Transport mechanism

A transport mechanism for silicon wafers comprises an elongate four arm linkage and a wafer support arm which is secured to the linkage through a gearing system. One of the short arms of the linkage is fixed, and as the linkage is rotated, the gearing system produces a different motion in the support arm. Preferably pinion gears are secured to the ends of the long arms and engage a pinion gear secured to one end of the support arm, so as to produce linear motion of a wafer placed on the support arm.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to apparatus for transporting silicon wafers or the 
like, and methods of using such apparatus. 
2. Introduction to the Invention 
In the manufacture of solid state electronics devices, silicon wafers are 
moved between different work stations, generally in cassettes which hold a 
number of wafers stacked one above the other in horizontal pockets, each 
pocket being in the form of a generally U-shaped horizontal groove into 
which a wafer can be placed. Careful handling of the wafers is essential; 
in particular insertion and extraction of wafers into and from cassettes 
require a linear motion in order to minimize damage to the wafers and the 
production of particulate contaminants. In many cases, it is desirable 
that the wafer should have a known orientation at the work station, and 
hence also a known orientation in the cassette. 
Currently used transport mechanisms for silicon wafers include belt 
transports and air tracks, but these mechanisms have been criticized as 
likely sources of particles which can contaminate the wafers. U.S. Pat. 
Nos. 4,433,951 and 4,483,654 (Koch and Peterson, assigned to Lam Research 
Corporation), the disclosures of which are incorporated herein by 
reference, describe a transport mechanism in which the rotational motion 
of a support arm is transmitted through a series of gears to a workpiece 
arm shaft having one end rotatably secured to the support arm, so that 
rotation of the support arm effects linear motion (or other predetermined 
motion) of a workpiece placed at the other end of the workpiece arm shaft. 
SUMMARY OF THE INVENTION 
I have discovered an improved transport mechanism which makes use of a 
combination of (a) a multi-arm linkage which can be caused to rotate about 
a fixed base and (b) a support arm which is secured to the linkage through 
(c) a gearing system, so that, when the linkage is rotated, the rotational 
movement of the linkage results in a related but different rotational 
movement of the support arm. In this way, a silicon wafer (or other load) 
placed on the support arm can be transported along a predetermined 
non-circular, preferably linear, path which depends upon the lengths of 
the arms and the gearing system. 
The linkage preferably comprises two long arms each having a first length, 
and two short arms which join the ends of the long arms and each of which 
has a second length which is less than the first length, eg. from 0.1 to 
0.5 times the first length; the four arms are rotatably secured to each 
other, and one of the short arms is fixed relative to a base. When the 
three rotatable arms are rotated (for example by means of a motorized 
driving shaft coaxial with the pivot point between one of the long arms 
and the fixed short arm), the area enclosed by the arms (typically a 
parallelogram) changes. If the second, third, fourth and support arms 
rotate in different (but parallel) planes about pivots which do not extend 
into adjacent planes of rotation, the four arm linkage can be rotated 
through two extreme positions in each of which all the pivot points 
between the arms are in a straight line. The arms of the linkage can be 
straight, but need not be, and the term "arm" is used herein in a broad 
sense to include any physical structure which will permit the desired 
operation of the apparatus. For example, the fixed arm can be provided by 
two pillars which are mounted on the base to provide fixed pivot points 
for the long arms, in which case the base and the pillars together provide 
the short fixed arm. 
The gearing system which connects the linkage and the support arm can be 
very simple, and preferably comprises (a) a first pinion gear which is 
secured to one of the rotatable arms and (b) a second pinion gear which is 
secured to the support arm and which is maintained in engagement with the 
first pinion gear as the rotatable arms of the linkage are rotated. The 
first pinion gear is preferably secured to one of the long arms at the end 
thereof remote from the fixed short arm. The second pinion gear is 
preferably secured to the support arm at one end thereof. The first and 
second pinion gears are preferably of the same size, so that rotation of 
the second arm results in an equal rotation of the support arm but in the 
opposite direction. This will result in linear motion of that section of 
the support arm (and/or of a wafer placed on the support arm) whose 
distance from the pivot point of the support arm is equal to the distance 
between the pivot points of each of the long arms. The linear motion 
passes through a point which is close to the pivot point between the first 
arm and the long arm carrying the pinion gear, but offset from that pivot 
point by a distance equal to the gear offset. 
When the linkage is rotated through a position in which all the pivot 
points between the arms are in a straight line, or close to such a 
position, the linkage tends to become unstable and positioning of the 
support arm tends to become less accurate. In a preferred embodiment of 
the invention, this tendency is controlled by using a gearing system in 
which a gear on the support arm is driven by gears on at least two of the 
rotatable arms of the linkage. Preferably first and third pinion gears 
(which must be of the same size) are secured to the long arms at the 
respective ends thereof remote from the fixed short arm, and a second 
pinion gear is secured to the support arm and is maintained in engagement 
with the first and third pinion gears as the linkage is rotated. 
