Optical alignment system in projection printing

Apparatus for placing a resist coated semiconductor wafer surface in the image plane of an optical system by means of three drivers (e.g. actuated flexure joints), with non-contact sensing (e.g. air gauges) at each driver providing individual feedback thereto to move the wafer into the image plane.

Description 
Technical Field 
This invention relates to optical focusing systems and, more particularly, 
to a system for accurately positioning a major planar surface of a 
semiconductor wafer in the image plane of a projected pattern such as 
employed in the fabrication of integrated circuits. 
As will appear from the following description, the focusing system of the 
invention, as well as the individual features of novelty of the system, 
may be employed for a variety of purposes. However, the invention is 
concerned primarily with projection printing of patterns in a photoresist 
coating on oxidized semiconductor wafers for fabrication of integrated 
circuits. For this reason, the invention will be described in connection 
with this primary application. 
Accordingly, one object of the present invention is to provide a 
positioning system which is capable of precise adjustment of the attitude 
of a planar surface concurrently with the alignment of the surface within 
a reference plane. 
Another object of the present invention is to provide a focusing system for 
aligning a planar surface within an image plane of a projected pattern. 
Another object of the present invention is to provide a positioning system 
which is capable of precise adjustment of the attitude of the surface of a 
resist coated semiconductor wafer concurrently with the alignment of the 
surface within a reference plane. 
Another object of the present invention is to provide a positioning system 
for accurately aligning a surface of a photoresist coated wafer in the 
image plane of a projected pattern such as employed in the fabrication of 
integrated circuits. 
Background Art 
The fabrication of integrated circuits typically requires successive 
treatments which involves a series of operations (e.g. etchings, 
diffusions, metallization, etc.) each of which employs a mask of 
predetermined design. The masks can be formed by use of a master array 
containing geometrical patterns which are projected on a photoresist 
coating, on a semiconductor wafer, by illumination from a suitable source 
to modify the photoresist where subjected to light to adapt the 
photoresist to etching operations in accordance with the nature of the 
photoresist, e.g. positive or negative. In projection printing, suitable 
illumination is passed through the master array to project an image of the 
geometrical patterns in a focus or image plane in which the resist coated 
wafer is positioned and aligned. See for example, the article 
"Semiconductor Levelling Systems" by J. W. Buechele, pp. 3255-56 of the 
IBM Technical Disclosure Bulletin, V.16, n.10, March 1974. In this 
respect, it may be noted that the fabrication of semiconductor devices may 
require as few as four or as many as twenty-five or more individual 
photoresist steps employing different master arrays or masks. Conversely, 
each wafer when processed may have several hundred to several thousand 
devices thereon with from 5 to 700 circuit elements in each device. Also, 
each individual device may measure as small as a few thousands of an inch 
long and wide, with electrodes and individual indicia thereon measured in 
a few ten thousands of an inch. In view of such densities and in light of 
trend for increased miniaturization, it is critical that each successive 
mask pattern must coincide in exact registration with the wafer relative 
to the prior processing stages of fabrication. 
Apparatus for positioning of workpieces or wafers is known in the art. One 
particularly effective system is that shown in U.S. Pat. No. 4,068,947 
issued Jan. 17, 1978 to J. D. Buckley et al. and with which the present 
invention can be employed; and thus accordingly the teachings of this U.S. 
Pat. No. 4,068,947 are incorporated herein by reference thereto. This 
patent describes a projection printing system and the alignment or 
registration of a semiconductor wafer in four coordinates: two lateral 
dimensions X and Y, a vertical dimension Z that is perpendicular to the 
face of the wafer, and rotation .theta. about the center of the wafer. 
Vertical orientation Z of the wafer to bring its face into the image 
plane, of the projection system, is obtained by abutment of peripherally 
spaced edge portions of a wafer against the faces of equiangularly spaced 
reference pads of an adapter ring mounted on an XY.theta. plate, which pad 
faces lie in the image plane in a co-planar relationship. Since the 
reference pad faces are co-planar with the image plane, it can be 
logically concluded that the proper attitude of the wafer is obtained 
concurrently with the orientation of its surface in the image plane. 
