Analysis of Semiconductor Laser-Induced Impurity Activation Using
Free
Carrier1Absorption
1
1
2
3
3
K. Ukawa , Y. Sakakura T. Sameshima , N.Sano , M.Naito and N. Hamamoto

1

Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
2
Hightec Systems Corporation, Yokohama, 222-0033, Japan
3
Nissin Ion Equipment Co., Ltd. Koka, Shiga 528-0068, Japan
E-mail : tsamesim@cc.tuat.ac.jp

Keywords: free carrier absorption, infrared light, microwave, sheet resistance, carrier concentration

Abstract. We report analysis of impurity activation using infrared light and microwave free carrier
absorption method. We carried out implantation of boron clusters at 6.0×1014 cm-2 and 6keV,
phosphorus ions at 2.6×1015cm-2 and 70keV, and BF3 ions at 4.4×1014cm-2 and 10keV. 200-nm-thickcarbon films were formed on the silicon surface as a photo absorption layer using sputtering method.
The analysis infrared light absorption revealed that 940-nm-laser irradiation with a diameter of 180 m
at 25 W for 3.6 ms decreased the sheet resistance to 608 Ω/□ in the case of boron clusters. The carrier
concentration was estimated to be 5.3×1014 cm-2 and the activation ratio was 0.89. Microwave
absorption analysis also gave decrease in sheet resistance to 564 Ω/□. The sheet resistance and thecarrier concentration were 104 Ω/□ and 2.2×1015 cm-2, respectively, for phosphorus implantation in thecase of laser irradiation for 2.6 ms at 25 W. They were 921 Ω/□ and 4.4×1014 cm-2 for BF3 implantation.
The PN diodes fabricated in boron cluster doping region showed a characteristic with an ideal factor of
1.42.
Introduction
We have investigated activation of impurity atoms by CW infrared semiconductor laser using carbon
layer as a optical absorption layer [1, 2]. Infrared semiconductor laser is attractive to heating processwith a high throughput because of its high power of ~10 kW, its high conversion efficiency ~50% andits stable emission. In order to solve the problem of low optical absorbance of Si in the infrared region,
carbon was used as absorption layer. Carbon layer has a high optical absorbance and a high thermal
durability to temperature around 5000 K. Therefore carbon layer can act as heat source to rapidly heat
underlying silicon substrate to a high temperature.
In this paper, we report investigation of electrical properties of impurity doping regions formed bysemiconductor laser annealing. We present measurements of infrared light and microwave absorption
and its analysis using free carrier absorption theory. Though the experimental and theoretical results,
we demonstrate non-destructive and non-contact analysis method of electrical properties ofsemiconductor, which will be useful during device fabrication. We also report diode properties
fabricated in doped region.
Experimental
Boron clusters with a boron concentration of 6.0×1014cm-2 were implanted with an accelerationenergy of 6keV into n-type silicon substrates with a sheet resistance of 133 Ω/□ and a thickness of 720
μm. Phosphorus atoms with a concentration of 2.6×1015cm-2 were also implanted with an accelerationenergy of 70keV into p-type silicon substrates with a sheet resistance of 192 Ω/□ and a thickness of 720
μm. Moreover, BF3 ions with a boron concentration of 4.4×1014cm-2 were also implanted with anacceleration energy of 10keV into n-type silicon substrates with a sheet resistance of 85 Ω/□ and a
thickness of 720 μm. The total boron and phosphorus concentrations given above were measured bysecondary ion mass spectroscopy (SIMS) analysis. The junction depths determined by the depth at animpurity concentration of 1018cm-3 were 9, 251 and 48 nm for samples implanted with boron clusters,
phosphorus and BF3, respectively. Then 200-nm-thick carbon films were formed on the silicon surface
as a photo absorption layer by the sputtering method. Optical measurement revealed that the opticalabsorbance was 75 % at 940 nm. Samples mounted on a mobile stage were irradiated with a 940 nm
CW infrared semiconductor laser with a power ranging from 21 to 25 W. The laser beam had adiameter of 180 μm at the sample surface and a Gaussian like power distribution. After laser irradiation,
the carbon layer was removed by oxygen plasma treatment. Optical spectra in ultra-violet and visible
region revealed that the surface region of each samples were completely recrystallized by the laser
annealing.
The rear surfaces of the samples were polished for the measurement of infrared transmissivity optical
spectra by conventional Fourier transform infrared (FTIR) spectrometry. Experimental spectra wereanalyzed by fitting process of calculated spectra obtained by a numerical program using free carrier
absorption and optical interference theories. The sheet resistance and carrier concentration of the doped
layer were estimated. The transmissivities of microwave were also measured using aninterferometer-type equipment, as shown in Figure 1 [3]. Sample and reference were inserted in thegaps A and B, respectively. The microwave was
A Sample
introduced using a waveguide and split into two
branches by T-type waveguide. Then the microwaves
transmitted through the samples were combined. The
IA-B
transmissivity of samples was calculated by theintensities of the sum IA+B and difference IA-B of theIA+B
microwaves. The detection accuracy of the present
Oscillator
Detectorsystem in the transmissivity was ± 0.1 %. The
9.35GHzexperimental transmissivities were analyzed bynumerical finite element program using the free
B Reference
carrier absorption theory. Diodes were fabricated toFig. 1: Schematic equipment of the freeevaluate electrical properties of pn junction.
carrier absorption measurement system
Free Carrier Absorption
The free carrier absorption effect is explained by the Drude theory. The real part εr and theimaginary part εi of the complex dielectric constant are expressed as,
2 2

