A new laser test stand for simulating charged-particle tracks
Y. Unno1, Y. Iwata2, T. Ohsugi2, T. Kohriki1, T. Kondo1, S. Terada1, H. Iwasaki1 and Y. Yamada3
1Physics Division, National Laboratory for High Energy Physics (KEK),
Tsukuba, Ibaraki 305, Japan
2Department of Physics, Hiroshima University,
Higashi-Hiroshima, Hiroshima 739, Japan
3Astro Design Co. Ltd,
Kawasaki, Kanagawa 211, Japan
ABSTRACTWe report on the construction of a new laser test standequipped with a 1064nm pulsed infrared YAG laser for
simulating the passage of a charged particle in a silicondetector. The standard semiconductor repairing tool, the socalled laser-cutter, has been modified to have a newly
developed optics which has the ability to reduce the light by
the order of 106 ~108 and to adjust the spot size to several
microns. As an application we measured the position
dependence of the induced signals in a silicon-strip detector
when the laser light hits in the region between the strips. The
measurement has shown that this device is very effective in
evaluating the detailed response of a silicon detector without
using charged particles generated by accelerators.

I. INTRODUCTION
Normally, in order to evaluate a silicon detector we use
charged particles produced by particle accelerators [1]. It
would be desirable to have alternative equipment which could
perform a similar job in the laboratory [2]. A charged particle
passing through silicon generates electron-hole pairs along its
passage in the bulk. Laser light of 1064 nm from a YAG
laser is suitable for simulating the signals generated bycharged particles in a silicon detector. Since the bandgap ofsilicon is 1.1 eV, corresponding to a wavelength of 1130 nm,
1064 nm light has an attenuation length of about one millimeter in silicon bulk [3 ], and induces electron-hole pairs
along the passage of the light nearly uniformly in a bulk of
several hundred microns.
In the semiconductor industry the YAG laser is being used
to evaporate aluminum traces on LSI chips to repair a short,or a mis-designed, trace in the chip. The laser is mounted ona microscope situated on a probe station (or prober), and isusually equipped with a collimator to limit the spot size fromseveral tens of microns to several microns in order toevaporate the target traces. The setup is called"Semiconductor repairing equipment".
The setup is most suitable for our application of generating
the signals of charged particles. Although a spot size ofseveral microns is appropriate, the laser power is too high forthis application, even though an order of magnitude of lightwould be lost in the collimating process. Laser power ofabout 50 µJ is required to evaporate a trace having a width ofabout 10 µm. In order to generate about 20,000 e-h pairs inthe silicon bulk we require power of about 3.5 fJ. Counting
losses in collimating process and quantum efficiency, we
would require power of 5 pJ. Thus, a reduction on the orderof about 10 7 is required. Although a low-power laser of afew 10 mW is available with a very short pulse-width of 50 ps
to a nano-second, this power would be not enough to tolerate
the losses in the collimating process.
If we could reduce the light power of the standard"semiconductor repairing equipment", we could use theequipment for both repairing and generating signals. We
have built such reduction optics and have used the system toevaluate induced signals in a double-sided silicon-strip
detector. We give in this paper a description of the reduction
optics and the result of the measurement.

Fig. 1. Overview of the laser test stand. The prober can beseen on the left-hand side with the Nd:YAG laser at the topof the device. A monitor screen displays the object and thelaser light spot.

II. SETUP
Fig. 1 shows a photograph of the prober equipped with a1064 nm laser [ 4 , 5 ]. The specifications of the laser are
summarized in Table 1. The microscope was equipped with
Infra-Red transparent lenses and ports for the laser and for a
CCD monitor [ 6 ]. A laser power-reduction unit (the laser

mounting adapter) was inserted between the microscope andthe laser.
A silicon strip detector was put on an X-Y stage of theprober and moved manually. The position of the laser spot
was measured on a CCD video screen. The collimation andthe focus of the laser spot was also monitored on the screen.

