Guide wing for a furnace paddle

A furnace paddle adapted to hold semiconductor wafers therein and having means attached thereto for supporting the paddle for movement along the inside of a cylindrically-shaped tube of a semiconductor processing furnace has means attached to the paddle adjacent the supporting means for limiting the lateral movement of the supporting means up the inside wall of the tube. The means includes a guide wing having two ends for contacting respectively the inside wall of the tube at intersecting points of a chord thereof and having a stem attached to the paddle adjacent the supporting means for maintaining the ends proximate the inside wall of the tube.

This invention relates to a furnace paddle adapted to hold semiconductor 
wafers therein and having means attached thereto for supporting the paddle 
for movement along the inside of a cylindrically-shaped tube of a 
semiconductor processing furnace, such as a diffusion furnace. 
In manufacturing semiconductor devices, semiconductor wafers are placed 
into high-temperature processing furnaces comprising relatively long 
cylindrically-shaped tubes. The doping process is typically performed in 
this manner wherein conductivity modifiers are diffused into the wafers 
through selectively-defined openings in masks formed adjacent the wafer 
surfaces. In order to transport the wafers into and out of the tubes, 
appropriately designed paddles are frequently utilized which traverse the 
tubes while minimizing the introduction of undesirable impurities into the 
furance atmosphere. Each paddle has means attached thereto for supporting 
the paddle for movement along the inside of the tube. Such supporting 
means typically comprises one or more wheels, or a skid, attached to the 
end of the paddle which enters the processing tube. 
A problem associated with the paddle while traversing the tube is that the 
supporting means, i.e., the wheels or skid, tends to track up the inside 
of the tube during entry or exit. The wafers are held upright in a 
stack-like configuration, usually by slots in a boat which rests in a 
depression in the paddle. Consequently, when the supporting means tracks 
up the inside of the furnace tube, the tops of the wafers scrape the upper 
portion of the tube and are damaged by breakage or warpage, contributing 
to a significant economic loss. Warped wafers are not able to be held 
securely by a vacuum chuck and will spin off and break. Such wafers also 
scratch photomasks and result in a low yield due to poor resolution on the 
warped wafers during the photolithographic process. The present invention 
provides a practical and efficient way to reduce such breakage and 
warpage, and thereby achieve a significant savings in resources.

