yuta tsukii , libo zhou2,b, hirotaka ojima2,c, jun...
TRANSCRIPT
Development of wireless dynamometer for rotary infeed surface grinding
Yuta Tsukii1,a*, Libo Zhou2,b, Hirotaka Ojima2,c, Jun Shimizu2,d,
and Teppei Onuki2,e
1Graduate School, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Japan 316-8511
2School of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Japan 316-8511
[email protected], [email protected], [email protected], [email protected],
Keywords: grinding force, dynamometer, rotary infeed surface grinding machine, grinding
Abstract. Many functional materials like Si, SiC, ฮฑ-Al2O3, GaN fall in wafer form. For such wafer
manufacturing, rotary infeed surface grinding (RISG) is used. However, RISG dynamics, in which
both grinding wheel and wafer rotate around their own axes, is different from that of the conventional
reciprocating surface grinding. For this reason, the conventional wired dynamometers are not able to
be applied in RISG. Consequently, the material removal mechanism of RISG is not sufficiently
clarified. In this research, we have developed a wireless dynamometer to measure grinding force in
RISG. The dynamometer consists of an aluminum octagonal ring, 2 ร 4 strain gages, Wheatstone-
bridge circuits, operational amplifiers, a 12bit AD converter, a Bluetooth transmitter and an internal
data storage. The dynamometer is installed in one segment of grinding wheel, can measure the
grinding force components (๐น๐ก, ๐น๐ ) in both tangential and normal directions during the contact
between the wheel segment and wafer. The estimated measurement range and resolution of the
dynamometer are 1~100[N] and 0.05[N] respectively. In addition, the dynamometer can measure the
distribution of grinding force along the wafer radical distance. The dynamometer was then applied
in silicon wafer grinding. ๐น๐ก, ๐น๐ and ๐น๐/๐น๐ก are evaluated. Both d๐น๐ก and d๐น๐ are linearly proportional
to the wafer radius. ๐น๐/๐น๐ก is found to be about 2.
Introduction
Wafer formed functional materials are often used as substrates by semiconductors in
manufacturing ICs and LEDs. In recent years, due to high integrity and miniaturization of electronic
devices, the demand for thin wafers is increasing. For such wafer processing, rotary infeed surface
grinding (RISG), as shown in Fig. 1, is often used. RISG dynamics, in which both grinding wheel
and wafer rotate around their own axes, is different from that of the conventional reciprocating surface
grinding. Our knowledge on conventional grinding dynamics is insufficient to cover RISG. When
the grinding force is concerned, which is an important factor of elucidation of the material removal
mechanism, the conventional wired dynamometers are not able to be applied in RISG. As the
grinding force affects damage and warpage during thinning process of wafer [1~3], it is therefore
essential to develop a dynamometer which is able to measure the grinding force components in RISG.
Although similar wireless systems have been proposed by M. Sakakura and M. Shindou to measure
the temperature in cylindrical plunge grinding and end-milling [4~5], in this article, we have
developed a wireless dynamometer for RISG, as illustrated in Fig. 1, and evaluated the change of
grinding force during the contact zone with silicon wafer.
Wireless dynamometer development
The outline of the system of developed wireless dynamometer is shown in Fig. 2. The
dynamometer consists of an aluminum octagonal ring, 2 ร 4 strain gages, two Wheatstone-bridge
circuits, operational amplifiers, a 12bit AD converter, a Bluetooth transmitter and an internal data
storage. Octagonal ring has two sets of 4-straingages affixed, in which A1~A4 strain gages constitute
Wheatstone-bridge circuit to measure normal grinding force (๐น๐) while B1~B4 strain gages constitute
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Proceedings of the 20th International Symposium on Advances in Abrasive Technology 3-6 December, Okinawa, Japan
Wheatstone-bridge circuit to measure tangential grinding force (๐น๐ก). A built-in battery powers the
system.
The resistance of strain gages in each circuit changes proportionally to the deformation of
octagonal ring in either normal or tangential direction, which is in turn proportional to ๐น๐ or ๐น๐ก respectively [6]. The Wheatstone-bridge circuits would then output response voltages corresponding
to ๐น๐ or ๐น๐ก [8]. The output voltages are first amplified by ร 200~1000, then converted into a digital
signal by a 12-bit AD converter at sampling frequency of 2 kHz. The digitized signals can be either
sent out by the Bluetooth transmitter or saved into the internal data storage [9]. The specifications of
developed dynamometer are listed in Table 1.
The calibration results of the developed dynamometer are shown in Fig. 3, in which the
amplification factor was fixed at ร 500. It was found both response voltages are linearly proportional
to grinding force components ๐น๐ and ๐น๐ก within the full range of 3.3 volts. The cross-talk of each
component was negligible.