Although the system of this U.S. Pat. No. 4,068,947 provides an excellent 
means for orientation of a semiconductor wafer in the XY.theta. 
coordinates, the system is, however, characterized with disadvantages in 
operations directed to positioning and alignment of photoresist coated 
wafer surfaces in the image plane of the projection system. Not only do 
the reference pads eliminate the availability of the contacted wafer 
portion for device fabrication, but deviations are encountered in the 
positioning of wafer surface in the image or focal plane where the wafer 
includes tapered or "roll off" portions at its periphery. Also, when the 
reference pads contact photoresist coating of the wafers, transfer of the 
resist to the pad faces occurs with eventual buildup of the resist on the 
reference pad faces to a point which interferes with the accurate 
positioning of the wafer surface in relationship to the optimum focal or 
image plane of the system. This necessitates frequent pad cleaning for 
removal of the resist to prevent unacceptable device loss, as well as 
resulting in down-time which reduces yield in device fabrication.

Disclosure of the Invention 
By means of a precision air sensing technique and three sets of flexure 
joints, driven by an electropneumatic system, the front faces or surfaces 
can be placed within .+-.20 microinches of the focal plane of an optical 
system. 
Referring to the drawings, particularly FIG. 2 where the levelling and 
focusing system is shown, the system comprises a levelling plate or chuck 
pedestal 1 which is mounted for reciprocation on three pneumatically 
actuated flexure units 2 (FIGS. 1, 2 and 3) equiangularly disposed (e.g. 
120.degree. apart) relative to each other. In turn, the flexure units are 
mounted within a chuck cup or socket 3 which is mounted to a support plate 
4 for prerequisition of the system relative to a focal plane. As will be 
described below, support plate 4 is more specifically defined as an 
XY.theta. or aperture plate which can be employed in the system of the 
aforesaid U.S. Pat. No. 4,068,947 for planar and rotational alignment of a 
semiconductor wafer in the image or focal plane of a projection printing 
system. 
Mounted on support plate 4, substantially opposite each flexure unit, are 
respective ones of three equiangularly spaced sensing heads 5, each 
including air sensor probes or nozzles 6, which are pivotally reciprocable 
over and away from the top surface of wafer 7 vacuum mounted on chuck 
pedestal 1. The air sensors operate in a conventional manner to sense 
pressure variations as ejected air is throttled by an approaching or 
receding surface of a workpiece, such as wafer 7. The sensors are 
pneumatic and function with pressure differential switches 8 (FIG. 2), 
which are suitably activated in accordance with the distance of the 
workpiece surface from the end of the sensing nozzles 6. The output of 
each pressure switch 8 is coupled to a suitable driving mechanism 9 which 
actuate the flexure units to move the chuck pedestal 1, and conversely 
wafer 7 to level and align its top surface within the focal or image plane 
of the optical system. 
Best Mode for Carrying Out the Invention 
As shown in FIG. 1, the support plate 4 is embodied as in XY.theta. plate 
which is incorporated onto a carriage plate 10 which is shown within the 
environment of a projection system as illustrated in FIG. 12 of the 
aforesaid U.S. Pat. No. 4,068,947, which as indicated is incorporated 
herein by reference thereto. The support plate is provided with an 
aperture 11 defining access to an image beam of any optical system such as 
a conventional projection system normally employed in the fabrication of 
semiconductor devices. The bottom surface of support plate 4 is normally 
planar which can conveniently be positioned to define a reference plane 
such as the image or focal plane 12 of the optical system in which the top 
surface of a wafer 7 is to be placed. 
Formed in the bottom of supported plate 4 are vacuum grooves 13 which, when 
activated, function to hold a chuck assembly 14 in place against the 
bottom of the plate. Disposed on the top of XY.theta. plate 4 are three 
sensor heads 5 equiangularly spaced thereon. Each of the sensor heads 5 is 
pivotally mounted on a suitable axis such as a shaft 15 coated with a 
lubricating polymer (FIG. 2A). A ring spring 16 is enclosed in sensor head 
wells 5A by a cover plate 17 secured to shaft 15 by fastener 18 to bias 
the head against support plate 4. The opposite end of shaft 15 is flanged 
so as to enable it to be secured to the bottom of XY.theta. plate 4 by 
fasteners 19 together with a corresponding flange of registration pin 20. 
