Method of and apparatus for mirror-like polishing wafer chamfer with orientation flat

A method for chamfer mirror-like polishing a wafer having an orientation flat by rotating the wafer in a state of being pressed by a rotating buffering wheel with a predetermined pressure, is disclosed. Mirror-surface polishing a stable wafer chamfer can be obtained with a relatively simple control system. The invention is predicated in the fact that the wafer rotation speed N.sub.s has low inertial mass and low rotation speed so that the wafer rotation speed control can be obtained with high response property and high accuracy compared to pressing pressure control and buffering wheel control, and it features detecting intrinsic peripheral part, corners and orientation flat part of wafer according to a detection signal of detection means for detecting the wafer mirror-like polishing position and controlling the wafer rotation speed N.sub.s according to the detected wafer mirror-like polishing position.

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.sub.2. At corners W.sub.3 between the intrinsic 
peripheral part W.sub.1 and orientation flat part W.sub.2, the curvature 
radius r.sub.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.sub.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.sub.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.sub.1 and orientation flat part W.sub.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 
EQU C=a.sub.1 pV.sub.b T 
where a.sub.1 is a constant (a.sub.2, . . . , a.sub.n appearing in the 
following being the same), p is the contact pressure, V.sub.b is the 
relative speed .varies.N.sub.b (N.sub.b being the rotation speed of the 
buffing wheel), T is the contact time .varies.1/N.sub.S (N.sub.S being the 
rotation speed of the wafer). Hence, 
EQU N.sub.S =a.sub.2 pN.sub.b /C 
As for p (approximated by two-circle contact between wafer circle and 
buffing wheel circle) 
EQU p=a.sub.3 {F(1/R1+1/R2)}.sup.1/2 (F being the pressing pressure). 
Hence, 
EQU N.sub.S =a.sub.4 N.sub.b {F(1/R.sub.1 +1/R.sub.2)}.sup.1/2 /C 
Assuming that a.sub.4, N.sub.b, C and F are constant, we have 
EQU N.sub.S =a.sub.5 {(1/R.sub.1 +1/R.sub.2)}.sup.1/2 
where R.sub.1 (diameter of the buffing wheel) is constant. Taking R.sub.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.sub.2 W.sub.1 W.sub.3 
______________________________________ 
R.sub.2 Large (.infin.) 
Medium (r.sub.1) 
Small (r.sub.3) 
N.sub.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.sub.S and N.sub.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.sub.2 as noted above, the processing time of the corner W.sub.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.sub.1 to corner W.sub.3 and from corner W.sub.3 to 
orientation flat part W.sub.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.sub.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 Nb 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.sub.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.sub.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.sub.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 
chamber 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.sub.S according to a detection signal from the wafer mirror-like 
polishing position detector; 
the wafer rotation speed N.sub.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 rotating mechanism may be a stepping motor. The wafer mirror-like 
polishing position detector may be a photo-sensor or the like, which is 
disposed at a position deviated from the mirror-like polishing position by 
a predetermined angle in the circumferential direction of the wafer to 
detect the intrinsic peripheral part, corners and orientation flat part of 
the wafer. This is by no means limitative, however; for instance, it is 
possible to use an angle detector, which detects the wafer rotation angle 
from a pulse output of a stepping motor. 
According to the invention having the above constitution, the wafer 
rotation speed N.sub.S is about one minute per one round, which is very 
low compared to the buffing wheel rotation speed N.sub.b. This means that 
it is possible to obtain accurate wafer rotation control without response 
delay by using a stepping motor or a pulse motor. Thus the wafer rotation 
speed N.sub.b can be quickly and accurately increased and reduced when the 
wafer mirror-like polishing position goes from intrinsic peripheral part 
W.sub.1 to corner W.sub.3 and from corner W.sub.3 to orientation flat part 
W.sub.2, and stable mirror-like polishing capacity C can be maintained 
over the entire wafer circumstance as shown in FIG. 4(A). 
According to the invention, a mirror-like polishing system thus can be 
provided, which is adapted to control the rotation of wafer with less 
inertial momentum and lower rotation speed, thus permitting wafer chamfer 
mirror-like polishing with superior response property and with a 
comparatively simple control system. 
In addition, according to the invention, in addition to the above effect, 
the wafer rotation speed N.sub.S is controlled by detecting the 
mirror-like polishing position of the wafer and providing correction 
according to the detected position. It is thus possible to obtain uniform 
speed chamfer mirror-like polishing of the intrinsic peripheral part, 
orientation flat part and corners of wafer. Particularly, it is possible 
to prevent excessive corner mirror-like polishing or buffing wheel wedging 
.

