Polishing method for SOI

In a dielectric isolation substrate, an end point of a polishing process for selective polishing for forming an SOI layer is detected with a high precision. When polishing a wafer with a polishing pad, the temperature of a region of the polishing pad having polished the wafer at a position immediately thereafter is detected by a temperature sensor and the selective polishing process is ended by discriminating that the rate of variation in the detected temperature has changed from a positive to a negative state and then to a fixed saturated state.

CROSS REFERENCE TO RELATED APPLICATION 
This application is based upon and claims the benefit of priority of the 
prior Japanese Patent Applications No. 6-319832 filed on Dec. 22, 1994, 
No. 7-286171 filed on Nov. 2, 1995, the contents of which are incorporated 
herein by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a polishing method for polishing an SOI 
(Silicon On Insulator) substrate or the like and a method for 
manufacturing a semiconductor substrate by use of this polishing method 
and, more particularly, to a substrate bonding technique and a method for 
manufacturing a thin film SOI substrate by use of selective polishing. 
2. Related Arts 
As a manufacturing method for manufacturing an SOI substrate, a method of 
bonding two silicon substrates to each other with an insulator film such 
as a silicon dioxide film interposed therebetween and polishing one of 
these substrates down to a prescribed thickness from the surface thereof 
to thereby form an SOI layer is known. However, there has been the problem 
that when attempting to form a thin SOI layer 0.5 .mu.m or less in 
thickness, the variation in the thickness of SOI layer increases because 
of the limitations in polishing precision. As a result, a desired 
semiconductor device can not be formed. 
Meanwhile, as a method for forming such a thin SOI layer with excellent 
thickness precision, the method illustrated in, for example, FIGS. 17A to 
17C is known. 
To explain the outline of this method, after forming a recess 2 in a mirror 
surface 1a of a first semiconductor substrate (single crystal silicon) 1, 
a silicon dioxide insulator film 3 is deposited and subsequently a 
polycrystalline silicon 4 is deposited (FIG. 17A). Thereafter, the 
polycrystalline silicon 4 is polished and planarized and a second 
semiconductor substrate (single crystal silicon) 5 is bonded to the 
polished surface thereof (FIG. 17B). Then, the other surface of the first 
semiconductor substrate 1 is ground and polished. During this polishing 
process, wherein the silicon dioxide film 3 formed in the recess 2 is made 
to function as a polishing stopper, i.e. selective polishing, whereby a 
thin film SOI substrate with minimum variation in the thickness of the SOI 
layer 6 can be obtained (FIG. 17C). 
In this method, since selective polishing is performed by causing the 
insulator film 3 such as a silicon dioxide film to function as a stopper, 
the precision of and variation in the thickness of the SOI layer 6 can be 
improved compared to ordinary polishing performed without using a stopper. 
However, although the polishing rate of the insulator film 3 as a stopper 
is low when compared with that of silicon, since the insulator film 3 and 
silicon are simultaneously polished, as the polishing time increases after 
the insulator film 3 has been exposed, the thickness of the SOI film 6 
becomes less than the desired thickness. Also, the longer the polishing 
time period, the more the polishing unevenness of the SOI layer 6 occurs 
with the result that the variation in thickness thereof increases. The 
larger the area of the SOI layer 6 becomes, the more prominent this 
tendency. 
Accordingly, it is necessary to end the polishing process when in an entire 
surface of the substrate all the portions of the insulator film 3 as a 
stopper have been exposed on the surface thereof for the purpose of 
decreasing the variation in thickness of the SOI layer 6. 
However, a polished state thereof, that is, an exposed state of the 
insulator film 3, cannot be confirmed by directly observing the polished 
surface during the polishing process. This requires the performance of 
repeated polishing operations in such a manner that after having performed 
polishing for a short period of time, the substrate is removed from the 
polishing device and the polished state thereof confirmed, then the 
substrate is polished again and the polished state thereof is reconfirmed. 
This raises the problem of the necessity of a great deal of time and 
labor. 
Also, even when polishing materials other than the above-mentioned SOI 
substrate, selective polishing for polishing a material to be polished by 
using as a stopper a material whose polishing rate is lower than the 
material to be polished incurs the problem that it is similarly impossible 
to detect an end point of the polishing process. 
The following examples can be taken as such selective polishing. 
FIGS. 18A to 18C and FIGS. 19A to 19C illustrate examples of a method for 
dielectric isolation of a semiconductor substrate 40. In the case of 
forming a dielectric isolation region by forming a groove portion (trench) 
40a in the semiconductor substrate 40 and filling an insulator material 
therein, selective polishing of a silicon dioxide film 42 or deposited 
material 421 is performed using as a stopper a nitride film 41 formed on 
the surface of the substrate prior to filling. In some cases, a metal film 
43 is used as the stopper. 
