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Grinding characteristics using freezing pin chuck for warped quartz glass wafer
K. Yoshitomi1, a *, A. Une1 and M. Mochida1 1National Defense Academy, Japan
Keywords: freezing pin chuck, warpage, non-deformation holding, grinding, quartz glass wafer
Abstract. The warpage of a thin substrate adversely affects manufacturing processes. Generally, a
substrate is held by a vacuum chuck for machining, therefore the chuck flattens the substrate by
vacuum pressure. So it is difficult to remove the warpage efficiently. To resolve this problem, we
have developed the freezing pin chuck. This paper describes the grinding characteristics using the
freezing pin chuck for a warped substrate. The grinding experiments were carried out for quartz glass
wafers with the diameter of 300 mm which had the warpage of about 30 μm. As a result, it was
clarified that the freezing pin chuck could be used for grinding without exfoliation.
Introduction
It is expected to provide a substrate which hardly has the warpage in semiconductor or flat display
panel manufacturing processes. However, it is difficult to fabricate a substrate reduced in the warpage
since typical chucking systems cannot chuck substrates without any deformation [1, 2, 3]. For
example, a vacuum chuck flattens a substrate with vacuum pressure, so the warpage is restored when
the machined substrate is unloaded from the chuck. Thus, it is necessary for reducing the warpage of
a wafer by grinding or polishing to prevent a thin substrate from deformation generated by the
holding process. However, non-deformation holding technique hasn’t been developed yet.
A quartz glass wafer is one of thin and large-sized substrates, which is used for the
photolithography process as a photo mask. Hence, we have developed the freezing pin chuck system
in order to hold a quartz glass wafer without any deformation [4, 5, 6, 7]. In this system, a wafer is
held by the sticking force of coagulation of freezing liquid which is applied on pins of the freezing pin
chuck.
This paper describes the results of grinding experiments using the freezing pin chuck for a warped
quartz glass wafer.
Experimental apparatus and grinding procedure for freezing pin chuck
Figure 1 shows the grinding machine (Okamoto machine tool Co. Ltd., PNX400) for 300 mm
wafers which uses a 450 mm cup wheel type grinding tool. The freezing pin chuck is installed on the
rotary table. As the spiral flow passages are provided inside of the chuck, the chuck is cooled to less
Figure 1 Photo of grinding machine with freezing pin chuck system
Rotary table
Freezing pin
chuck
Wafer
Grinding
head
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than 5 °C by passing cooling liquid through the spiral passages. Cooling liquid is cooled by a chilling
unit. The flow rate of cooling liquid was 2000 ml/min. In addition, this chuck can also be used as a
vacuum pin chuck. Figure 2 shows the schematic diagram of the holding principle using freezing
liquid. Low temperature coagulant (EMINENT supply Co. Ltd., MW-1) is used as freezing liquid,
which has freezing point of 17 °C. As shown in this figure, the wafer is held by the sticking force of
coagulation when the freezing liquid is frozen by the cooled freezing pin chuck. The sticking force of
the frozen freezing liquid gets larger as the temperature of the frozen freezing liquid gets lower. In
this experiment, the frozen freezing liquid is cooled to less than 5 °C. The variation in the volume due
to coagulation of the freezing liquid does not deform a wafer since it is enlarged in the horizontal
direction. In this principal, only the warpage of the wafer can be removed by grinding or polishing.
Figure 3 shows the enlarged photos of the freezing pin chuck and the state of the frozen freezing
liquid between the top of a pin and the underside of a wafer. Large number of pins is provided on the
chuck and the pin area is 300 mm in diameter. The diameter and the height of pins are 0.5 mm and 0.8
mm, respectively. The pitch of pins is 1 mm. The right photo shows that the frozen freezing liquid on
the pin held the wafer. In this state, the thickness of the frozen freezing liquid was about 50 μm.
Figure 4 Holding procedure using a freezing pin chuck
(<5°C)
(Room temperature)
(>17°C)
(a) Freezing liquid applying (b) Freezing liquid coagulating (c) Wafer holding
Atomizer
Freezing liuid Frozen freezing liquid
Wafer
Melted freezing liquid
Wafer
Cooled freezing pin chuck
Frozen freezing liquid
Warpage
Pin
Figure 2 Schematic diagram of holding principle using freezing liquid
Figure 3 Enlarged photos of freezing pin chuck and frozen freezing liquid between pin and wafer
Pin
Pin pitch 1 mm
0.1mm
Pin
Frozen freezing liquid
Wafer
Trench
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Proceedings of the 20th International Symposium on Advances in Abrasive Technology 3-6 December, Okinawa, Japan
Figure 4 shows the holding procedure using the freezing pin chuck. First, the freezing liquid is
applied on the pins as shown in Fig. 4(a). The spray apparatus is a centrifugal type atomizer which
atomized the freezing liquid by the turbine that rotates at high rotational speed. The size of mist of the
freezing liquid gets smaller as the rotational speed gets higher. Table 1 shows the applying condition.