In the Figures, reference numeral 1 designates a wafer, 2 a buffing wheel, 
3 an air cylinder, 11 a wafer rotation speed sensor, 12 a buffing wheel 
rotation speed sensor, 13 a pressing pressure sensor, 14 a wafer 
mirror-like polishing position sensor, 20 a stepping motor, 100 a 
controller, 121 a wafer rotation speed setter, 122 a wafer rotation speed 
comparator, and 123 a wafer rotation speed calculator. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the invention will now be described in detail with 
reference to the drawings. It is to be construed that unless particularly 
specified, the sizes, materials, shapes and relative dispositions of 
described parts of the embodiments are not limitative but mere examples. 
FIG. 1 shows the structure of a wafer chamfer mirror-like polishing 
apparatus according to the invention. FIG. 2 is a view taken in the 
direction of arrow Z in FIG. 1. FIG. 3 is a block diagram illustrating 
control of a wafer drive stepping motor. FIG. 5 is a plan view showing a 
wafer to be chamfer mirror-like polished according to the invention. 
Referring to FIGS. 1 and 2, reference numeral 1 designates a wafer, which 
is set such that it is attracted to a suction board 21 secured to a wafer 
drive shaft 22. 
Reference 20 designates a stepping motor for step-by-step driving the wafer 
drive shaft 22. 
Reference numeral 4 is an arm, which has a central portion pivoted on a 
pivotal pin 23, one end fitted on the wafer drive shaft 22 and the other 
end capable of being contacted by a piston rod 3a of an air cylinder 3 to 
be described later. 
The air cylinder 3 is operable by operating air from a change-over valve 7. 
Its piston rod 3a and one end in contact with the corresponding end face 
of the arm 4. 
When the air cylinder 3 is operated to push the arm end with the piston rod 
3a, the arm 4 is pivoted about the pivotal pin 23 in the direction of 
arrow Y in FIG. 2 to generate a pressing pressure F between a buffing 
wheel 2 to be described later and the wafer 1. 
The buffing wheel 2 is for chamfer mirror-like polishing the wafer 1. It is 
driven for rotation at a rotation speed N.sub.b from a motor 6 via a shaft 
5. 
Reference numeral 11 is a wafer rotation speed sensor for detecting the 
rotation speed N.sub.S of the wafer drive shaft 22 (i.e., the rotation 
speed of the stepping motor 20). Reference numeral 12 designates a buffing 
wheel rotation speed sensor for detecting the rotation speed of the 
buffing wheel drive shaft 5. 
The wafer 1 has a shape as shown in FIG. 5, having an intrinsic peripheral 
part W.sub.1 with radius r.sub.1, an orientation flat part W.sub.2 formed 
as a flat notch, and corners W.sub.3 with radius r.sub.3 between the 
intrinsic peripheral part and orientation flat part. 
Reference numeral 14 designates a photo-sensor serving as a wafer 
mirror-like polishing position sensor, which detects the mirror-like 
polishing position of the wafer having the shape as described above and 
provides a detection signal as its input to a controller 100 to be 
described later. The photo-sensor 14 is disposed at a position deviated 
from the mirror-like polishing position by a predetermined angle in the 
circumferential direction of the wafer. It can detect the intrinsic 
peripheral part, corners and orientation flat part of wafer. 
Reference numeral 13 is a pressing pressure sensor for detecting the 
operating air pressure in the air cylinder 3, i.e., pressing pressure 
between the buffing wheel 2 and the wafer 1. 
The controller 100 receives data of the operating air pressure in the air 
cylinder 3, i.e., the pressing pressure F between the wafer 1 and buffing 
wheel 2, from the pressing pressure sensor 13, data of the buffing wheel 
rotation speed N.sub.b from the buffing wheel rotation speed sensor 12, 
data of the rotation speed of the stepping motor 20, i.e., the wafer 
rotation speed N.sub.S, from the wafer rotation speed sensor 11, and data 
of the mirror-like polishing position of the wafer from the wafer 
mirror-like polishing position sensor 14, and it calculates the rotation 
speed of the stepping motor 20 by a method to be described later, the 
calculated data being outputted to the stepping motor 20. 
Wafer rotation speed control means according to the invention will now be 
described. 
The controller 100, as shown in the block diagram of FIG. 3, includes a 
mirror-like polishing position judging unit 125, a wafer rotation speed 
setter 121, a wafer rotation speed comparator 122 and a wafer rotation 
speed calculator 123. 
The wafer rotation speed setter 121 sets a reference wafer rotation speed 
N.sub.O (i.e., a rotation speed of the wafer periphery) from the pressing 
pressure F between the wafer 1 and buffing wheel 2 as detected by the 
pressing Pressure sensor 13 and the buffing wheel rotation speed N.sub.b 
as detected by the buffing wheel rotation speed sensor 12 by a method to 
be described later. 
The wafer rotation speed comparator 122 calculates the difference .DELTA.N 
between the reference wafer rotation speed N.sub.O and the detected wafer 
rotation speed N.sub.W of wafer 1. 