FIGS. 20A to 20B illustrate an example of a wiring technique for wiring 
semiconductor devices. A groove portion or bore portion (recess portion) 
51 is formed in an insulator film 52 and a metal film 53 such as Cu or W 
is embedded therein. Thereafter, selective polishing is performed on the 
metal film 53 by using the insulator film 52 as a stopper. 
Further, in addition to the above-mentioned case of selective polishing, 
where planarizing the surface of an interlayer dielectric film by 
polishing as a multi-layer wiring technique for a semiconductor substrate, 
the problem it is impossible to detect the end point of the polishing 
process. Namely, as illustrated in FIG. 21A, when depositing an insulator 
film (silicon dioxide film) 62 over a wiring pattern 61 on a semiconductor 
substrate (silicon) 60, irregularities occur on the surface of the 
insulator film 62. Even when this surface of the insulator film 62 is 
planarized by polishing in the process step of FIG. 21B, it is difficult 
to detect an end point (a point in time at which the planarization of the 
surface to be polished as a whole has been completed) of the polishing 
process. Although the polishing process is at present performed through 
time management, since the polishing rate varies due to, for example, how 
worn the polishing pad is, there is the problem that stable planarization 
by means of a polishing process is impossible. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above-mentioned problems 
and an object thereof is to perform the end point detection of a polishing 
process when the above-mentioned polishing processes are performed. 
The inventors of the present invention have experimentally discovered that 
the behavior of variation in temperature of a polishing pad during a 
polishing process differs according to the kind of a material to be 
polished, and have conceived a method for performing the end point 
detection of a polishing process by utilizing the variation in temperature 
of the polishing pad. Namely, the present invention is characterized in 
that in a polishing process performed with the use of a polishing pad, the 
temperature of a portion of this polishing pad having polished a surface 
to be polished at a point in time immediately thereafter is measured and 
end point detection of the polishing process is performed according to 
variations in the temperature thus measured. 
Accordingly, it is possible to perform end point detection of a polishing 
process without performing the operations of, for example, detaching the 
substrate from the polishing machine and confirming the polished state 
thereof during the polishing process. 
In this case, since a portion of the polishing pad having polished a 
surface to be polished is measured at a point in time immediately 
thereafter, it is possible to perform end point detection of the polishing 
process with high precision without the effect of temperature variations 
at different points of the polishing pad.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS 
An explanation will now be given of a method of detecting the end point of 
polishing by variations in temperature of a polishing pad during a 
polishing process, on the basis of the results of experiments performed by 
the present inventors. 
FIG. 2A illustrates the construction of a polishing device used when 
selective polishing is performed. FIG. 2B illustrates a state where a 
wafer is polished with the use of the polishing device. 
In FIG. 2A, a polishing pad 12 is adhered onto a turn table 11. Also, a 
wafer 10 with respect to which selective polishing is to be performed is 
mounted on a chuck table 13, and the resulting wafer is mounted on the 
polishing pad 12. The turn table 11 and chuck table 13 are rotated (the 
chuck table 13 is rotated by means of a drive unit not illustrated) while 
dropping a polishing slurry 14 onto a position 12a of FIG. 2B at which the 
polishing slurry is to dropped, whereby the wafer 10 is polished by the 
polishing pad 12. 
The present inventors performed their experiment on the temperature 
variations of the polishing pad 12 when the polishing process was 
performed with the use of this polishing device. 
Polishing slurry `Nalco 2350 (trade name)` which was made to have a 
grinding particle density of 0.05 wt % and a temperature of 25.degree. C. 
was used as the polishing slurry 14. Also, `Suba 600 (trade name)` was 
used as the polishing pad 12 and was adhered to the turn table 11, the 
temperature of which was adjusted to 20.degree. C., to perform the 
polishing process. The speed of rotation of the turn table 1 was set at 60 
rpm. Note that the polishing slurry 14 used in this experiment was made to 
have a grinding particle density lower than that of a polishing slurry 
used in ordinary polishing. This is for the purpose of allowing a silicon 
dioxide film 3 to function as a stopper by decreasing the polishing rate 
thereof and preventing the occurrence of polishing unevenness of an SOI 
layer 6 by decreasing the polishing rate of the silicon. 
The temperature of the polishing pad 12 during selective polishing is 
detected at a portion of the polishing pad 12 where the wafer 10 has been 
polished, that is, at a portion (a temperature measuring point 12b of FIG. 
2B) of the polishing pad 12 immediately after contact thereof with the 
wafer 10. Note that in a case where selective polishing is performed while 
the position of the wafer 10 on the polishing pad 12 is being moved 
relative to the polishing pad 12, the position of the temperature 
measuring point 12b is moved accordingly. In order to perform the 
above-mentioned temperature detection, a non-contact type temperature 
sensor is used which measures the radiation energy at the temperature 
measuring point in a non-contact manner and converts it to a temperature 
measurement. 
Also, in this experiment, the thickness of the silicon layer on the silicon 
dioxide film 3 which is to become the stopper prior to start of selective 
polishing is made to be 0.8 .mu.m. 