The applying distance is the distance between the spray nozzle and the surface of the chuck. Applying
position is the distance between the center of the nozzle and the center of the chuck. The height of
applied freezing liquid is about 100 μm under this applying condition. In addition, the freezing pin
chuck oscillates in order to apply the freezing liquid on the whole of pin area uniformly. The
temperature of the chuck is a room temperature at this time. Next, the freezing pin chuck is cooled to
coagulate the freezing liquid before the wafer placement. Then the wafer is placed on the frozen
freezing liquid as shown in Fig. 4(b). At this process, the wafer is not held with the frozen freezing
liquid. Next, water of room temperature is supplied in the spiral passages of the chuck to melt the
frozen freezing liquid. In this way, the wafer floats on the freezing liquid without deformation and the
freezing liquid does not overflow from the gap between a pin and a wafer as shown in Fig. 4(c).
Finally, the chuck is cooled to less than 5 °C again to hold the wafer.
For using the freezing pin chuck to grinding, it is necessary to prevent the frozen freezing liquid
from melting by the grinding heat and from crushing by the grinding force. To resolve these problems,
cooling liquid of less than 5 ℃ is supplied among pins. Figure 5 shows the state of cooling liquid
flowing from the center to the periphery of the chuck. Cooling liquid flows out through holes which
are installed at the periphery of the chuck. The flow rate of cooling liquid was 100 ml/min. The
observation of this state showed the cooling liquid flowed among pins without the damage of the
frozen freezing liquid. Further, this flow clarifies the area where the freezing liquid overflows from
pins. The overflowed area causes the large wafer deformation. From these results, it was considered
that this cooling method could cool the frozen freezing liquid not to be melted by grinding heat.
In order to measure the rigidity of the frozen freezing liquid, the compression test was carried out.
Figure 6 shows the rigidity measuring method. A quartz wafer was held by freezing pin chuck and
Table 1 Applying condition
Rotational speed of turbine 20000 min -1
Rotational speed of chuck 10 min -1
Flow rate of freezing liquid 10 ml/min Applying distance 100 mm
Applying time 90 s Applying position 60 mm
Figure 5 State of coolant liquid flowing from center to periphery of chuck
0
0.04
0.08
0.12
0.16
0.2
0 50 100 150 200 250 300 350
Dev
iati
on
fro
m a
vera
ge
po
sit
ion
μm
Pessure kPa
Figure 7 Rigidity of frozen freezing liquid Figure 6 Rigidity measuring method
Quarz glass wafer
Push pull gage
Displacement sensor
Freezing pin chuck
Pin
Coolant liquid
Frozen freezing liquid
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pressed by the push pull gage. The displacement sensor measured the shift of the location of the wafer
surface. The compressive pressure was from 31 kPa to 312 kPa. The measurement times were five for
each pressure. Figure 7 shows the deviation from the average location for each pressure. The
deviations of all pressure were less than 0.1 μm. It was considered that the rigidity of the frozen
freezing liquid was enough high for the grinding force.
Grinding experiment
Table 2 shows the grinding condition. Diamond type grinding tools of 100 and 800 grit were used
since the grinding force varied for grit size. The grinding experiments were carried out under the
condition of dry and wet. The work pieces were quartz glass wafers. The diameter and the thickness
of the wafer were 300 mm and 1.2 mm, respectively. The shape of wafers was convex.
Figure 8 shows the profile change using the 100 grit grinding tool. Wafer profiles were measured
by displacement sensor which was installed on an air slider type linear stage. Initial profiles were
measured profiles of the wafer which was placed on the chuck without holding. The warpage in the
direction of 0 degree was approximately 25 μm and the one in the direction of 90 degrees was also
approximately 25 μm. The wafer profile changed for the initial profile after holding the wafer by the
freezing pin chuck. However, the amount of deformation was small and the wafer was not flattened as
is the case with a vacuum pin chuck. The maximum amounts of deformation were approximately 15
μm and 7 μm in the direction of 0 degree and 90 degrees, respectively. As a reason for this
deformation, it is assumed that the meniscus force deformed the wafer at the area where the applying
quantity of the freezing liquid was large. For grinding characteristics, the wafer was not removed and
cooling liquid among pins were filled during grinding. Moreover, the wafer was grinded regularly. It
means that cooling ability is enough to prevent from melting by grinding heat and the frozen freezing
liquid has the adequate rigidity against the grinding force. The ground profile was concave shape and
the flatness was approximately 5 μm in the both direction. In this case, the stock removal was about
15 μm. Therefore, the periphery of the wafer was not grinded.