Fig.1 Overview of rotary infeed surface grinding and wireless dynamometer
Fig.2 Configuration of developed wireless dynamometer
Table 1 Specifications of developed dynamometer
Amplification 200 ~ 1000
AD converter 12 bit (LPC1768) [7]
Bluetooth device RN-42XVP
Resolution [N] 0.05
sampling frequency [kHz] 2
Measurement range [N] Up to 100
wafer
Wireless dynamometer
Grinding wheel segment
n1
n2
f
Strain gage
Octagonal
Dynamometer
A2 A3A1
B2
A2A2
B4B3
B1
1. Power supply
2. Amplifier
3. ADC
4. Bluetooth transmitter
5. Data storage
Grinding
segment
Wafer
A1
A4A2
A3
B1
B4B2
B3
Fn
Ft
Bridge circuit A
Power supply
Bridge circuit B
Amplifier
Amplifier
ADC
Bluetooth
transmitter
PC
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The developed dynamometer is able to measure both ๐น๐ and ๐น๐ก simultaneously in real
grinding time. As the segment moves across the wafer, the developed dynamometer also records the
distribution/variation of grinding force along the cutting path of wheel segment traced.
Installation and test
The developed dynamometer is installed as one of wheel segments, as shown Fig. 4. The
octagonal ring together with SD1000 underneath is fixed to the wheel spindle by a designed jig. The
size of SD1000 attached to the dynamometer is 4 ร 5 mm2. Grinding test was performed on a Si
wafer while the forces were measured by using the developed dynamometer. The grinding conditions
are shown in Table 2.
Fig. 5 records the typical grinding force (a) at wheel speed ๐1 = 200 m/min while (b) at ๐1 =400 m/min, for two turns of wheel rotation. When the wheel revolution speed is doubled, it is found
that both grinding interval and grinding forces become halved.
Fig.3 Calibration of developed dynamometer
Vt = 0.1467Ft + 0.546
Vn = -0.0344Fn + 3.0503
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
Outp
ut
Volt
age
V
n, V
t(V
)
Force Fn , Ft (N)
Ft
Fn
Ft
Fn
Fig.4 Implementation of developed wireless dynamometer
Table 2 Grinding conditions
Wheel segments, size (mm2) SD1000M, 4 ร 5
Wheel rotation speed (m/min) 200, 400
Wafer rotation speed (min-1) 25
Wheel down-feed rate (ยตm/min) 2
Coolant Dry
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Proceedings of the 20th International Symposium on Advances in Abrasive Technology 3-6 December, Okinawa, Japan
Results and discussion The wafer volume removed in one wheel revolution in RISG is illustrated in Fig. 6 (a), in
which, the wafer is constantly removed in a spiral manner. At the stable state of grinding, therefore,
the depth of wheel engagement is given as;
โ=๐
๐2 (1)
where ๐ is the feed rate and ๐2 is the wafer revolution. The wafer can be considered as a bunch of
aggregated concentric annulus with different radical distance. The engagement between the wheel
and the annulus located at the wafer radical distance [๐2, ๐2 + d๐2 ] is zoomed up in Fig. 6 (b). The
side surface area ๐2 removed in one wheel revolution is express in Eq. (2).
๐2 = 2๐๐2๐2๐1โ= 2๐๐2
๐
๐1 (2)
During one revolution of wheel, the number of cutting edge passing through that side surface
๐2 is given as ๐ฟ โ ๐ โ ๐๐/cos๐, where ๐ฟ,๐ are the length of wheel circumference and wheel width
and ๐๐ is the density of effective cutting edge in a specific area. Therefore, the average cross-
sectional area cut by an individual grain at the wafer radical distance ๐2 is calculated as below;
๐๐(๐2) =๐2cos๐
๐ฟ โ ๐ โ ๐๐=2๐
๐ฟ
cos๐
๐ โ ๐๐
๐
๐1๐2 (3)
It is obvious that the average cross-sectional area of chip is proportional to the in-feed rate ๐,
reversely propotional to the wheel revolution speed ๐1. Also, ๐๐ increases as an increasing in the
wafer radical distance ๐2. The cutting force can be considered as the product of the specific removal
energy ๐ถ๐ and chip cross-sectional area ๐๐. Therefore, the grinding force created by an individual
