Abrading – Precision device or process - or with condition responsive... – With indicating
Reexamination Certificate
2000-08-31
2003-06-03
Wilson, Lee (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
With indicating
C451S007000, C451S041000, C451S288000, C451S287000, C451S006000
Reexamination Certificate
active
06572444
ABSTRACT:
TECHNICAL FIELD
The present invention relates to apparatus and methods of automated wafer-grinding of semiconductor wafers, and more particularly to automatically grinding by monitoring a grinding parameter.
BACKGROUND OF THE INVENTION
The source material for manufacturing semiconductor chips is usually a relatively large wafer of silicon. Such wafers may be produced by slicing a silicon crystal ingot to a suitable thickness to obtain a number of nearly disk-shaped semiconductor wafers. Both surfaces of each wafer are subjected to abrasive machining, and then etched in a suitable mixed acid solution. One surface of each wafer is then polished to obtain a mirror surface. Circuits are fabricated in the mirror surface of the resulting semiconductor wafer by known processing steps, such as, for example, printing, etching, diffusion, or doping.
When the silicon wafers are sliced from the crystal ingot, the thickness of the wafers is usually greater than desirable for a finished integrated circuit product so as to provide a more robust wafer to stand up to the rigors of the integrated circuit fabrication process. Relatively thick silicon wafers may be necessary, for example, during certain integrated circuit fabrication steps to prevent warpage and breakage of the wafer as a result of heating, handling, and other circuit fabrication processes. Because the thickness of the wafer after the circuit fabrication process is usually greater than desirable for device packaging restrictions, it is typically necessary to grind a backside surface of the wafer opposite from the surface on which the integrated circuits are formed to reduce the wafer thickness.
Automated grinding machines for grinding the backside surfaces of wafers are known. Conventional grinding machines generally include a plurality of chuck tables that secure a plurality of wafers to be ground by one or more grinding wheels. A conventional grinding wheel typically includes a plurality of diamonds embedded in a resinous binder, with some of the diamonds exposed and some unexposed. As the grinding progresses, the exposed diamonds wear down to the level of the binder. The binder is selected to erode during grinding to expose fresh diamonds. The rate of wear of the grinding wheel may be dependent on the composition of the binder, the grinding rate, or other factors, as described more fully below.
FIG. 1
is a side elevational view of an automated grinding machine
10
for grinding a backside surface
25
of a wafer
12
in accordance with the prior art. The grinding machine
10
includes a spindle housing
14
disposed about a spindle
16
having a rotatable grinding shaft
18
. A grinding wheel
20
is rigidly secured to the end of the shaft
18
. A spindle motor
22
rotates the shaft
18
and the grinding wheel
20
at conventional speeds of 2400-3200 RPM during the grinding process, causing the grinding wheel
20
to grind away semiconductor material from the backside surface
25
of the wafer
12
. The spindle housing
14
is coupled to a feed mechanism
26
that allows the placement and the feed rate of the grinding wheel
20
to be adjusted relative to the wafer
14
to provide, for example, different grinding rates.
A controller
27
, such as a computer, is electrically connected to the grinding wheel
20
by electrical conductor
29
to receive feedback signals, and to a feed rate motor
31
by electrical conductor
33
to send control signals thereto. The controller
27
is also connected to a shaft speed sensor
19
by electrical conductor
35
, to a spindle motor current detector
21
by electrical conductor
37
, and to the spindle motor
22
by electrical conductor
23
. The wafer
12
is secured to a chuck table platform
30
of a chuck table
28
by a suitable securing mechanism, such as vacuum suction, with the front side of the wafer
12
that includes the integrated circuits positioned against the chuck table platform
30
. The chuck table platform
30
is secured to a shaft
32
which is driven by a chuck table motor (not shown) at conventional speeds of between 50-300 RPM.
FIG. 2
is a bottom plan view of the grinding wheel
20
of the grinding machine
10
of FIG.
1
.
FIG. 3
is a partial cross-sectional radial view of the grinding wheel
20
of FIG.
2
. As shown in
FIGS. 2 and 3
, the grinding wheel
20
includes a disk portion
40
and an annular shoulder
42
depending downwardly from the peripheral edge
41
of the disk portion
40
. The annular shoulder
42
includes a lower surface
47
. A plurality of cylindrical cavities
44
are formed in the lower surface
47
of the annular shoulder
42
and a cylindrical grinding tooth
46
is disposed in each cavity
44
. Each cavity
44
is connected to a central shaft-receiving bore
43
by a pressure signal transmission pathway
45
.
As best shown in
FIG. 3
, each grinding tooth
46
includes a body
48
having a first end
50
, which includes a grinding surface
24
, and a second end
52
. The second end
52
is disposed in the cavity
44
. A pressure sensor
54
is disposed in the cavity
44
between the second end
52
and the disk portion
40
. The pressure sensors
54
may include, for example, a piezoelectric element
60
that produces an electrical voltage when it is squeezed. Thus, the pressure sensor
54
may convert mechanical pressure on the grinding teeth
46
into an electrical signal, the strength of which increases or decreases with the pressure exerted by the grinding wheel
20
against the backside surface
25
of the wafer
12
. The grinding surface
24
may include a plurality of diamonds suspended in a resinous binder. As disclosed, for example, in U.S. Pat. No. 5,827,112 to Ball, incorporated herein by reference, the binder may be selected to be reactive with wheel dressing and to dissolve, either mechanically, or chemically or both. As the binder dissolves, the dull diamonds from the grinding surface
24
are released and washed away, leaving freshly exposed sharp diamonds.
The controller
27
may receive input signals from the pressure sensors
54
to indicate the pressure exerted by the grinding wheel
20
against the wafer
12
. The controller
27
may also receive input signals from the speed sensor
19
indicative of the rotational speed of the shaft
18
, and input signals from the current detector
21
which indicate the amount of current being drawn by the spindle motor
22
. Based on these input signals, the controller
27
may adjustably control various operating parameters of the automated grinding machine
10
, including, for example, the feed rate of the feed rate motor
31
, the rotational speed of the spindle motor
22
, or the release of wheel dressing for sharpening the grinding wheel
20
.
FIG. 4
is a schematic view of a typical grind recipe
80
of a grinding machine
10
in accordance with the prior art. During the grinding process shown in
FIG. 4
, the grinding wheel
20
descends along a z-axis as a function of time t (shown as the horizontal axis in FIG.
4
), allowing the grinding teeth
46
to grind away the backside surface
25
of the wafer
12
. During a first or “rapid descent” phase
82
, the grinding wheel
20
maintains a relatively high rate of descent between times t
0
and t
1
. During a second or “F
1
removal” phase
84
, the rate of descent of the grinding wheel
20
is decreased (typically 40 microns per minute) between times t
1
and t
2
. Finally, during a third or “F
2
removal” phase
86
, the rate of descent of the grinding wheel
20
is further decreased (typically 20 microns per minute) between times t
2
and t
3
. Thus, in the representative grind recipe
80
, the time required to remove a wafer layer of thickness z
0
-z
3
is the time t
3
-t
0
. The times t
1
, t
2
, and t
3
are typically selected to avoid stress cracks or other defects in the wafer
12
.
In addition to descent rate of the grinding wheel, other operating conditions of the grinding machine
10
may be varied during the phases
82
,
84
,
86
. For example, the rotational rate of the grinding wheel may be varied, or
Ball Michael B.
Cobbley Chad A.
Dorsey & Whitney LLP
Micro)n Technology, Inc.
Wilson Lee
LandOfFree
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