Figure 9 shows the profile change with the 800 grit grinding tool. The warpage in the direction of
0 degree was approximately 15 μm and the one in the direction of 90 degrees was approximately 17
μm. The maximum amounts of deformation in the holding process were approximately 8 μm and 5
μm in the direction of 0 degree and 90 degrees, respectively. Since the deformation characteristics of
the both direction were similar, it is assumed that the applying amount of freezing liquid was uniform
and not too much on the entire pin area. In order to further reduce the deformation, it is considered to
Figure 8 Profile change using grinding tool of 100 grit under dry condition
0
5
10
15
20
25
30
35
40
-150 -100 -50 0 50 100 150
Pro
file
μ
m
Wafer radius position mm
Initial profile
Holed profile
Grinded profile
0
5
10
15
20
25
30
35
40
-150 -100 -50 0 50 100 150
Pro
file
μ
m
Wafer radius position mm
Initial profile
Holed profile
Grinded profile
(a) 0 degree (b) 90 degree
Table 2 Grinding condition
Rotational
speed
Tool 1200 min -1
Stock removal 20 μm
Wafer 100 min -1
Spark out 10 s
Feed rate 10 μm/min Coolant Water, Dry
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Proceedings of the 20th International Symposium on Advances in Abrasive Technology 3-6 December, Okinawa, Japan
increase the applying amount of freezing liquid on pins only. From the grinding result, the wafer was
grinded without exfoliation even under the wet condition. It was said that grind fluid could be used for
grinding with the freezing pin chuck. The flatness was approximately 5 μm in the both direction.
Further, the whole of the wafer was grinded in this case. The locus density of the grinding tool causes
of the recess on the center of the wafer.
According to these results, it was clarified that the freezing pin chuck could be used grinding for a
quartz glass wafer, regardless of the grit size.
Summary
The possibility of grinding for a quartz glass wafer using the freezing pin chuck was studied. The
key technique of the grinding with the freezing pin chuck is to supply cooling liquid among pins to
prevent the frozen freezing liquid from melting. And the rigidity of the frozen freezing liquid is
enough high for the grinding force. From experimental results, it was clarified that the freezing pin
chuck makes it possible to grind a quartz glass wafer with reducing the wafer deformation and
without exfoliation.
Acknowledgment
The present study was supported by a Grant-in-Aid for Scientific Research (C), JSPS (No.
15K05748).
References
[1] S. Matsui et al, Development of Pin-type Vacuum Chuck for High Precision Machining, Journal
of JSPE, Vol.63, No.12, (1997)1705-1709(in Japanese).
[2] K. Kitajima et al, Fundamental Characteristics of a Freezing Chuck System for Brittle Material
Machining, Key Engineering Materials, Vol.257-258, (2004)113-118.
[3] Yoshiro Kubota, Next generation processing technology for a large diameter wafer. Electrostatic
chuck and its application., Electronic materials, Vol.35, No.7, (1996)51-57(in Japanese).
[4] K. Yoshitomi, et al, Development of a freezing pin chuck for polishing to fabricate a nonwarped
substrate, Proc. of the 13th euspen int. conf., Berlin (2013) 133-136 V2.
[5] A. Une, et al, Development of a nondeforming chucking technique for EUV lithography,
Microelectronic Engineering, Vol.112, (2013)278-281.
[6] K. Yoshitomi, et al, Development of a Freezing Pin Chuck for Polishing (2nd Report), Journal of
JSPE, Vol.82, No.1, (2016)82-86(in Japanese).
[7] K. Yoshitomi, et al, Development of a freezing pin chuck system to prevent a thin substrate from
deformation, Proceedings of the 19th International Symposium on Advances in Abrasive Technology,
(2016)253-256.
0
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35
40
-150 -100 -50 0 50 100 150
Pro
file
μ
m
Wafer radius position mm
Initial profile
Holed profile
Grinded profile
0
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20
25
30
35
40
-150 -100 -50 0 50 100 150
Pro
file
μ
m
Wafer radius position mm
Initial profile
Holed profile
Grinded profile
Figure 9 Profile change with grinding tool of 800 grid under wet cndition
(a) 0 degree (b) 90 degree
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