 110 and surface 101. 
After the desired extent of contact between surface 101 and stamping 
surface 110 has been achieved, flexible stamp 106 is removed from article 
102 by adding inert gas 130 to pressure-controlled chamber 112 at a 
controlled rate and in a continuous manner so that flexible stamp 106 
peels off surface 101 without distorting the predetermined pattern of the 
layer of fluid. In this particular embodiment, and as illustrated in FIG. 
12, SAM 134, which includes a plurality of SAM-forming molecular species, 
is formed, and remains, on surface 101 and has the predetermined pattern 
of stamping surface 110, which is shown in exaggeration in FIG. 12. 
The alignment pressure and printing pressure between a hard mask and wafer 
of a prior art contact printer/aligner are between about 0-500 mm Hg. The 
vapor pressure of a given SAM solution, at the temperature of the stamp, 
may be relatively high, when compared to this range of alignment and/or 
printing pressures. If contact between flexible stamp 106 and article 102 
is achieved by lowering the pressure between flexible stamp 106 and 
article 102, the pressure required to provide the desired contact may be 
sufficiently low so as to result in undesired vaporization of the SAM 
solution. In this particular method, the pressure gradient across the 
stamp varies because, as the contacting proceeds from the center outward, 
the pressure is decreased continuously to provide the controlled contact. 
This varying pressure gradient may result in nonuniformities in the 
printing conditions (and, therefore, in the printed pattern) as the 
printing proceeds. That is, vapor pressures and solution content on the 
surface of the flexible stamp vary with position on the stamp/article. In 
these situations, when the properties of the SAM-forming 
solution/molecular species require it, it is desirable to maintain a 
constant, predetermined pressure between the flexible stamp and the 
article throughout the contacting process, as contact proceeds from the 
center of the stamp, outward. Such a function is provided by an embodiment 
of an apparatus for stamping a surface in accordance with the present 
invention and described below with reference to FIGS. 13 and 14. 
Referring now to FIG. 13, there is depicted a side elevational view of 
another embodiment of an apparatus 300 for stamping a surface 301 of an 
article 302 in accordance with the present invention. A stamping surface 
310 of a flexible stamp 306 is wetted with a solution containing a 
SAM-forming molecular species by, for example, a method such as that 
described with reference to FIG. 9. Apparatus 300 further includes a 
second pressure-controlled chamber 313, which is positioned above wetted 
flexible stamp 306 and in which is positioned an outer surface 308 of 
flexible stamp 306. Article 302 is positioned on a support structure 304 
and within a first pressure-controlled chamber 312, so that a surface 301 
of article 302 is directly opposed to stamping surface 310 of flexible 
stamp 306 which has a predetermined pattern to be printed on surface 301. 
Apparatus 300 is utilized in an embodiment of a method for stamping a 
surface of an article in accordance with the present invention. In this 
embodiment of the method, stamping surface 310 of flexible stamp 306 is 
positioned in close proximity to surface 301, so that a printing gap, G, 
of about 100 micrometers is established between stamping surface 310 and 
surface 301. Flexible stamp 306 is aligned with article 302 and immovably 
secured at edges 307 by a mechanical attachment 314. An inert gas 330 is 
provided within first pressure-controlled chamber 312 to define a first 
printing pressure, P.sub.1. Inert gas 330 is also provided within second 
pressure-controlled chamber 313 to define a second printing pressure, 
P.sub.2. As will be described in greater detail below, first printing 
pressure, P.sub.1, and second printing pressure, P.sub.2, establish a 
pressure differential across flexible stamp 306 which is manipulated to 
provide controlled contact between stamping surface 310 and surface 301 
and to provide controlled pressure conditions within first 
pressure-controlled chamber 312. Initially, as illustrated in FIG. 13, the 
pressure differential across flexible stamp 306 is zero so that printing 
gap, G, is maintained. Then, second printing pressure, P.sub.2, is 
increased in a controlled manner by adding inert gas 330 in a controlled 
manner (as indicated by the upper arrow of FIG. 13) while first printing 
pressure, P.sub.1, is decreased in a controlled manner so that contact 
between stamping surface 310 and surface 301 commences substantially at 
the center of flexible stamp 306 and proceeds in a controlled manner 
outwardly from the center thereby preventing entrapment of inert gas 330 
between stamping surface 310 and surface 301. The resulting configuration 
of apparatus 300 during this step is depicted in FIG. 14. First printing 
pressure, P.sub.1, is maintained at a constant value by providing a 
suitable rate of removal of inert gas from pressure-controlled chamber 
312, as indicated by the outward pointing arrow in FIGS. 13 and 14. In 
this manner the conditions within pressure-controlled chamber 312 are kept 
constant, and vaporization of the fluid is prevented, thereby ensuring 
uniform printing conditions throughout the printing process. The rate of 
increase of second printing pressure, P.sub.2, can be a predetermined 
rate. After the desired extent of contact between surface 301 and stamping 
surface 310 is achieved, flexible stamp 306 is removed from article 302 in 
a controlled manner by simultaneously decreasing second printing pressure, 
P.sub.2, by removing inert gas 330 from second pressure-controlled chamber 
313, and maintaining the constant value of first printing pressure, 
P.sub.1, by adding inert gas 330 to first pressure-controlled chamber 312 
at an appropriate rate. This removal step is performed so that flexible 
stamp 306 peels off of surface 301 without distorting the predetermined 
pattern of the SAM which is formed on surface 301. The appropriate control 
schemes to provide the desired pressure control of first printing 
pressure, P.sub.1, and second printing pressure, P.sub.2, will occur to 
one skilled in the art. 
While we have shown and described specific embodiments of the present 
invention, further modifications and improvements will occur to those 
skilled in the art. We desire it to be understood, therefore, that this 
invention is not limited to the particular forms shown and we intend in 
the appended claims to cover all modifications that do not depart from the 
spirit and scope of this invention.