Referring to FIG. 1 of the drawings, there is shown a typical furnace 
paddle 10 having a size and shape that will allow the paddle 10 to 
traverse a cylindrically-shaped tube 12 of a semiconductor processing 
furnace. The paddle 10 may be fabricated by forming and machining a 
silicon carbide billet to the desired shape after a presinter heat 
treatment. The machining would also include cutting a depression 14 in the 
paddle 10 in which a semiconductor wafer boat 16 adapted to hold 
semiconductor wafers 18 is placed. Alternatively, the silicon carbide 
paddle 10 can be formed to the desired shape and size by slip casting. For 
example, the paddle 10 can be slip cast to the shape shown in FIG. 1, 
having hollow end portions 24 and 22 and a boat-carrying section 20. The 
paddle 10 also has means attached to one end 28 thereof which enters the 
tube 12 for supporting the paddle 10 for movement along the inside wall 30 
of the tube 12. Such supporting means typically comprises a skid or a pair 
of wheels 32, as illustrated in FIG. 1. The wheels 32 are formed 
separately and assembled with the finished paddle 10 utilizing an axle and 
axle caps 34. The semiconductor wafer boat 16 may be fabricated in similar 
manner to have a multitude of slots 36 which hold the wafers 18 upright in 
a stack-like configuration, as shown in FIG. 1. A more detailed 
description of silicon carbide furnace paddles is contained in Alliegro et 
al., U.S. Pat. No. 3,951,587 which issued Apr. 20, 1976, the disclosure of 
which is incorporated herein by reference. 
Attached to the paddle 10 adjacent to the supporting means, i.e., wheels 32 
in the present example, is means for limiting the lateral tracking 
movement of the supporting means up the inside wall 30 of the tube 12. In 
the present embodiment the novel limiting means comprises a guide wing 38 
having two ends 40 and 42 of a member 54 for contacting respectively the 
inside wall 30 of the tube at intersecting points of a chord thereof. The 
guide wing 38 also has a stem 44 attached to the paddle 10 adjacent the 
wheels 32 for maintaining the ends 40 and 42 proximate the inside wall 30 
of the tube 12 in a manner such that the adjoining chord (46) intersects 
the diameter (48) meeting the area where the wheels 32 contact the tube 
12. The chord referred to above is diagrammatically shown in FIG. 2 as 
dotted line 46 which intersects the side wall 30 of the cylindrical tube 
12 at points A and B. Line 46 thus has a dimension equal to a chord of 
tube 12. Preferably line 46 extends along the longitudinal axis of 
elongated member 54. The diameter meeting the wall area where the wheels 
32 contact the tube 12, shown as dotted line 48 in FIG. 2, intesects the 
chord 46 at point C. Diameter 48 is coincident with the longitudinal axis 
of stem 44 when the paddle 10 is moving only along the desired 
longitudinal path through the tube 12. 
The geometrical language used in the foregoing paragraph is intended to 
specifically define the geometry of the structure of the novel means 
utilized for limiting the lateral tracking movement of the wheels 32 up 
the inside wall 30 of the tube 12. As the paddle 10 is pushed into the 
tube 12, the wheels 32 have a tendency to track tangentially up the inside 
wall 30 of the tube 12 by moving in a lateral and upward direction, for 
example, in the positive X-direction and positive Y-direction. When this 
happens, the end 42 of the guide wing 38 contacts the inside wall 30 of 
the tube 12 at point B, making it possible for the tube 12 to exert on the 
guide wing 38 a counteracting force having components in the negative 
X-direction and negative Y-direction at point B, and thereby prevent 
further lateral movement of the wheels 32. In order to prevent this 
lateral movement in both the positive and negative X-directions, it is 
necessary that the chord 46 extending through member 54 intersect the 
diameter 48 so that a counteracting force vector in both X-directions will 
be developed when the member 54 contacts the wall of tube 12 . 
In the preferred embodiment, the chord 46 extending through member 54 and 
the two lines connecting the ends thereof with the area where the wheels 
32 contact the tube 12, shown as dotted lines 50 and 52 in FIG. 2, form an 
acute triangle. An acute triangle is formed whenever the geometrical 
longitudinal center of the tube 12 lies within the triangular boundaries. 
The formation of such a triangle allows the chord 46 to lie above the 
geometrical center, so that a counteracting force vector exerted by the 
inside wall 30 of the tube 12 in the negative Y-direction at points A and 
B may be available to prevent further lateral movement. Whether or not 
such a counteracting force vector in the negative Y-direction is required 
will depend upon such factors as the magnitude of the downward force 
exerted by gravity on the paddle 10, the magnitude of the frictional force 
between the guide wing 38 and the tube 12 at the contact points, and the 
amount of lateral tracking movement of the wheels 32 which can be 
tolerated. 
Utilizing the aforementioned geometrical guidelines, the novel guide wing 
38 may comprise any shape or design including T-shape, Y-shape, or any 
combined configuration. In the present embodiment, the guide wing 38 is 
T-shaped wherein the chord 46 is substantially orthogonal to the diameter 
48. As illustrated in FIG. 3, the two ends 40 and 42 comprise the ends of 
a substantially rectangular parallelepiped member 54, and the stem 44 
comprises a hollow cylinder 56 having one base thereof attached to a 
surface 58 of the parallelepiped member 54. Preferably, the ends of the 
parallelepiped member 54 are slightly rounded in order to facilitate their 
movement adjacent to and prevent binding with the inside wall 30 of the 
tube 12 as the paddle 10 traverses the tube 12. In the present embodiment, 
the other base of the cylinder 56 is attached to a U-shaped yoke 60 
adapted to fit over the one end 28 of the paddle 10. There is a hole 62 in 
each side of the yoke 60 through which the axle is inserted prior to 
assembling the wheels 32 and axle caps 34. This type of arrangement allows 
the guide wing 38 to be held in a relatively fixed position with respect 
to the paddle 10 and still be removable. Preferably, the three parts of 
the guide wing 38, i.e., the parallelepiped member 54, cylinder 56 and 
yoke 60, are constructed separately of either quartz or silicon carbide, 
and then fused together so as to minimize the introduction of impurities. 
The novel guide wing 38 provides relatively simple means for limiting the 
lateral movement of the paddle wheels 32 as they move along the inside 
wall 30 of the tube 12. The guide wing 38 is a practical and efficient way 
to eliminate warpage caused by the semiconductor wafers 18 scraping the 
inside wall 30. Such a guide wing costs only a few dollars to construct, 
but achieves estimated yearly savings in the thousands of dollars.