The result of this experiment is shown in FIG. 3. As shown in FIG. 3, the 
temperature of 20.5.degree. C. when polishing is started rises up to 
23.3.degree. C. after 15 minutes and thereafter does not vary (region A). 
In this state, the silicon dioxide film 3 as a stopper is not exposed and 
the silicon is polished over the entire surface of the substrate (the 
state illustrated in FIGS. 4A and 5A). Thereafter, after 15 minutes, the 
temperature drops, that is, the rate of change in temperature relative to 
the lapse of time becomes negative (region B). This indicates that, as 
illustrated in FIGS. 4B and 5B, part of the silicon dioxide film begins to 
be exposed. Then, after 40 minutes, the temperature becomes unvaried at a 
fixed value, that is, the rate of temperature change falls within a 
prescribed range around zero (region C). This indicates that the silicon 
dioxide film has become exposed over the entire surface of the wafer 
(FIGS. 4C and 5C). This fixed point becomes a terminal or end point of the 
selective polishing, that is, a point in time at which the selective 
polishing is to be terminated. Note that FIGS. 4A to 4C illustrate 
sectional states of the wafer corresponding to the regions A, B and C 
(FIG. 3) and FIGS. 5A to 5C illustrate upper surface states of the wafer 
corresponding to FIGS. 4A to 4C. 
It can be proven from the following experiment as well that when the 
silicon dioxide film becomes exposed on the surface of the wafer, the 
temperature of the polishing pad decreases. A non-processed silicon wafer 
having no silicon dioxide film formed on its surface and a wafer having a 
silicon dioxide film formed on its surface by thermal oxidation were 
prepared, and polishing was performed under the same conditions as in the 
case of the above-mentioned experiment. The results are shown in FIG. 6. 
When the non-processed silicon wafer is polished, as indicated by a solid 
line in FIG. 6, the temperature of the polishing pad rises to 23.degree. 
C. and then becomes unvaried. This state of temperature corresponds to one 
in the region A of FIG. 3. On the other hand, when the wafer having a 
silicon dioxide film formed on its surface is polished, as indicated by a 
broken line in FIG. 6, the temperature of the polishing pad becomes 
unvaried at 21.degree. C. which is lower than in the case of the 
non-processed silicon wafer. This state of temperature approximately 
corresponds to one in the region C of FIG. 3. Note that in region C of 
FIG. 3 the surface of the wafer is not entirely the silicon dioxide film 
and silicon regions (SOI regions) co-exist thereon. 
It can be seen from the results of this experiment that even if polishing 
is performed under the same conditions, the temperature of the polishing 
pad during the polishing process exhibits the difference between polishing 
silicon and polishing silicon dioxide film and is higher by around 
2.degree. C. in the polishing of silicon than in the polishing of silicon 
dioxide. The reason for this is as follows. The friction heat generated 
between the polishing pad and the surface (silicon, oxidized film) to be 
polished differs in the case of polishing silicon and in the case of 
polishing the silicon dioxide film. In addition, the polishing speed of 
oxidized film is much lower than that of silicon. Therefore, the reaction 
heat generated between the polishing slurry and the surface to be polished 
differs between when silicon is polished and when silicon dioxide film is 
polished (the reaction heat when silicon is polished is greater). 
Accordingly, it is possible to detect the end point of selective polishing 
by utilizing the temperature difference of the polishing pad 12. Namely, 
in the case of selective polishing, when the silicon dioxide film begins 
to be exposed, the temperature of the polishing pad begins to decrease. 
When the silicon dioxide film has become exposed over the entire surface 
of the wafer, the temperature of the polishing pad becomes fixed. The end 
point of polishing can be determined by this temperature fixation. In 
other words, when the temperature of the polishing pad is in a saturated 
state, the amount of heat generated during polishing is also in a 
saturated state. 
Also, even when a selective polishing technique used in the dielectric 
isolation technique for device isolation of a semiconductor substrate 
illustrated in FIGS. 18A to 18C and FIGS. 19A to 19C and a selective 
polishing technique used in the wiring technique illustrated in FIGS. 20A 
and 20B are performed on the same principle as in the case of the 
above-mentioned selective polishing of SOI substrate, that is, a material 
to be polished (metal, insulator film, silicon film) or the like is 
polished by using as a stopper a material (metal, insulator film, or the 
like) whose polishing rate is lower than that of the material to be 
polished, it is possible to detect an end point of polishing by the 
variation in temperature of the polishing pad in the same manner as in the 
case of the above-mentioned selective polishing of an SOI substrate. 
Also, even when in the multi-layer wiring technique for a semiconductor 
substrate illustrated in FIGS. 21A and 21B the irregularities of an 
interlayer dielectric film are planarized by a polishing process, it is 
possible to perform an end point detection of polishing by the variation 
in temperature of the polishing pad during the polishing process. 
In this case, although no stopper exists, in a case where irregularities, 
or recessed and protruding portions, exist on a surface to be polished, in 
the polishing process the protruding portions are selectively polished. 