Preferably the second pinion gear is of the same size as the first and 
third pinion gears, so that linear motion of the support arm is obtained. 
If desired, a similar gearing system, but without a support arm, can be 
used to stabilize the linkage at the fixed arm. 
With a simple support arm, the transport of a wafer (or like object) along 
a defined, preferably linear, path will be accompanied by a defined degree 
of rotation. In some cases this is not a problem; in other cases, it is 
valuable if the wafer can be delivered in a different orientation (or 
close to it), so that the step of orienting the wafer at the work station 
is eliminated or made quicker and cheaper. This desirable result can be 
achieved by using a wafer support arm which is an auxiliary four arm 
linkage comprising one arm which is in a fixed position relative to the 
travelling short arm of the main linkage. If the auxiliary four arm 
linkage is identical to the main linkage, rotation of the wafer can be 
eliminated entirely. If the auxiliary linkage is different, then there 
will be a predictable degree of rotation which is less than that with a 
simple support arm. 
The support arm can if desired comprise two or more auxiliary linkages 
successively connected through each other to the main linkage in the same 
way as is described above, in order to provide linear (or other 
predictable) motion over longer distances. However, care must be taken to 
ensure that the arm is sufficiently strong and rigid. 
The gear on the support arm must be maintained in engagement with the gear 
or gears on the linkage. This is preferably achieved by spring loading the 
gears into each other or by using other known anti-backlash methods, which 
helps to make the apparatus more stable and to eliminate or reduce 
backlash.

DETAILED DESCRIPTION OF THE INVENTION 
A preferred embodiment of the invention is an apparatus which comprises 
(1) a base, 
(2) a four arm linkage comprising 
(a) a first arm which is fixed relative to the base and which has a first 
end and a second end, 
(b) a second arm which is substantially longer than the first arm and which 
has a near end and a far end, the near end being rotatably secured to the 
first end of the first arm; 
(c) a third arm which is of the same length as the second arm and which has 
a near end and a far end, the near end being rotatably secured to the 
second end of the first arm, and 
(d) a fourth arm which is of the same length as the first arm and which has 
a first end rotatably secured to the far end of the second arm and a 
second end rotatably secured to the far end of the first arm; 
(3) a first pinion gear which is secured to the second arm at the far end 
thereof; 
(4) a wafer support arm which has a pivot end and a wafer support section, 
the wafer support section being adapted to carry a silicon wafer; 
(5) a second pinion gear which is secured to the wafer support arm at the 
pivot end thereof and which is maintained in engagement with the first 
pinion gear as the second arm is rotated relative to the first arm; 
whereby, when a wafer is placed on the wafer support arm and the second arm 
is rotated relative to the first arm, the wafer is transported along a 
linear path. 
Referring now to FIGS. 1-3 of the drawing, these illustrate an apparatus 
which comprises a base 1; a four arm linkage 2 which comprises a first 
short arm 21 which is fixed to the base, a second long arm 22, a third 
long arm 23, and a fourth short arm 24; and a motor 8 for rotating the 
linkage about the base. The arms and the pivots between them are arranged 
so that the arms rotate in parallel but different planes. Secured to the 
end of arm 22 is pinion gear 3, and secured to the end of arm 23 is 
identical pinion gear 6. Wafer support arm 4 has a pivot end 41 and a 
wafer support section 42 which can carry a silicon wafer 19 (shown in FIG. 
2 only). Secured to the pivot end 41 is a pinion gear 5 which is identical 
to pinion gears 3 and 6. The pinion gear 5 is rotatably mounted within a 
bracket 7 and is spring-loaded into engagement with pinion gears 3 and 6 
by means of shoulder screws 71 and Belleville springs 72 which secure the 
bracket 7 to the short arm 24. Alternative means for springloading the 
pinion gears into engagement can of course be used. 
Referring now to FIG. 4, this shows a four arm linkage 2 and a support arm 
4 as in FIGS. 1-3 with an auxiliary four arm linkage 9 between linkage 2 
and support arm 42. The auxiliary linkage comprises short arms 91 and 94 
and long arms 92 and 93. Short arm 91 is maintained in a fixed position 
relative to short arm 24 by means of identical pinion gears 97 and 98 
which are maintained in engagement with pinion gears 6 and 3. Arms 92 and 
93 are secured to identical pinion gears 93 and 96 which engage the pinion 
gear 5 secured to the support arm 4.