The mirror-like polishing position judging unit 125 calculates the wafer 
mirror-like polishing position from a detection signal X.sub.W inputted 
from the wafer mirror-like polishing position sensor 14 to judge that the 
intrinsic peripheral part W.sub.1, orientation flat part W.sub.2 or corner 
W.sub.3 is at the mirror-like polishing position, and sends out a judgment 
signal representing the wafer mirror-like polishing position (i.e., the 
intrinsic peripheral part SW.sub.1, orientation flat part SW.sub.2 or 
corner SW.sub.3) to the wafer rotation speed calculator 123. 
The wafer rotation speed calculator 123 has a memory 123a , in which 
predetermined correction values are stored. It reads out correction value 
data SW from the memory according to the judgment signal noted above 
(representing the intrinsic peripheral part SW.sub.1, orientation flat 
part SW.sub.2 or corner SW.sub.3) and calculates a corrected wafer 
rotation speed N.sub.S after the following formula, the calculated data 
being outputted to the stepping motor 20. 
EQU N.sub.S =N.sub.O (1+SW) (1) 
SW: SW.sub.1 =0, SW.sub.2 =-0.3, SW.sub.3 =+0.7. 
The operation of the wafer chamfer mirror-like polishing apparatus having 
the above constitution will now be described. 
The pressing pressure sensor 13 detects the operating air pressure pa in 
the air cylinder 3, and calculates the pressing pressure F between the 
wafer 1 and buffing wheel 2 from the arm ratio of the arm 4, sectional 
area of the air cylinder 3, etc., the calculated data being inputted to 
the wafer rotation speed setter 121. 
The reference wafer rotation speed N.sub.O which is a basis in the above 
equation (1) is 
EQU N.sub.O =a.sub.6 N.sub.b F.sup.1/2/ C (2) 
The wafer rotation speed setter 121 thus calculates the reference wafer 
rotation speed N.sub.O corresponding to the inputted detected pressing 
pressure F and detected buffing wheel rotation speed N.sub.b from F, 
N.sub.b and desired mirror-like polishing capacity C using equation (2), 
the calculated data being inputted to the rotation speed controller 122. 
The rotation speed controller 122 calculates the difference .DELTA.N, i.e., 
(N.sub.O -N.sub.W), between the desired reference wafer rotation speed 
N.sub.O and the detected wafer rotation speed N.sub.W inputted from the 
wafer rotation speed sensor 11, the calculated data being inputted to the 
wafer rotation speed calculator 123. 
The wafer mirror-like polishing position sensor 14 may, for instance, use a 
photo-sensor. 
When the intrinsic peripheral part W1 is passing by the photo-sensor, light 
from a light emitter 14a is blocked by the part W.sub.1 and does not reach 
a light receiver 14b. When the orientation flat part W.sub.2 is passing by 
the photo-sensor, on the other hand, light from the light emitter 14a 
reaches the light receiver 14b. The photo-sensor as the wafer mirror-like 
polishing position sensor 14 thus detects the orientation flat part 
W.sub.2 from light received by the light receiver 14b. 
The corner W.sub.3 is detected as locality corresponding to the instant of 
switching from the state, in which light is blocked, over to the state, in 
which light is received. 
The wafer position detection signal X.sub.W which is obtained during the 
mirror-like polishing of wafer in the above way, is inputted via the wafer 
mirror-like polishing position judging unit 125 to the wafer rotation 
speed calculator 123. 
The wafer rotation speed calculator 123 takes out correction value data 
from the memory 123a according to SW.sub.1, SW.sub.2 or SW.sub.3 judgment 
signal, and calculates the wafer rotation speed N.sub.S according to the 
taken-out correction data using the equation (1) 
EQU N.sub.S =N.sub.O (1+SW). 
When SW is, for instance, SW.sub.1 =0, SW.sub.2 =-0.3 and SW.sub.3 =+0.7, 
the wafer rotation speed N.sub.S is reduced to 0.7 N.sub.O when the wafer 
mirror-like polishing position detection signal represents the orientation 
flat part W.sub.2, when the signal represents the intrinsic peripheral 
part W.sub.1, the speed N.sub.S can be corrected to just N.sub.O and 
maximized to 1.7 N.sub.O for the corners W.sub.3. 
The wafer rotation speed N.sub.S which is thus corrected is as shown in 
FIG. 4(A). This wafer rotation speed N.sub.S is set so that the stepping 
motor 20 is driven at this speed. 
FIGS. 4(A) and 4(B) compare the response in wafer mirror-like polishing 
according to the invention and that in the prior art. 
FIG. 4(B) shows an example of control of the pressing force between the 
buffing wheel and wafer in the prior art. In this case, a response delay 
is generated as shown by the broken plot. According to the invention, as 
shown in FIG. 4(A), owing to the above control of the wafer rotation speed 
N.sub.S, the response delay is hardly generated, and high response 
characteristic can be ensured.