The setting or positioning of the registration pin 20 is insured by means 
of a stanchion 21, on the chuck assembly, which mates within an axial bore 
22 on shaft 15. 
The sensor heads 5 have two positions "in" and "out" under the control of 
two associated pneumatic cylinders 23 and 24 having plungers which act on 
respective shoulders 25 and 26 formed on sensor heads 5. When cylinders 23 
are activated, via flexible tubing 28 to a source of pressure at manifold 
29, the extension of their plungers against the shoulders 25 rotates the 
heads 5 to the "in" position which locates the sensor nozzle tips 6 over 
an associated flexure unit 2. Conversely, actuation of the "out" cylinders 
24, via flexible tubing 30 to a pressure source at manifold 29, extends 
its plungers against the head shoulders 26 to rotate the head 5 on 
bearings 16 out of the field of the image beam of the optical system. 
Formed in the top of the XY.theta. plate 4 below the bottom of sensor 
heads 5 are grooves 27 which are connected (not shown) by flexible tubing 
to an alternate source of vacuum or air pressure. During rotation of 
sensor heads 5, the grooves 27 can be pressurized to emit a layer of air 
to form an air bearing to facilitate rotation of the heads. In the "read" 
and "rest" positions of heads 5, grooves 25 are connected to a source of 
vacuum to clamp the heads 5 in fixed positions on the XY.theta. plate 4. 
For the "read" position, over wafer 7, the sensor nozzle tips 6 are preset 
at a distance of 0.003.+-.0.0002 inches from the reference plane defined 
by the bottom of the XY.theta. plate 4. The sensor nozzle 6 constitutes an 
air flow rate detecting element which provides a measure of the spacing of 
a workpiece surface from the end of the nozzle 6. 
Positive air flow is provided to sensor nozzle 6 through a passageway 30, 
in sensor head 5, which extends through flexible tubing 31 to manifold 29, 
in turn connected by flexible tubing 31', to a sense outlet 32 of a 
pressure differential (P/D) switch 8. 
Three pressure differential switches 8 are used to establish the focal 
plane for this system, one for each of sensor heads 5 and flexure elements 
2, with it being understood that the focal plane is optically and 
mechanically aligned before the P/D switch 8 calibration by means of an 
optical flat (not shown) which is initially substituted for the wafer 
front surface. 
The P/D switch 8 comprises a dielectric or insulating body 34 having an 
interior chamber in which is mounted a sensitive conductive and metallic 
bellows 35. The bellows defines two air chambers 36 and 37. A source of 
regulator air pressure (e.g. 16 psi) is connected to an inlet 38 which is 
bifurcated into two lateral passage runs 39 and 40. In this manner, a 
portion of air flows through a fixed restrictor 41 to the switch sense 
outlet 32 and to chamber 36. The remaining portion of the air flow passes 
through a fixed restrictor 42 (substantially equal to restrictor 41) to 
chamber 37 and to a control outlet 43 connected by flexible tubing 58 to a 
bias control valve 44. 
The valve unit 44 is constructed of a single mass of metal 45 (e.g. 302 
stainless steel) split into two cantilevered control arms 46 and 47 by 
means of kerfs 48 and 49. The spacing between the bights of kerfs 48 and 
49 is thinned by means of suitable bores 50 and 51 to form a flexure joint 
52 which provides proportional strength which provides adjustment 
capabilities by means of a coarse adjust set screw 53 and a fine adjust 
screw 54. Setting of the set screws 53 and 54 controls the spacing between 
a base leg 55 and lever arm 46 for control positioning of a valve pad 56 
relative to an air bore 57 which is connected by flexible tubing 58 to the 
control outlet 43 of the P/D switch 8. Adjustment of the bias control 
valve 44 adjusts the division of air flow in P/D switch 8 in correlation 
with the desired distance of a workpiece surface from sensor nozzle end 6, 
e.g. when the surface is in the focal or image plane. 