<10 -2 [7]. The polarizer could withstand a maximum laser
power of 10 MW/cm2 pulsed. The laser power was about
5kW/cm2.

laser head

Table 1. Specifications of the 1064-EHQ laser

beam shutter

Energy/Pulse
Wavelength
Gain material
Pulse width(*)
Peak power(*)
Q-switch PRF
Pulse-pulse stability
Beam diameter
Beam divergence
Polarization ratio
Timing jitter(*)
Warm-up time(*) measured at 0~1 kHz PRF

55 µJ
1064 nm
Nd:YAG
11~15 ns
3.7~5 kW0~25 KHz<3% rms
0.5 mm<4 mrad>100:1<10 ns<15 minpolarizer unit

potentiometerpolarizer-unit fixture
tilt-angle controller
ND filter A

tilt-angle fixture

ND filter B

x-aperture fixture
optical fiber

y-aperture controller
CLOSE
OPEN

optical fiber fixture
x-aperture controller

y-aperture fixture

III. THE LASER MOUNTING ADAPTER
to microscpoe

We adopted two reduction components: ND filters and apolarizer. The major power reduction was achieved by theND filters. A variation in the power was produced using apolarizer. The laser power could be reduced by adjusting its
power supply. However, the diode-pumped YAG laser that
we used had a characteristic of pulse-width widening when
the pumping power was reduced. In order to keep the pulse
width in the range of 10 ns, we needed to maintain the laser
output at its maximum.
A schematic drawing of the adapter is shown Fig. 2. The
adapter was made of: a Glan Laser Polarizer, a worm-gearwith a potentio-meter for rotating the polarizer, a tilt-angle
adjuster, ND filters (A, B), a mirror for the reference light,
and an aperture controller. In the adapter, a reference light
was introduced so that the reference and the laser light would
pass through the same path so as to visually confirm thecollimation and focus of the spot. Since the visual light andthe laser had different wavelength, all of the opticalcomponents had to have similar characteristics for both
wavelengths. The maximum transparency of the adapter was
about 85%, which still enabled the laser to evaporate
aluminum traces. The maximum reduction ratio was about
108 , and was variable in the range of 106 ~108. Below webriefly describe the components, and summarize their
specifications in Table 2.

Glan Laser Polarizer
Using the polarization of laser light, an external polarizer
can control the power of laser light. The 1064-EHQ laser hasa polarization ratio of >100:1. The maximum transparency ofthe polarizer was 92% and the maximum reduction rate wasFig. 2. Laser mounting adapter, placed between the laser
head and the microscope. The reference light is introduced
to this adapter.

Worm-gear with a potentio-meter
A worm-gear was attached to the Glan polarizer. The
rotation was controlled by a potentio-meter connected to thegear. The polarizer rotated 90° as the potentio-meter rotated
10 times.

Tilt-angle controller
A controller was used to adjust the tilt-angle of the laser
head so that the laser light hit the optics at the normal angle.

ND filters
These filters were the main power reducer. Two filters (Aand B) were used in series. Each had a visible-lighttransparency of about 0.3%. There was no available
measurement of the transparency for IR light. We assumed
that the transparency for IR light was similar to that of visible
light.

MirrorA mirror combined the laser-light and the visible-light into
the common optical axis. The transparency for 1064nm lightwas 92%. The reflection rate was 90~92% for 600~900 nm
light.

IV. APPLICATION - RESPONSE IN SILICON STRIPDETECTORSAperture controller
Aperture controllers controlled the openings and thepositions of the openings in the X and Y directions. The
range of the opening was 0 to 10 mm in both directions. With
the full opening the laser spot size was about 50µm in
diameter. By closing the aperture the spot could be made as
small as 2µm, which was eventually limited by diffraction.
Table 2. Specifications of the components of the laser
mounting adapter
Glan Laser Polarizer
Clear aperture
Surface quality
Wavelength range
Transparency
Reduction rate
ND filters
Aperture
Transparency (visible light)
MirrorTransparency (1064nm)
Reflection rate (600~900 nm)
Aperture controller
X-Range
Y-Range

8 mm
20/10
215 nm - 2.3 µm
92 %<1×10-2
φ15 mm~0.3%
92%
90~92%
0~10 mm0~10 mmTable 3. Specifications of the silicon strip detectors
SDC prototype SVX II test detector
Type
Double-sided
Double-sided
Coupling
AC
AC
Substraten-typen-typeChip size
60.0mm×34.1mm 78.85mm×17.12mm
Wafer thickness
300 µm300 µmn-sideStrip pitch
50 µm
74.5 µm
Number of strips
6401024
Implant strip width
12 µm
18 µm
Al strip width
6 µm
12 µm
p-isolation width
26 µm
45 µm
Bias resistor
250 kΩ
1 MΩ
Readout
Single metal layer Double metal layer
2nd metal pitch
59 µm
p sideStrip pitch
50 µm
60 µm
Number of strips
640
258
Implant strip width
12 µm
14 µm
Al strip width
6 µm
8 µm
Bias resistor
250 kΩ
1 MΩ
Readout
Single metal layer Single metal layer
Stereo10 mrad90°