nf

r

2

2

p

f

Si
2

2n f

1

2 2

(1),

p

(2)
(1 2 2 ), where, ω is the angular frequency, εsi is the dielectric constant of intrinsic silicon, nf is the refractive
index of silicon, kf is the extinction coefficient induced by free carrier optical absorption, τ is the
lifetime of carriers and ωp is the plasma angular frequency, which is proportional to square root of thecarrier density. In the microwave case, ω is 9.35 GHz and ω2τ2 is much lower than 1. εr and εi are
therefore approximated as,
i

r ~

f

2
Si

p

2

(3),

2
i ~

p

(4).

εr is lower than the initial value by ωp2τ2, and does not depend on the microwave angular frequency. εican be very high for low ω, the free carrier absorption is sensitively observed at the 9.35 GHz
microwave frequency. εi is also proportional to the electrical conductivity (product of the carrier density
and the carrier lifetime).
On the other hand ω2τ2 is generally much higher than 1 for frequencies in infrared regions. They areapproximated as follows,

2
r ~

p

Si

(5),

2

2
i ~

p

(6).

3

εr is lower than the initial value by ωp2/ω2, and does not depend on the carrier lifetime. Although εi is
low because of the high angular frequency, it is interesting that εi increase as τ decreases. FTIR
measurement will be suitable for analysis of free carrier effect of heavy doped layers with a low carrier
lifetime, low carrier mobility as well as a high carrier density. Eqs. (5) and (6) mean that the carrier
mobility and carrier density can be determined by analysis of FTIR spectra with different angularfrequencies.
Figure 2 shows calculated transmissivity and absorbance of a 9.35 GHz microwave as well as for 45
THz infrared light (1500 cm-1) as a function of the sheet resistance for a 10nm p-type doped layer
formed in n-type silicon substrates with a substrate sheet resistance of 133 Ω/□ and a thickness of 720
μm. For the infrared case, transmissivity and absorbance were calculated with two different carrier
mobilities of 12.5 and 50.0 cm2/Vs in the doped layer. The transmissivities of the infrared light and themicrowave monotonously decreased for decreasing sheet resistance. The transmissivity at 45THz(1500cm-1) for a mobility of 12.5 cm2/Vs decreased much more than that for 50.0 cm2/Vs. There issubstantial absorbance of the microwave caused by free carriers in silicon substrate. The absorbance ofthe microwave monotonously decreased as the sheet resistance in the top doped region decreased
because the reflectivity increased as the sheet resistance decreased. On the other hand, there is anabsorbance peaks of infrared light at 90 Ω/□ for 12. 5cm2/Vs and 25 Ω/□ for 50.0 cm2/Vs. The peak
60