This is because the protrusions more strongly contact the polishing pad 
than the recessed portions. In an initial stage of polishing during 
execution of the multi-layer wiring technique, the surface of an 
interlayer dielectric film is in a state having recessed and protruding 
portions and therefore the protruding portions more strongly contact the 
polishing pad and the recessed portions are less likely to contact it. 
When compared to a plane surface to be polished which has no 
irregularities (this state corresponding to an end point of polishing), 
the surface of the insulator film constituting a surface to be polished 
reacting with the polishing pad via a polishing slurry is small in area 
and less reaction heat is also generated in an initial stage of polishing. 
However, due to variations in degree of level of the recessed and 
protruding portions occurring when the polishing process proceeds, the 
reaction area also varies with the result that the amount of reaction heat 
generated varies. Also, the amount of heat generated due to friction 
between the surface to be polished and the polishing pad also varies 
according to the degree to which the recessed and protruding portions are 
levelled as the polishing process proceeds. Namely, since the degree of 
level of the recessed and protruding portions becomes less as the 
polishing process proceeds, the reaction area between the surface to be 
polished and the polishing pad increases with the result that the reaction 
heat and also the friction heat vary. 
As mentioned above, since the amount of heat generated varies as the 
duration at which the polishing process proceeds, the temperature of the 
polishing pad also varies. When the recessed and protruding portions are 
levelled, with the result that the surface to be polished uniformly 
contacts the polishing pad, the amount of heat generated becomes fixed, 
with the result that the temperature of the polishing pad is saturated. 
The present invention has been achieved on the basis of the above-mentioned 
various experiments and considerations performed and completed by the 
present inventors. 
More specifically, the present invention is characterized in that, in a 
polishing process using a polishing pad, the temperature of a portion of 
the polishing pad having polished a surface is measured at a position 
immediately after the performance of this polishing operation and end 
point detection of a polishing process is performed on the basis of 
variations in the temperature thus measured. 
Accordingly, it is possible to detect an end point of a polishing process 
without performing an operation of, for example, confirming a polished 
state of the substrate during the polishing process by detaching this 
substrate. 
In this case, since the temperature of a portion of polishing pad after 
polishing a surface at a position immediately after the performance of 
this polishing operation is measured, it is possible to detect the end 
point of the polishing process with high precision without the effect of 
variations in temperature of different portions of the polishing pad. 
Also, the present invention is characterized in that an end point of a 
polishing process is detected utilizing the relationship between a total 
amount of heat calculated from the temperature of a polishing pad and the 
time duration of polishing, that is, the total amount of heat generated by 
polishing as measured from a time when polishing has been started, and the 
volume of a portion of a material to be polished which has been removed by 
selective polishing. 
By utilizing the correspondence relationship between the total amount of 
heat as measured from the time when polishing is started and the volume of 
a material to be polished, it is possible to detect an end point of the 
polishing process with high precision irrespective of the conditions under 
which polishing is performed. 
Also, the present invention is characterized in that adjustment of 
temperature is performed so that the temperature of the turntable on which 
the polishing pad is mounted becomes fixed. 
When adjustment of the temperature of the turn table is not performed, it 
becomes less likely that the temperature (or amount of heat generated) of 
the polishing pad during a polishing process will become saturated. 
However, by adjusting of the temperature of the turntable, the temperature 
(or amount of heat generated) thereof can be saturated, whereby it is 
possible to detect the end point of the polishing process with precision. 
Further, the present invention is characterized in that a polishing process 
is performed while the polishing position is being swung in the radial 
direction of the pad. 
When the polishing position is fixed, the temperature of the polishing pad 
continues to rise as the polishing process proceeds, with the result that 
the polishing precision decreases due to, for example, variation in the 
polishing rate. However, by swinging the polishing position, it is 
possible to improve polishing precision. Even in a polishing process 
wherein the polishing position is swung, by measuring the temperature of a 
portion of the polishing pad having polished at a position immediately 
after the performance of this polishing operation, it is possible to 
detect the end point of the polishing process with precision. In this 
case, if the temperature detecting means is also moved simultaneously with 
the movement of the polishing position in the same manner, the position of 
a temperature measuring point can be set at all times to a position 
immediately after a portion of the polishing pad where polishing has been 
performed. 
Also, in such a polishing process wherein the polishing position is swung, 
if adjustment of the temperature of the turntable is made, it is possible 
to detect the end point of the polishing process with higher precision 
because the reference polishing pad temperature becomes fixed even if the 
temperature measuring position varies. 
The present invention will now be explained on the basis of the embodiment 
illustrated in the relevant figures. 
FIGS. 1A to 1G illustrate process steps for manufacturing an SOI substrate 
in this embodiment mode. 