One of our main interests in the silicon-strip detector is the
hit-position dependence of signals in neighboring strips. If
the signal of a strip is proportional to the distance of the hit
position from the strip, the position resolution can be
improved by measuring its pulse heights. The laser test standproduces a laser-beam spot of several microns, which is sharp
enough for such a study with a strip pitch of 50 microns, forexample.
In a silicon-strip detector with a strip pitch of several tensof microns, a large number of readout channels are packed ina small volume. To reduce the number of readout channels, asparse-readout or a ganged-readout method are often applied.
In the case of a sparse readout, every other (or, third, fourth,...) strip is read out. In the case of a ganged readout, two (or afew) strips are ganged into one readout channel. We scanned
the laser light from one strip to the other and measured thehit-position dependence of induced signals for the non-sparse(i.e., every channel), sparse, and ganged readout methods
with the laser test stand.
Two types of silicon-strip detectors were tested: a doublesided silicon-strip detector with a single metal-layer readout
and a double-sided detector with a double metal-layer
readout. The former was an SDC prototype detector [8 ] andthe latter was a test detector for the SVX II [9 ]. Their
specifications are briefly listed in Table 3.

Hit-position dependence of signals
The spot positions and raw induced signals are shown in
Fig. 3. The laser light, with a spot size of 3 µm × 3 µm (at the
top surface), was scanned over the n-side of the SDC
prototype silicon-strip detector. The divergence of the laser
light was about 100 mrad. The distances between the readout
strip and the laser spots are indicated in the figure. In themeasurement, only one strip was connected to a readout
electronics [10] and the neighboring strips were floated. The
integration time of the electronics was of order of µsec. The
detector was reverse-biased at 85 V, while the full depletion
voltage of the detector was 75 V. The intensity of the laser
was adjusted so that the same order of the output was
obtained as that of beta rays from 90Yttrium. There was no
signal when the laser spot hit the readout electrode. This was
proof that the laser spot size was smaller than the strip width(6µm). From these observations we could say that the nearer
was the hit the larger was the induced signals. We shouldnote, however, that the measurement was performed for thecase where neighboring strips were floating, resulting that thepulse heights in Fig. 3(e) and Fig. 3(f) were the same.

(a) 0 µm(b) 5 µm(c) 10 µm(d) 20 µm(e) 35 µm(f) 60 µm

Fig. 3. Spot positions of the laser light on the SDC prototype silicon strip detector
and the induced signals at the n-side on the oscilloscope. The spot size was 3 µm by 3 µm.
The inset figures show the readout strips and the spot. The strip pitch was 50 µm andthe left-hand side strip was the readout. Spot from the readout at, (a) 0 µm, (b) 5 µm,(c) 10 µm, (d) 20 µm, (e) 35 µm, and (f) 60 µm.

Fig. 4 (a) and (b) show the pulse-height distributions as afunction of the position. In these measurements, two
successive strips were read out while the electrodes outside ofthese two strips were grounded. The origin of the abscissa
was defined at the left edge of the left-side readout aluminum
strip. The signals were not linear to the distance from thereadout strip. There were steps at the middle point of the
strips, where the electric-field lines were separated so as to go
to the respective implant strips. The steps were clearer for
signals of the p side (i.e., junction-side) than for that of the nside. A further study of the electric field in the bulk, thecharge collection, and an investigation of the optics of thelaser light, i. e. focus, defocus and attenuation in the bulk, arenecessary in order to understand the difference between thesesteps in detail.

1000

(a)

Signal [mV]
Arbitrary unit
Signal [mV]

p11ch.
n10ch.

Step

800

600

400

200

0

1- 20 0 0
- 5 0 (b)050

Step [µm]Position

800

p12ch 1 0 0
n11ch

600

400

200

0

1- 20 0 0
- 5 0( c )0Sum of two strips

50

Position [µm]800

p-sum 1 0 0
n-sum

600

400

200

0

-200
-50

0

50

100

Position [µm]Fig. 4. Hit-position dependence of signals to the laser light in
the p- (circle) and n-side (cross) of the SDC double-sided
single-metal-layer detector. Two successive strips were readout while aluminum electrodes of the neighboring strips weregrounded: (a) one readout channel, (b) next readout channel,(c) sum of the two channels with software.