60
50
40
30
20

Absorbance [%]10

Infrared light
45THz(1500cm-1)
12.5cm2/Vs
Microwave
9.35GHz0
P type doped layer
Microwave
45
10nm
9.35GHz40
35 N type Si
30
Infrared light
25
45THz
20
(1500cm-1)
15
12.5cm2/Vs
10550.0cm2/Vs
0
102
103
104
10
Sheet resistance [Ω/□]Fig. 2 ． Calculated absorbance andtransmissivity of a 9.35 GHz microwave as
well as for 45 THz infrared light (1500 cm-1) asa function of the sheet resistance for a 10 nm
p-type doped layer on n-type silicon substrates
with a sheet resistance of 133 Ω/□ and a
thickness of 720 μm.

(a)1000cm2/Vs

50
40
Transmissivity [%]Transmissivity [%]50.0cm2/Vs

100cm2/Vs
10cm2/Vs

30

Carrier density
1×1015cm-2
N type Si

2050

1×1015cm-2

40
30

(b)

1×1014cm-2

2×1015cm-2

Carrier mobility
12.5cm2/Vs
N type Si

20
1000

1500 2000 2500 3000
Wavenumber [cm-1]

3500

Fig. 3. Calculated transmissivity spectra ininfrared region of a doped layer for different
carrier mobilities and a carrier density of 1×1015
cm-2 (a) as well as for different carrier
concentrations and a carrier mobility of 12.5
cm2/Vs (b).

absorbance for 12.5cm2/Vs was higher than that for 50.0cm2/Vs according to serious change in the
dielectric constant.
Figure 3 shows calculated transmissivity spectra in infrared region for different carrier mobilities (a)as well as for different carrier concentrations (b) in the doped layer. The transmissivity decreased as thecarrier mobility decreased as shown in figure (a) according to high optical absorption given by eq (6). It
also decreased as the carrier concentration increased as shown in figure (b). The fitting process of
calculated spectra to experiment ones gives values of carrier mobility and carrier concentration.
Results and Discussion
Figure 4 shows the FTIR transmissivity spectra of the samples as-implanted and the laser irradiatedfor implantations of boron clusters at 6 keV. Laser irradiations at 21 and 25 W for 3.6 ms wereconducted. Calculated spectra are also plotted in Fig.4. The calculated spectra were obtained by anumerical program using free carrier absorption and optical interference theories. The carrier
distribution obtained by a finite element method was also used in calculation of transmissivity spectra.
The transmissivity decreased especially in the low wavenumber region because of high free carrier
absorption effect for laser annealed samples. Especially it was low in the case of laser annealing at 25
W. Agreement between experimental and calculated spectra gave sheet resistance, carrier density andcarrier mobility for doped region as shown in Fig.5. The sheet resistance decreased from 987 to 608
Ω/□ as the laser power intensity increased from 21 to 25 W. Our analysis resulted in the fact that thecarrier concentration markedly increased from 2.9×1014 to 5.8×1014 cm-2 as laser power intensity
increased. Heating at high temperature effectively generated hole carriers. The activation ratio wasturned out to be 0.89 for a laser power of 25 W.
Figure 6 shows the transmissivity of microwave for the laser irradiated samples for laser powers of
21 and 25 W, which were the same samples measured by FTIR. The transmissivity of microwave werealso analyzed and the sheet resistance of the doped layer was estimated. They were presented in thehorizontal axis in Fig.6. The calculated curve of the transmissicity as a function of sheet resistance wasalso plotted in Fig.6. The transmissivity of microwave and the sheet resistance decreased from 9.2 to8.4% and 856 to 564 Ω/□, respectively, as the laser power intensity increased from 21 to 25 W.
Effective carrier generation was also observed by the microwave absorption method. The analyzed
sheet resistances were also presented in Fig.5 for comparison to FTIR results. There was rather good
agreement between values given by both methods. The carrier density was also presented when the
carrier mobility obtained by FTIR method was used.
1200