The P/D switch 8 sensing function is obtained by means of suitably formed 
contacts which form the basic electrical circuit of FIG. 5. The contacts 
include a low sense electrical contact 60 which extends into the chamber 
36 within bellows 35 and an axially aligned high sense electrical contact 
61 which extends into the switch chamber 37 externally of bellows 35. The 
circuit is completed by a common terminal 62 positioned in contact with 
the flange of the metallic bellows 35, which together with suitable 
circuitry (graphically illustrated as elements 64 and 65) in any suitable 
electronic control unit 66 activates the driver unit 9 for control of 
flexure elements 2. 
In operation, with air pressure applied at the P/D switch inlet 38, the gap 
between the workpiece surface and probe nozzle end 6 dictates the degree 
of expansion of bellows 35 within the switch. When the gap of the surface 
at the probe nozzle 6 exceeds the callibrated distance to the focal or 
image plane (e.g. greater than 0.003"), the pressure in switch chamber 37 
rises to a higher level than in switch chamber 36, to compress bellows 35 
in contact against the low screw contact 60 to signal the driver unit 9 to 
continue to move the workpiece surface into closer proximity with the 
sensor probe 6. Inversely, if the workpiece surface is positioned at a 
closer gap beyond the focal or image plane (e.g. less than 0.003"), the 
pressure in bellows chamber 36 will rise to a point higher than that in 
switch chamber 37 to extend the bellows 35 into contact with the high 
electrical contact 61 to signal the driver unit 9 to move the workpiece 
surface away from the probe tip 6. When the three points of the surface 
are properly spaced at the three equiangularly spaced nozzle tips 6, the 
planar surface of the workpiece is levelled within the reference plane 
(e.g. optical or image plane). 
A driver unit 9 is associated with each pair of probes and flexure units 2, 
and it is comprised of an air regulator 67 (e.g. a 1 to 60 psi Fairchild 
Model 70B) having a geared head 68 (e.g. 84 teeth) meshed with the teeth 
(42 teeth) of a gear 70 mounted on a drive motor 69 (e.g. Globe Motor 
Model #43A144-27). The rotation of motor 69 determines the pressure 
regulation of a suitable pressure source (e.g. 70-80 psi) to the flexure 
units 2, in response to the directional control from the electronic 
control unit 66 as specified by the P/D switch 8. 
The details of the flexure units 2 are shown in FIG. 4 mounted within the 
cup-shaped chuck frame 3. As will be understood, three equiangularly 
flexure units are employed, each in association below a respective one of 
probe or sensor nozzles 6. These flexure units are formed from metal block 
80 (e.g. 302 stainless steel) slit lengthwise as by a kerf 81 with a 
thinned portion 82 adjacent the kerf bight 83 to form a flexure joint to 
provide controlled deflection of the unit. The base leg 84 of the device 
is provided with a boss 85 having a bore 86 for mounting therein a bellows 
87. The bellows is sealed in bore 86 by a cover cap 88 having a passageway 
89 communicating from the cap port 91 to the bellows chamber 90. The 
opposite end of bellows 81 is sealed with a top plate 92 having a concave 
seat 93 for receiving a thrust ball 94 which is normally held in 
non-stressing abutment against the lever arm 95 of the flexure element 80. 
The base leg 84 of the flexure element is secured to chuck frame yoke 96A 
wall 96 by fasteners 97. 
Provided at the end of lever arm 95 is a concave seat 98 for a pivot ball 
99 which is mounted against the concave seat 101 of a bearing screw 100 
secured in a threaded bore 302 of chuck pedestal 1. The flexure element 80 
is pivotally clamped against pivot ball 99 at the shoulder portion 102, of 
lever arm 95, by means of a spring leaf retainer 103, which is secured to 
the bottom of the chuck pedestal 1 at a boss 104. Also provided at the 
periphery of the chuck pedestal 1, by fastener 106, is a locator pin 105 
for mating with locator notches of a semiconductor wafer in preliminary 
alignment thereof on chuck pedestal 1. For securing a wafer on chuck 
pedestal 1, its top surface is provided with vacuum grooves 107 which are 
connected to a suitably controlled vacuum source by flexible tubing 108. 