First, as illustrated in FIG. 1A, at least one surface of a first 
semiconductor substrate 1 consisting of a single crystal silicon is 
polished to a mirror surface 1a. Part of this mirror surface 1a is 
selectively etched by wet chemical etching or dry etching such as reactive 
ion etching (RIE) to thereby form a recessed portion 2. The depth of the 
recessed portion 2 is from, for example, 0.01 to 1 .mu.m and this recessed 
portion 2 is formed as a scribing area or dielectric isolation region. 
Next, a silicon dioxide film (insulator film) 3 having a thickness of, for 
example, 0.1 .mu.m to 5 .mu.m is formed by thermal oxidation, CVD or the 
like. Note that the insulator film 3 need not be a silicon dioxide film 
but may be a silicon nitride film formed with the use of a CVD method or 
the like. 
Next, as illustrated in FIG. 1B, a polycrystalline silicon 4 is deposited, 
for example, to 3 .mu.m or more with the use of an LPCVD method or the 
like. Next, as illustrated in FIG. 1C, the polycrystalline silicon 4 is 
polished and planarized to thereby remove depressions and protrusions 
therefrom and form a mirror surface 4a. 
Next, as illustrated in FIG. 1D, a mirror surface 5a of a second 
semiconductor substrate 5, consisting of a single crystal silicon and 
which has at least one surface polished to a mirror surface, and the 
mirror surface 4a of the above-mentioned polycrystalline silicon are 
bonded to each other by using a known wafer bonding technique. 
Next, as illustrated in FIG. 1E, the first semiconductor substrate 1 is 
ground at a surface 1b side thereof and then, as illustrated in FIG. 1F, 
is polished. The polishing performed at this point in time may be one 
which is ordinarily performed because it is for the purpose of eliminating 
surface depressions and protrusions and the crushed layer by grinding. 
In this polishing process step of FIG. 1F, 2 to 5 .mu.m of the silicon 1 is 
eliminated by ordinary polishing. In this polishing process, in order to 
lessen the amount of silicon eliminated by selective polishing to the 
largest possible extent, polishing is performed up to a point in time 
immediately before the silicon dioxide film 3 as a stopper is exposed, for 
example, so that the thickness of the silicon on the silicon dioxide film 
3 which is to be a stopper becomes 1 .mu.m or less. Also, it is preferable 
that polishing variation within the substrate be minimized. 
The reason for this is as follows. Namely, when selective polishing is 
performed, the polishing rate for silicon is made lower than that when 
ordinary polishing is performed, and therefore it is intended to shorten 
the time period in which selective polishing is performed. Also, while a 
point in time when the silicon dioxide film 3 as a stopper has been 
exposed on the surface of the substrate is the end point of polishing, it 
serves to increase the precision of end point detection and also decrease 
the variation in silicon thickness if all portions of the silicon dioxide 
film 3 are exposed as simultaneously as possible. 
Next, as illustrated in FIG. 1G, the silicon dioxide film 3 formed in the 
recessed portion 2 is exposed on the 1b side surface of the substrate to 
thereby form an SOI layer 6. At this time, the polishing process is 
performed by selective polishing wherein the silicon dioxide film 3 is 
made to function as a stopper. 
Thereafter, although not illustrated, a MOSFET (semiconductor device) is 
formed in the SOI layer 6 of the semiconductor substrate manufactured 
through the performance of the above-mentioned process steps by use of a 
general MOSFET manufacturing technique. 
This embodiment is characterized in that in the above-mentioned selective 
polishing, an end point of detection of the polishing process is performed 
by monitoring variations in the temperature of the polishing pad used 
therein. 
A detailed construction of the polishing device is illustrated in FIG. 7. 
Also, a portion where polishing is performed as viewed from a side thereof 
is illustrated in FIG. 8. 
In FIG. 7, the turntable 11 has the polishing pad 12 mounted on its surface 
and is rotated at a prescribed speed. This turntable 11 is adjusted to a 
prescribed temperature (for example, 20.degree. C.) by a temperature 
control unit 15. Namely, water (temperature-controlled water) is 
circulated as illustrated from the temperature control unit 15 through a 
groove (passage) provided in the turntable 11, whereby the temperature of 
the turntable 1 is maintained. 
Also, the chuck table 13 for retaining the wafer 10 has a retention surface 
which opposes the turntable 11 and on which the wafer 10 is fixed, and is 
rotated at a prescribed speed. A drive unit 16 rotatably retains the chuck 
table 13, presses the wafer 10 fixed on the retention surface of the chuck 
table 13 against the turntable 11, and swings the chuck table 13 in the 
radial direction (radial direction of the turntable 11) of the polishing 
pad 12 as indicated by the arrow mark in the figure. Also, a polishing 
slurry and deionized water are dropped onto the polishing pad 12 by a 
polishing slurry supply system 17 and a deionized water supply system 18. 