cutting edge at the wafer radical distance ๐2 is expressed as;
{
๐๐ก = ๐ถ๐ โ ๐๐(๐2) =2๐๐ถ๐
๐ฟ
cos๐
๐ โ ๐๐
๐
๐1๐2
๐๐ = ๐ โ ๐๐ก =2๐๐๐ถ๐
๐ฟ
cos๐
๐ โ ๐๐
๐
๐1๐2
(4)
where ๐ is the force component ratio of ๐น๐/๐น๐ก. Within wafer radical distance [๐2, ๐2 + d๐2], the total
number of grains simultaneously engaged into the wafer is given as ๐๐ โ ๐ โ d๐2/cos๐. The grinding
force generated at [๐2, ๐2 + d๐2] is therefore calculated as;
(a) ๐1=200 (m/min) (b) ๐1=400 (m/min)
Fig.5 Recorded grinding force
-1
0
1
2
3
4
5
6
0 0.1 0.2 0.3 0.4 0.5 0.6
Gri
nd
ing
forc
eF
n, F
t(N
)
Grinding time t (s)
Fn
Ft
n
t
-0.5
0
0.5
1
1.5
2
2.5
0 0.05 0.1 0.15 0.2 0.25 0.3
Gri
nd
ing
forc
eF
n, F
t(N
)
Grinding time t (s)
Fn
Ft
n
t
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Advances in Abrasive Technology XX
{
d๐น๐ก = ๐๐ โ ๐ โ ๐๐2
cos๐โ ๐๐ก =
2๐๐ถ๐
๐ฟ
๐
๐1๐2d๐2
d๐น๐ = ๐๐ โ ๐ โ ๐๐2
cos๐โ ๐๐ =
2๐๐๐ถ๐
๐ฟ
๐
๐1๐2d๐2
or
{
๐น๐ก =๐๐ถ๐
๐ฟ
๐
๐1๐22
๐น๐ =2๐๐๐ถ๐
๐ฟ
๐
๐1๐22
(5)
(a) Wafer removal in one revolution of wheel (b) Detailed engagement at ๐๐2
Fig. 6 Wheel/wafer interference
(a) Force component ๐น๐
(b) Force component ๐น๐ก
Fig. 7 Change of grinding force along wafer radial distance
0
1
2
3
4
5
6
0 20 40 60 80 100
Gri
nd
ing
fo
rce
F
n(N
)
Wafer radical distance r2 (mm)
Measured V1=200 (m/min)
Measured V1=400 (m/min)
Analyzed V1=200 (m/min)
Analyzed V1=400 (m/min)
Measured V1=200 (m/min)
Measured V1=400 (m/min)
Analyzed V1=200 (m/min)
Analyzed V1=400 (m/min)
0
0.5
1
1.5
2
2.5
3
0 20 40 60 80 100
Gri
nd
ing
forc
eF
t(N
)
Wafer radical distance r2 (mm)
Measured V1=200 (m/min)
Measured V1=400 (m/min)
Analyzed V1=200 (m/min)
Analyzed V1=400 (m/min)
Measured V1=200 (m/min)
Measured V1=400 (m/min)
Analyzed V1=200 (m/min)
Analyzed V1=400 (m/min)
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Proceedings of the 20th International Symposium on Advances in Abrasive Technology 3-6 December, Okinawa, Japan
d๐น๐ก,๐ stands for the distribution of grinding force along the wafer radical distance, while ๐น๐ก is
the total grinding force generated across the whole wafer. It is found that both d๐น๐ก,๐ and ๐น๐ก,๐ are
proportional to the in-feed rate ๐, reversely propotional to the wheel revolution speed ๐1. However,
d๐น๐ก,๐ is proportional to ๐2, but ๐น๐ก,๐ is proportional to the squares of ๐2.
Fig. 7 plots two sets of grinding force as a function of the wafer radical distance ๐2; one is
measured in actual grinding experiment as shown in Fig.5, the other is analyzed via Eq. (5). They
well agree one to another, for two different wheel speeds. The force components ratio ๐น๐/๐น๐ก is found
to be about 2. However, the experimental results were slightly larger than analyzed data at the wafer
radical distance of 40 ~60 mm. This difference mainly attributes to that the initial wafer surface was
not flat enough to keep the depth of wheel engagement consistent. It can be therefore concluded that
the developed dynamometer is functional correctly and accurately in RISG.
Summary
In this paper, a wireless dynamometer is designed and developed to monitor the grinding force
in RISG. The obtained results can be summarized as below:
1) The wireless dynamometer is able to measure grinding force components (๐น๐ก, ๐น๐ ) in both
tangential and normal directions in RISG dynamics.
2) Not only the total grinding force but also the distribution of grinding force in the wafer radical
direction can be measured correctly and accurately as compared with analyzed results.
3) The grinding force is funded to be proportional to the wafer radical distance and the in-feed rate
๐, reversely propotional to the wheel revolution speed.
Acknowledgement
This research was financially supported by JSPS KAKENHI, Grant-in-Aid for Scientific Research
(A) (No. 15H02213), and Exploratory Research (No. 15K13840).
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