Abrading – Precision device or process - or with condition responsive... – By optical sensor
Reexamination Certificate
2000-07-11
2001-12-25
Rose, Robert A. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
By optical sensor
C451S044000
Reexamination Certificate
active
06332828
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of and an apparatus for mirror-like polishing a chamfer of a semiconductor single crystal wafer (hereinafter referred to as wafer having an orientation flat).
2. Description of the Prior Art
Wafer chamfer mirror-like polishing of a wafer comprising a semiconductor single crystal, is made for such purposes as preventing dust generation and coping with liquid pool when washing the wafer.
Such wafer, as shown in
FIG. 5
, has its periphery formed with an orientation flat part W
2
. At corners W
3
between the intrinsic peripheral part W
1
and orientation flat part W
2
, the curvature radius r
3
is very small, and in this locality the relative curvature radius with respect to the buffing wheel for mirror-like polishing the wafer
1
is extremely small compared to the other localities. Therefore, with a constant pressing pressure the contact pressure p is very high at the corners W
3
. In the meantime, when the wafer is rotating at a constant rotation number, the speed of movement of the point of contact between the wafer chamfer and the buffing wheel is greatly reduced at the corners, thus extending the process time at this locality. For the above reasons, the mirror-like polishing of the corners W
3
that is done under the same mirror-like polishing conditions (i.e., wafer rotation speed, pressing pressure between the buffing wheel and wafer, rotation speed of the buffing wheel, etc.) as for the intrinsic peripheral part W
1
and orientation flat part W
2
, results in excessive wear or wedging of the buffing wheel at the corners.
The capacity C of wafer chamfer mirror-like polishing is obtained from the following general approximation equation
C=a
1
pV
b
T
where a
1
is a constant (a
2
, . . . , a
n
appearing in the following being the same), p is the contact pressure, V
b
is the relative speed ∝N
b
(N
b
being the rotation speed of the buffing wheel), T is the contact time ∝1/N
S
(N
S
being the rotation speed of the wafer). Hence,
N
S
=a
2
pN
b
/C
As for p (approximated by two-circle contact between wafer circle and buffing wheel circle)
p=a
3
{F
(1
/R
1
+1
/R
2
)}
½
(F being the pressing pressure).
Hence,
N
S
=a
4
N
b
{F
(1
/R
1
+1
/R
2
)}
½
/C
Assuming that a
4
, N
b
, C and F are constant, we have
N
S
=a
5
{(1
/R
1
+1
/R
2
)}
½
where R
1
(diameter of the buffing wheel) is constant. Taking R
2
(diameter of the wafer) as a variable, relation as shown in Table 1 below is obtained in connection with the showing in FIG.
5
.
TABLE 1
Wafer
peripheral
position
W
2
W
1
W
3
R
2
Large (∞)
Medium (r
1
)
Small (r
3
)
N
s
Small
Medium
Large
When the pressing pressure F (Kgf) of the buffing wheel is constant, the area of contact between the wafer and the buffing wheel is small with a small relative curvature radius of the wafer and large with a large relative curvature radius.
It is thus possible to control the wafer chamfer mirror-like polishing capacity C through control of p, N
S
and N
b
noted above.
A technique of controlling the excessive wear of the corners of wafer through control of the contact pressure p between the wafer and buffing wheel while controlling the wafer chamfer mirror-like polishing capacity C, is shown by the applicant in Japanese Laid-Open Patent Publication No. 6-155263.
In this technique, when mirror-like polishing the wafer chamfer, the mirror-like polishing capacity C is made uniform for the orientation flat part, intrinsic peripheral part and corners by varying the pressing pressure between the wafer and buffing wheel according to a wafer position detection signal from wafer position detecting means, which makes a determination as to whether the wafer mirror-like polishing position corresponds to the orientation flat part, intrinsic peripheral part or corner.
The curvature radius of the corner is about 2 mm, and with an 8″ wafer (with a radius of about 100 mm) which has the orientation flat part W
2
as noted above, the processing time of the corner W
3
is usually reduced to a couple of seconds by setting the wafer rotation speed to about one minute per one round.
However, when the wafer mirror-like polishing position goes from intrinsic peripheral part W
1
to corner W
3
and from corner W
3
to orientation flat part W
2
, the mirror-like polishing capacity C is varied in these localities as shown by the solid plot in FIG.
4
(B) unless the pressing pressure is quickly raised and lowered. In the above technique of controlling the pressing pressure between the wafer and buffing wheel, the pressing pressure generating means employs an air cylinder which is inferior in the response property. Therefore, a response delay is generated as shown by the dashed plot in FIG.
4
(B). This frequently results in the occurrence of excessive mirror-like polishing or wedging into the buffing wheel particularly at the corner W
3
.
The follow-up property can be improved by using an oil hydraulic cylinder. In wafer mirror-like polishing, however, oil is undesired because it causes impurity introduction.
It is possible to control the rotation speed N
b
of the buffing wheel for the control of the mirror-like polishing capacity C. However, the buffing wheel is rotated at a high rotation number and has a high moment of inertia. The high momentum thus generated deteriorates the response property, so that it is difficult to obtain fine and accurate control.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method of and an apparatus for wafer chamfer mirror-like polishing, which permits satisfactory and accurate response in the chamfer mirror-like polishing control particular at the corners, can realize stable chamfer mirror-like polishing control with a relatively simple control system and can realize uniform chamfer mirror-like polishing for the corners, orientation flat part and intrinsic peripheral part.
The invention is predicated in the facts that the contact (or mirror-like polishing) time T corresponds to the wafer rotation speed N
S
and that the wafer rotation control provides for high response and high accuracy compared to the pressing pressure control or buffing wheel rotation speed control because the wafer rotation speed N
S
is low speed and has low mass of inertia.
The invention features a method of mirror-like polishing chamfer of a wafer having an orientation flat with a rotating buffing wheel pressed against the wafer chamfer with a predetermined pressure while rotating the wafer, wherein:
the wafer rotation speed N
S
is changed in correspondence to wafer mirror-like polishing positions of intrinsic peripheral part, corners and orientation flat part of the wafer according to detection signal from detecting means for detecting the wafer mirror-like polishing positions.
As a structure suitable for carrying out such a method, the invention features an apparatus for mirror-like polishing chamfer of a wafer having an orientation flat comprising a wafer rotating mechanism for rotating the wafer mounted thereon, a buffing wheel rotating mechanism for rotating a buffing wheel for mirror-like polishing the wafer, and a pressing mechanism for pressing the wafer and buffing wheel with a predetermined pressure, the rotating buffing wheel being pressed against the wafer chamfer with a predetermined pressure while the wafer is rotated by the wafer rotating mechanism, the apparatus further comprising:
a wafer mirror-like polishing position detector for detecting wafer mirror-like polishing positions; and
wafer rotation speed control means for controlling the wafer rotation speed N
S
according to a detection signal from the wafer mirror-like polishing position detector;
the wafer rotation speed N
S
is changed in correspondence to wafer mirror-like polishing positions of intrinsic peripheral part, corners and orientation flat part of the wafer according to detection signal from the wafer mirror-like polishing position detector.
The wafer ro
Hasegawa Fumihiko
Ichikawa Koichiro
Inada Yasuo
Kuroka Yasuyoshi
Tsuchiya Toshihiro
Crowell & Moring LLP
Rose Robert A.
Shin-Etsu Handotai & Co., Ltd.
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