Also, a temperature sensor 19 serving as temperature detecting means is 
mounted on the drive unit 16 and detects in non-contact manner the 
temperature of a temperature measuring point 12b, that is, the temperature 
of portion of the polishing pad 12 (the portion thereof at a position 
immediately after polishing on a downstream side of rotation from a 
portion of the polishing pad against which the wafer 10 has been pressed) 
at a position immediately after contact thereof with the wafer 10 during 
the selective polishing process. As mentioned above, the temperature 
sensor 19 is fixed to the drive unit 16 and is swung jointly with the 
swinging motion thereof. Preferably, the temperature sensor 19 is fixed to 
the drive unit 16 on the downstream side of rotation of the turn table 11. 
Also, more preferably, the temperature sensor 19 is fixed so that the 
temperature of the polishing pad within a swing range of the chuck table 
13 is detected. Preferably, the temperature measuring point 12b is set to 
a position of the polishing pad 12 immediately after polishing, that is, a 
substantially central position of the swing range of the chuck table 13 on 
the downstream side of rotation from a portion of the polishing pad 12 
where the wafer 10 has been pressed. 
A control system 20 rotates the turntable by a drive means, not 
illustrated, during the polishing process, and controls the driving 
operation of the drive unit 16 to thereby rotate the chuck table 13 
jointly with the wafer 10, and simultaneously swings this chuck table 13 
in the radial direction of the polishing pad 12. It also adjusts 
pressurization of the wafer 10, and causes supply of the polishing slurry 
and deionized water and the lift motion of the chuck table 13. 
This control system 20 controls the end of the polishing process on the 
basis of the temperature detected by the temperature sensor 19. 
FIG. 9 illustrates a schematic construction of a portion where control of 
the end of polishing process is performed. 
A temperature signal obtained by detection made by the temperature sensor 
19 is input to a measuring device 20a using a microcomputer or the like. 
The measuring device 20a performs a control for selective polishing on the 
basis of the input temperature signal. Examples of the control for 
selective polishing will be explained hereafter. 
A first example of control for selective polishing is illustrated in FIG. 
10. 
First, a temperature signal from the temperature sensor 19 is input at 
prescribed time intervals (step 100), the rate of variation in the 
temperature being calculated on the basis of this temperature signal (step 
101). On the basis of this rate of variation in temperature, whether or 
not the temperature has shifted from a state of the region A illustrated 
in FIG. 3 to a state of the region B and further to a state of the region 
C is determined (step 102). Namely, it is determined whether or not the 
rate of variation in temperature has shifted from a positive state to a 
negative state (region A-region B) and then to a saturated state where the 
rate of variation in temperature is below a prescribed value of around 0. 
When it has been determined from the temperature that the polishing state 
has shifted to the region C, the operation is placed in standby for a 
prescribed period of time (for several minutes) (step 103). When a 
prescribed period of time has lapsed, a polishing-process stopping process 
is performed (step 104) as the end point of the selective polishing. 
This polishing-process stopping process is performed by, when the polishing 
process is performed on a single wafer processing basis, causing deionized 
water to flow in place of the polishing slurry 14 and performing a rinsing 
process or by lifting the chuck table 13 and separating the wafer 10 from 
the polishing pad 12. Also, in the case of a device having a plurality of 
chuck tables, a measuring device is provided with respect to each table, 
whereby the table is lifted for each table to thereby separate the wafer 
from the polishing pad 12 and stop the polishing process. 
In this case, the above-mentioned polishing-process stopping process can be 
performed automatically by means of a polishing-process stopping device 
20b of FIG. 9 or manually by notification of an end point of selective 
polishing by means of a buzzer or the like from the measuring device 20a 
when it has been determined that selective polishing has been completed. 
Note that the measuring device 20a and the polishing-process stopping 
device 20b constitutes part of the control system 20. 
Through the performance of the above-mentioned end point detection of 
selective polishing, it is possible to form an SOI layer having a uniform 
thickness in a necessary minimum polishing time period while practicing 
only, and by means of a single process. 
Also, since the manner in which the wafer is polished varies depending upon 
the type, temperature and the like of the polishing slurry 14 and 
polishing pad 12, the polishing conditions such as the speed of rotations 
of the turntable 11, the pressure for pressing the substrate and the like, 
and further the pattern of the silicon dioxide film on the substrate 
becoming a stopper, and the area ratio between the silicon dioxide film 
and the silicon, etc., it is preferable that the method of determining a 
saturation point of temperature be set according to the applied 
conditions. 
Also, preferably, an end point of selective polishing is determined, for 
example, such that polishing is ended simultaneously with a time when the 
temperature becomes fixed or that polishing is ended when the temperature 
becomes fixed and a prescribed period of time (e.g., 5 minutes) has lapsed 
thereafter, so that for each condition as applied an optimum polished 
state is reached. 
Although in the above-mentioned first example of control the end of 
polishing was determined by the temperature of the polishing pad or the 
rate of variation in temperature having become a prescribed value, it may 
be determined by determining that the rate of variation in the temperature 
has become a minimum value, that is, the two times differentiated value of 
the temperature has become 0. Next, this determining method will be 
explained as a second example of control. 