When the laser spot hit more distant positions over theneighboring strip, there was essentially no signal in thereadout strip, in contrast to the Fig. 3. The signal wasshielded by the neighboring strip, since it was grounded. A
sum of the signals in the two strips were made with softwareand is plotted in Fig. 4 (c). The sum was essentially uniform
between the two readout channels.

Sparse readout and ganged readout
For a sparse readout, two strips were connected to readout
electronics with one intermediate strip being unconnected in
the SDC detector. The center strip is the so-called "floatingintermediate" strip. The signal pulse heights are shown in
Fig. 5, for (a) one readout channel, (b) next readout channel,
and (c) a sum of the two channels being added with software.
In the scanning, the origin of the position was defined as
being at the left edge of the intermediate aluminum strip.
Four regions ( -50 µm to -25 µm, -25 µm to 0 µm, 0 µm to+25 µ m and +25 µ m to +50 µm) showed different
characteristics in the p-side. There were signals in thereadout strip when the laser light hit the region beyond thefloating strip, similar to that shown in the Fig. 3. At the pside, there was a large signal in the first region (the nearest
region to the readout strips). The signals were almost flat in
the second and third regions. They were about 1/3 and 1/4 ofthe maximum signal in the first region, respectively. In the
fourth region it was diminishing. At the n-side, however, thesignals looked to be proportional to the distance from thereadout strip.
As clearly seen in the sum of the two channels, theobserved charges around the intermediate strip were smaller
than the charges observed around the readout strips. Since noamplifier was connected to the intermediate strip, charges
collected in the intermediate strip were coupled to theneighboring readout strips capacitively. The less charges in
the intermediate strip indicated that there were othercapacitively coupled regions, such as the backplane and otheradjacent strips from the intermediate strip. The amplifier
integration time of µsec was much slower than the chargecollection time of order of 10 ns; no ballistic deficit wasinvolved.
The sparse and ganged-readout methods were also
investigated in the n-side of the SVX II detectors. The
detector had a double-metal-layer structure on the n- (ohmic)
side. A bias voltage of 80 V, which was 20 V higher than the
full-depletion voltage, was applied. The resulting pulseheight distributions are shown in Fig. 6. In the sparse
readout, the strip at 0 µm was floated. In the ganged readout,
the strips at -75 µm and 0 µm, and the strips at +75 µm and+150 µm were ganged.
In the sparse readout, the signals were about 1/6 of the
maximum near to the intermediate strip, which were much
lower than those in the case of the SDC detector. The SVX II
detector had a double-metal structure and a larger interstrip
capacitance (10 pF) compared with that of the SDC single

700

(a)

p10ch.
n10ch.

ganged-n8ch.
sparse-n8ch.
800

400
300
200

600

200

1 2 000- 1 0 0 (b) - 5 0

0

50

100

150

200

Position [µm]1000

ganged-n7ch
sparse-n7ch.

100
800
0
700
-1
- 6 0(b)600-40-p12ch.
200n12ch.
Position

20

40

60

p7ch.

p10ch.

(a)
1000

400

500

Arbitrary unit

1200

Arbitrary unit

600

From these observations we could expect a higher efficiency
for the ganged readout than that of the sparse readout.

p7ch.

metal structure (7 pF) even though the strip pitch (74.5 µm)
was larger than that (50 µm) of the SDC detector. These
structural parameters may have caused a difference in thesignals. With the low pulse heights and non-linearity, we cannot expect a good efficiency nor a better resolution than thatof the readout pitch in the sparse readout of the SVX II
detector.

[µm]

600

400

500

p10ch.

200
400
1 2 000
-100

300

100
0

Sum of two strips

-1
700
-60
600

-40

-20

0

20

40

60

Position [µm]500Sum of two channels

200

-50

0

50

100

800

600

400

200

400

ganged0-100

300

150 (c) 200

Position [µm]

1000

-50

0

50

100

sparse150

200

Position [µm]

200
100
p-sum
n-sum

0
-100
-60

-40

-20

0

20

40

60

Position [µm]Fig. 5. Signals in the sparse readout with one intermediate
floating strip (p-side (circle) and n-side (cross)). The originof the position was at the left edge of the intermediate
aluminum strip: (a) one readout channel, (b) next readout
channel, (c) a sum of the two channels with software.