as-implanted

55
50
45

21W
25W

40
35
30
1000

experimental
calculated

Sheet Resistance [Ω/□]Transmissivity [%]

60

1000
800
600
400

5.8×1014cm-2

200

FTIR
Microwave

01500 2000 2500 3000
Wavenumber [cm-1]

3500

Fig. 4 ． The transmissivity spectra of thesamples as-implanted and the laser irradiatedfor implantation of boron clusters at 6 keVmeasured by FTIR. The calculated spectra arealso shown.

2.9×1014cm-2
13.1cm2/Vs
5.3×1014cm-2
11.8cm2/Vs
14-2
3.4×10 cm

21
25
Laser power [W]

Fig. 5. Sheet resistance of doped layer
analyzed by infrared light and microwave free
carrier absorption methods of the samples
as-implanted and the laser irradiated for
implantation of boron clusters at 6keV as afunction of laser power. Carrier concentrationand carrier mobility are also shown.

10
8
21W

25W

6
4

experimental
calculated

2
0
100

300
500
700
Sheet resistance [Ω/□]Electrical current density [A/cm2]

Transmissivity [%]

12

900

Fig. 6．The transmissivity of microwave of thelaser irradiated samples for implantation ofboron clusters at 6 keV as a function of the
sheet resistance of the doped layer. Calculatedtransmissivity is also shown.

100

p
SiO2

10-2

25W

Al
n

21W

10-4
10-6
10-8
-1.0

0
Voltage [V]1.0

Fig. 7 ． Electrical current as a function ofvoltage for pn diodes formed in the laser
irradiated regions for implantation of boron
clusters at 6 keV.

To evaluate the pn junction, the diodes with the mesa structure as shown by inset in Fig. 7. I-V
characteristics of pn junction formed for the laser irradiated samples for implantations of boron clusters
at 6keV are presented in Fig.7. The ideality factor and the reverse bias current were 1.80 and
4.7μA/cm2 for 21 W, and 1.42 and 8.6μA/cm2 for 25 W, respectively. This result revealed that a heavily
doped layer with a thin depth was well activated with a low defect density by CW infrared
semiconductor laser irradiation.
Figure 8 shows the FTIR transmissivity spectra of the samples as-implanted and laser irradiated for
implantations of phosphorus at 70 keV (a) and of BF3 ions at 10keV (b). Laser irradiations at 25 W for
1.2 and 2.6 ms were conducted. The transmissivity decreased by laser annealing. Calculated spectra
were also plotted and they agreed well to the experimental ones. The spectra were almost same between
laser irradiation for 1.2 and 2.6 ms for phosphorus and BF3 doping cases. It indicates the laser dwell
time of 1.2 ms was enough and dopant atoms were well activated. Figure 9 shows our precise analysesof sheet resistance, carrier concentration and carrier mobility. Sheet resistance slightly decreased from
121 to 104 Ω/□ as the dwell time increased 1.2 to 2.6 ms for phosphorus doping because the carrier
60

as-implanted (a)

50
40
1.2ms
30
2.6ms
20
10
1000

experimental
calculated

as-implanted
Transmissivity [%]Transmissivity [%]

60

(b)

55
50

2.6 ms

1.2 ms

45
40
35
30
1000

experimental
calculated

1500 2000 2500 3000 3500
1500 2000 2500 3000 3500
-1
Wavenumber [cm ]Wavenumber [cm-1]
Figure 8．The transmissivity spectra of the as-implanted sample and the laser irradiated samples for
implantations of phosphorus atoms at 70 keV (a) and BF3 ions at 10keV (b) measured by FTIR. The
calculated spectra are also shown.