FIG. 11A illustrates variations in the temperature of the polishing pad 
relative to the selective polishing time period, FIG. 11B illustrates a 
rate of variation in the temperature, and FIG. 11C illustrates a rate of 
variation in this rate of variations in the temperature. When the silicon 
dioxide film 3 as a stopper is locally exposed on the surface with the 
result that the area of silicon on the stopper decreases, the rate of 
variation in the temperature becomes negative (point in time indicated by 
.alpha. in FIGS. 11A and 11B). The rate of variation in the temperature 
after the point .alpha. in time has a minimum value and, before long, 
becomes 0 (at which the temperature of the polishing pad is fixed, a point 
in time indicated by .beta.). At the point .beta. in time, the heat 
generated due to excess polishing (variation of polishing) is also 
saturated. This means that between the points .alpha. and .beta. in time 
there exists a border point between the heat generated due to polishing of 
silicon on the stopper and the heat generated due to excess polishing 
(polishing variation). In this second example of control, end of polishing 
is determined on the basis of the minimum value (point .gamma. in time) of 
the rate of variation in the temperature. 
FIG. 12 illustrates a detailed process performed in the second example of 
control. The same reference numerals as those illustrated in FIG. 10 
represent the same processing. 
A temperature signal from the temperature sensor 19 is input in units of a 
prescribed time period .DELTA.t, the two times differentiated value 
(d.sup.2 T/dt.sup.2) of a time t relative to the temperature T of the 
polishing pad 12 is calculated (step 201), and it is determined whether or 
not this value d.sup.2 T/dt.sup.2 has become 0 (step 202). When the rate 
of variation in the temperature becomes minimum with the result that the 
value d.sup.2 T/dt.sup.2 becomes 0, polishing is ended. 
Also, preferably, the timing with which the temperature signal from the 
temperature sensor 19 is input is made to become a multiple of the cyclic 
period for swinging the substrate. This is because the temperature of the 
polishing pad immediately after contact thereof with the substrate varies 
(the shorter the distance from the center of the polishing pad is, the 
higher the temperature relatively becomes) according to the position of 
the substrate with respect to the polishing pad (the distance thereof from 
the center of the polishing pad). 
Also, polishing may be ended in a prescribed time period after the minimum 
value has been determined in step 202. This is because when the variations 
in thickness of the silicon on the stopper prior to performance of the 
selective polishing are large, the time period in which polishing of the 
silicon on the stopper and excess polishing (polishing variation) are 
simultaneously performed within the substrate becomes long with the result 
that the minimum value sometimes does not become the border point. 
Next, a third example of control will be explained. 
In the above-mentioned examples of control, there are cases where according 
to the conditions for selective polishing it is difficult to accurately 
end the polishing processes for all wafers, including for example, a case 
where the variations in thickness of the silicon on the stopper prior to 
selective polishing are large, a case where all portions of the stopper 
are not exposed simultaneously over an entire surface of the wafer during 
selective polishing, a case where the area of the SOI layer is so large 
that dishing of the area of the SOI layer readily occurs, or a case where 
selective polishing is performed under the polishing conditions wherein 
polishing variations are likely to occur (not adjusting the pH value of 
the polishing slurry by use of hydrogen peroxide, using a soft polishing 
pad, etc.). In the above-mentioned cases, sometimes the temperature of the 
polishing pad and the rate of variation in the temperature thereof vary 
between the wafers. Accordingly, in the case of a selective polishing from 
which a higher precision of end point detection is demanded, there are 
cases where controls in the above-mentioned examples of control are 
insufficient. 
As a countermeasure against this, in this third example of control, end 
point of polishing is detected using the relationship between a total 
amount of heat generated from a time of start of polishing and a volume of 
the substrate removed by selective polishing. 
First, the theory behind the third example of control will be explained. 
A total amount Q of heat generated when silicon is removed by selective 
polishing can be determined from an amount of silicon removed, that is, 
the volume V of the removed silicon portion. It can be determined from the 
following equation (1) by, for example, multiplying the volume V by a 
prescribed constant cl. 
EQU Q=c1.times.V (1) 
where the constant cl is set according to the conditions for selective 
polishing (polishing slurry used, polishing pad used, etc.). 
On the other hand, in a selective polishing, the total amount Q of heat 
generated actually is in proportion to the area S of a region hatched in a 
graph of FIG. 13 (corresponding to the graph of FIG. 3). Accordingly, 
assuming that a constant c2 represents the proportionality factor, the 
amount Q of heat generated actually is expressed by the following equation 
(2). 
EQU Q=c2.times.S (2) 
The following equation (3) is obtained from the above equations (1) and (2) 
. 
EQU S=(c1/c2).times.V (3) 
Accordingly, if the amount V of silicon removed is previously determined 
and when the area S determined from the polishing period of time and the 
temperature of the polishing pad has satisfied the equation (3) it is 
determined that polishing has been ended, it is possible to detect the end 
of polishing with a high accuracy. 