In the ganged readout, the laser light hit two regions: theregion inside the ganged pair, and the region between thereadouts. The signals inside the ganged pair were large andflat in position. The signals between the readouts gradually
decreased as the laser spot hit further from the readout strip.
In this region, the sums of two signals in the two readouts
were equal to the signals of the spot inside the ganged pair.

Fig. 6. Signals in the sparse readout (cross) and theganged readout (circle) at the n- (ohmic) side of the SVXII prototype double-metal-layer detector: (a) one readout
channel, (b) other channels, and (c) a sum of the readout
channels with software. In the sparse readout, the strip at 0µm was floated. In the ganged readout, the strips at -75µm and 0 µm, and the strips at +75 µm and +150 µm were
ganged.

V. SUMMARY
We have built a laser test stand which simulates the
response to charged particles for characterizing silicon-strip
detectors. A modification was made to semiconductor
repairing equipment with a pulsed Nd:YAG laser having awavelength of 1064 nm. Special effort was made to reduce
the power of the laser light to be on the order of 10 6 to 108 ,with filters and a polarizer, and to make a small light spot on
the order of several microns. The laser test stand has worked
not only as a detector test stand but also as an original

repairing tool when the filters are removed and the polarizer
is set at the maximum transparency.
We have applied the laser test stand to investigate the hitposition dependence of induced signals in silicon-strip
detectors with strip pitches being on the order of tens of
microns. Signals were generated by shinning laser lightbetween the strips. We have observed a similarity and adifference in response at the p-side and the n-side with nonsparse and sparse readout (with one intermediate strip). We
have taken quantitative data for two kinds of double-sided
silicon-strip detectors: a single metal-layer detector and adouble metal-layer detector, with a sparse readout (with one
intermediate strip) and a ganged readout in detail. The
double metal structure has shown very low pulse heights nearand over the intermediate strip in the sparse readout.
The results given in this paper have demonstrated the good
performance of this test stand and a quick characterization ofsilicon detectors without charged particles from accelerators.

REFERENCES[1 ] Y.Unno et al., "Beamtests of the double-sided silicon strip
detectors with fast binary readout electronics", International
Symp. on Development and Application of Semiconductor
Tracking Detectors at Hiroshima, Oct. 10-13, 1995.
[2 ] B.S. Avset et al., Nucl. Instr. and Meth. A310(1991)203; E.
Barberis et al., The Fermilab Meeting DPF92, C.H. Albrighted. (World Scientific, Singapore 1992), Vol 2, p1752; S.
Gadomski et al., Nucl. Instr. and Meth. A326(1993)239; K.
Ing et al., Nuc. Scie. Symp., 1994 IEEE Conf. Rec. p510.
[3 ] H.Melchior, "Demodulation and Photodetection Techniques,"
in F.T.Arecchi and E.O.Schulz-Dubois, Eds., Laser
Handbook, Vol. 1, North-Holland, 1972, pp725-835[4 ] Manual Prober Model SE-6001, ASTRO Design, Inc., 863
Kamikodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa 211,
Japan
[5 ] ALC Q-Switched Microlaser, Model 1064-EHQ with
programmable laser controller 120894, Amoco Laser
Company, 1251 Frontenac Road, Naperville, IL 60563,
U.S.A.
[6 ] Model VMU-3H, Mitsutoyo Co. Ltd.
[7 ] Tower Optical Corporation, Wayne, New Jersey 07470,
U.S.A.
[8 ] T. Ohsugi, et al. Nucl. Instr.and Meth. A342(1994)16.
[9 ] D. Bortoletto, S. Seidel, J. Spalding, International Symp. on
Development and Application of Semiconductor Tracking
Detectors at Hiroshima, Oct. 10-13, 1995.
[10] Hybrid charge-sensitive preamplifier, Model N012-1, Hoshin
Densi Co. Ltd., Hatanodai, Shinagawa, Tokyo 142, Japan,
and a spectroscopy amplifier, Model 472, EG&G ORTEC,
Oak Ridge, TN 37831-0895, U.S.A.