Also, as shown in FIG. 2, the flexure unit bellows 87 are actuated by 
suitable tubing 109 extending between the bellows port 91 and the driver 
unit 9. 
Equiangularly spaced about the periphery of the chuck frame 3 are location 
pins 110 having registration against for pre-mounting the XY.theta. plate 
registration pins 20 by gravity. The mounted chuck frame is secured to 
XY.theta. plate 4 by means of machined vacuum grooves 13. The XY.theta. 
plate 4 is mounted to carriage plate 112 through a focus ring 113 to which 
it is secured by means of at least three equiangularly spaced tension 
springs 114 extending through and secured at anchor pins 115 and 116 at 
the end of the bores 117 and 118 in the XY.theta. plate 4 and focus ring 
113, respectively. 
In turn, the focus ring 113 is mounted to the carriage plate 112 by means 
of a flexure blade or leaf spring 114 secured to the focus ring at point 
215 thereat and to a lever arm 116. The lever arm 216 is pivotally mounted 
against a fulcrum ball 117. An adjusting screw 118 is provided at one end 
of lever arm 216 to act against carriage plate 112 so as to controllably 
pivot the lever arm 216, about fulcrum ball 217, for alignment of the 
XY.theta. plate face 119 into the reference plane 12, e.g. focal or image 
plane of an optical system. After setting of lever arm 216, it is locked 
into place by clamp screw 120 extending through an oversized bore 121 in 
the lever arm 216, into carriage plate 112. Focus ring 113 is also 
provided with a passageway 130 having an inlet 131 connected to an 
alternate source of air pressure or vacuum, with the opposite end 
terminating at the adjacent surfaces of the focus ring 113 and the 
XY.theta. plate 4. The function of passageway 130 is to lock the focus 
ring 113 and the XY.theta. plate 4 together, by connection to a source of 
vacuum, as required, e.g. wafer stage alignment operation, and during 
exposure in a projection printing system. Alternatively, when relative 
motion between the focus ring 113 and XY.theta. plate 4 is required (e.g. 
X-Y-.theta. alignment as in U.S. Pat. No. 4,068,947) an air bearing can be 
provided between them by connection of the focus ring passageway 130 to a 
pressurized gas source. 
The pneumatic operation of the system is shown in FIG. 6 which illustrates 
the use of two chuck assemblies, one outboard chuck 3' on which a wafer is 
loaded while the inboard chuck 3 is being utilized in the optical system. 
Operation of the dual chuck arrangement is for all purposes comparable to 
that referred to in the aforesaid U.S. Pat. No. 4,068,947. As can be seen 
in FIG. 6, one set of drivers is provided for the two chuck arrangements. 
Although the arrangement shown involves concurrent pneumatic operation of 
the chuck assemblies, the arrangement does simplify the mechanical and 
electrical complexity substantially, while still maintaining a high 
performance level. 
In operation, the alignment of the XY.theta. plate surface 119 is preset in 
the reference plane 12 (e.g. image or focal plane) by means of the focus 
ring adjustment by adjustment of lever arm 116 utilizing an optical flat 
with the air probes 5 in position. The optical flat is retracted, and a 
semiconductor wafer inserted on the chuck pedestal 1. With the chuck frame 
3 in position against the XY.theta. plate 4 and held thereon by vacuum in 
groove 13, the alignment system is activated. Air probes 5 are pivoted 
over the wafer 7 which controls the three flexure units to level and 
position the wafer system into the focus or image plane 12, by means of 
the P/D switch 33, the electronic control 66 and driver 9, as described 
above. Performance curves are shown in FIG. 7 for three different flexure 
element designs. Illustratively, the flexure lever arm 95 can be designed 
to provide a 0.005 to 0.006 mil travel since semiconductor wafers may have 
up to 0.003 mils out of parallelism, and .+-.0.0015 inches in thickness 
variations. 
While the invention has been illustrated and described with reference to 
preferred embodiments thereof, it is to be understood that the invention 
is not limited to the precise construction herein disclosed and the right 
is reserved to all changes and modifications coming within the scope of 
the invention as defined in the appended claims.