While the value of (c1/c2) in the equation (3) is a constant, this value is 
determined according to the conditions for selective polishing, etc. 
Accordingly, by, for example, actually bare silicon wafers under the same 
conditions for selective polishing for a prescribed period of time, the 
value (c1/c2) is calculated from the amount V of silicon wafer removed and 
the area S obtained from a graph of the variation in temperature of the 
polishing pad relative to the time duration of polishing performed thereby 
and is thereby set beforehand. 
A detailed process in the third example of control based on the 
above-mentioned theory is illustrated in FIG. 14. 
First, prior to selective polishing, as illustrated in FIG. 15, the 
thickness of the silicon on the silicon dioxide film 3 becoming a stopper 
is measured over an entire surface of the substrate. This measurement is 
performed by non-contact measurement which uses an optical method. Note 
that it serves to increase the precision of end point detection by 
measuring as many points as possible. 
The measuring device 20a (FIG. 9) inputs data of the measured thickness 
from an input device not illustrated (step 300). 
Next, the volume V of the silicon to be removed by selective polishing is 
calculated from the data of the measured thickness (step 301). Further, 
the area So (quantity of heat) for ending the selective polishing is set 
from this volume V by use of the equation (3) (step 302). 
Thereafter, a temperature signal from the temperature sensor 19 is input 
(step 303) and, from an integrated value thereof, the area S illustrated 
in FIG. 13 is determined (step 304). Then, it is determined whether or not 
the area S has become equal to the area So as set (step 305). Then, when 
the area S has become equal to the area So, an end of polishing is 
determined. 
By using the correspondence relationship between the total amount of heat 
as measured from the start of polishing and the volume of the polishing 
material as mentioned above, it is possible to detect an end point of 
polishing with a high precision regardless of the polishing conditions. 
In the first to third examples of control, detection of the temperature is 
not limited to one wherein the temperature of a portion 12b of the 
polishing pad having polished at a position immediately thereafter is 
detected. As illustrated in FIG. 16, the temperature of a portion 12b of 
the polishing pad having polished the substrate at a position immediately 
thereafter and that of a portion 12c of the polishing pad having polished 
the substrate at a position immediately prior thereto may be measured 
simultaneously, whereby the difference between the both temperatures may 
be used. As a result of this, even when the temperature of a portion 12b 
of the polishing pad having polished the substrate varies due to swing 
thereof during the polishing process, it is possible to detect an end of 
polishing with a high precision. 
The above-mentioned polishing and the detection of an end point thereof 
performed and made using the polishing pad 12 can be applied quite 
similarly to the polishing process performed by the techniques illustrated 
in FIGS. 18A to 18C, FIGS. 19A to 19C, FIGS. 20A and 20B and FIGS. 21A and 
21B. 
Also, even when as mentioned above the position of the wafer 10 on the 
polishing pad 12 is moved relatively thereto, by simultaneously moving the 
temperature sensor 19 as well, it is possible to set the position of the 
temperature measuring point 12b at all times at a position immediately 
after a polishing portion. Accordingly, it is possible to detect the 
temperature of the polishing pad 12 with a high precision. 
In the embodiment illustrated in FIGS. 18A to 18C, selective polishing is 
performed by forming the insulator film 41 (silicon dioxide film or 
nitride film) or metal film 43 over an entire surface of the semiconductor 
substrate 40, forming the trenches 40a, thereafter depositing the 
insulator film 42 over the surface of the resulting structure, and 
thereafter performing selective polishing by using as a stopper the 
insulator film 41 or metal film 43 formed in the regions of the 
semiconductor substrate 40 having no trenches formed therein. Also, when 
the polycrystalline silicon 421 is deposited on the insulator film 41 
(silicon dioxide film or nitride film), it is preferable that the 
insulator film is further formed in the trenches 40a as illustrated in 
FIGS. 19A to 19C. 
Also, although in the above-mentioned embodiments, the saturated state of 
an amount of heat generated during the polishing process was detected by 
measuring the temperature of the polishing pad 12, an end point of 
polishing may be also detected similarly by measuring an amount of heat 
generated in other portions than the polishing pad. 
Also, although it is preferable that measurement of the temperature of the 
polishing pad 12 be non-contact measurement such as measurement made using 
radiation energy, the measurement thereof may be contact measurement such 
as that made using a thermocouple. 
Also, although in the above-mentioned embodiments the chuck table 13 is 
swung by the drive unit 16 in the radial direction of the polishing pad 12 
as rotated, this swing may be performed in a direction different from the 
rotation direction of the turntable 11 (polishing pad 12). For example, 
the swing motion may be performed in parallel or slantwise with respect to 
the radial direction of the turntable 11 as rotated. 
While the present invention has been shown and described with reference to 
the foregoing preferred embodiments, it will be apparent to those skilled 
in the art that changes in form and detail may be made therein without 
departing from the scope of the invention as defined in the appended 
claims.