Summary

1400
Sheet resistance [Ω/□]concentration and the carrier mobility slightly
increased from 2.1×1015 to 2.2×1015 and 24.4 to26.4 cm2/Vs, respectively. The slight decrease in
sheet resistance was also observed in BF3 dopingfrom 945 to 921 Ω/□, which resulted fromincrease in the carrier concentration from
4.2×1014 to 4.4×1014. The activation ratio for 2.6
ms for phosphorus and BF3 were estimated to be
0.83 and 1.0, respectively. These results indicatethat low annealing has an advantage of carrier
generation, although the changes in those values
with laser dwell time are close to our resolution
limit.

1200

4.2×1014cm-2
14.5cm2/Vs

1000
800

BF3
Phosphorus

600
400
200

4.4×1014cm-2
14.1cm2/Vs

2.1×1015cm-2
24.4cm2/Vs

2.2×1015cm-2
26.6cm2/Vs

0
1.2
2.6
Dwell time [ms]Figure 9. Sheet resistance of doped layer
analyzed by infrared light free carrier absorption
method for implantations of phosphorus atoms at
70 keV as well as BF3 ions at 10keV as a functionof dwell time. Carrier concentration and carrier
mobility are also shown.

We report analysis of impurity doping using
infrared light and microwave free carrier
absorption method. We carried out implantationof boron clusters at 6.0×1014 cm-2 and 6keV,
phosphorus ions at 2.6×1015cm-2 and 70keV, and
BF3 ions at 4.4×1014cm-2 and 10keV.
200-nm-thick carbon films were formed on the silicon surface as a photo absorption layer using
sputtering method. The samples were irradiated with a 940 nm CW infrared semiconductor laser with abeam diameter of 180 μm to activate impurity atoms. Laser irradiations with a laser power of 21 and 25
W for 3.6 ms were conducted to the samples for implantations of boron clusters at 6keV. The sheet
resistance decreased from 987 to 608 Ω/□ as the laser power intensity increased from 21 to 25 W. The
carrier concentration markedly increased from 2.9×1014 to 5.8×1014 cm-2 as laser power intensity
increased. The activation ratio for 25 W was estimated to be 0.89. With microwave free carrier
absorption analysis the sheet resistance decreased from d 856 to 564 Ω/□. The diodes were fabricatedwith the mesa structure. The ideality factor and the reverse bias current were 1.80 and 4.7μA/cm2 for 21
W, and 1.42 and 8.6μA/cm2 for 25 W, respectively. This result revealed that a heavily doped layer witha thin depth was well activated with a low defect density by CW infrared semiconductor laser
irradiation. Laser irradiations with a laser power of 25 W for 1.2 and 2.6 ms were conducted to thesamples for implantations of phosphorus atoms at 70keV as well as BF3 ions at 10keV. Sheet resistanceslightly decreased from 121 to 104 Ω/□ as the dwell time increased 1.2 to 2.6 ms for phosphorus
doping owing to the slight increases in carrier concentration and carrier mobility. The slight decrease in
sheet resistance was also observed in BF3 doping from 945 to 921 Ω/□ owing to the slight increase incarrier concentration. The activation ratio for 2.6 ms for phosphorus and BF3 were estimated to be 0.83
and 1.0, respectively.
Acknowledgement
This work was partially supported by CASIO Science Promotion Foundation.
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L620-L622.
[2] T.SAMESHIMA,M.MAKI, M.TAKIUCHI, N. ANDOH, N. SANO, Y. MATSUDA and Y. ANDOH,
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