P-16Carbon Heating Tube Rapid Heating System for Fabricating Silicon Solar Cells
Tomoyoshi Miyazaki1,3, Go Kobayashi2, Izumi Serizawa2, Toshitaka Kikuchi3, Takuma Uehara3, Takuji Arima3,Masahiko Hasumi3 and Toshiyuki Sameshima3
1

Techno Research., Ltd., Koganei, Tokyo, 184-0012, Japan2

Orc manufacturing Co., Ltd. 4896 Tamagawa, Chino, 391-0011, Japan

3

Tokyo University of Agriculture and Technology, Graduate School of Engineering, 2-24-16, Naka-cho, Koganei, Tokyo, 184-8588, Japan
Email: tsamesim@cc.tuat.ac.jp

We report 2.45-GHz microwave heating system with a carbon heating tube (CHT) made by a 4-mm-diameter and 60-mm-long-quartz tube
filled with conductive carbon particles and Ar gas at 1400 Pa set in a 300-mm-diameter-sized metal cavity. The three-dimensional finite
element numerical simulation method resulted in the most effective electrical conductivity of CHT ranged from 10 to 55 S/m to absorb themicrowave power. A proportional-integral-differential feedback CHT temperature control system was used to activate 1.0x1015-cm-2-boron
and phosphorus implanted regions in n-type crystalline silicon substrate from 1000 to 1200oC. The CHT heating at 1200oC realized decreasein the sheet resistivity to 146 Ω/sq, decrease in the density of defect states to 1.3x1011 and 9.2x1010 cm-2 for boron (p+) and phosphorus (n+)
implanted surfaces, and solar cell characteristic with a conversion efficiency of 15% under illumination of air mass 1.5 at 0.1 W/cm2.
Introduction
We recently proposed a new heating system with a carbon

irradiation analyzed by a numerical simulator constructed with theheating tube (CHT) lamp made with a quartz tube filled withthe effective condition of microwave absorption by the CHT. Then

carbon particles and Ar gas.1,2) 2.45-GHz microwave at 200 W
introduced into a metal cavity with the CHT was effectively

we demonstrate silicon solar cell fabrication using PID temperature
controlled CHT heating. The effective lifetime（τeff）, and solar cellabsorbed by carbon in the CHT and heated it to a high temperature

characteristics are reported.

1.

three-dimensional finite element moment method12) to determine

oof 1279 C for 33 s irradiation. Moreover, we achieved thetemperature

controloftheCHTusinga3)proportional-integral-differential feedback (PID) circuit which

2.

Experimental

regulated the magnetron power source with a signal of a radiation

2-1. CHT heating system
Figure 1 shows a schematic image of a CHT heating system.

type thermometer. Successful control of a constant temperature at

For fabricating CHTs, quartz tubes with 60-mm long and 4-mm

1100oC was demonstrated. We also achieved crystallization ofinter diameter and 6-mm outer diameter were made.

amorphous silicon films with a high crystalline volume ratio of

2-μm-diameter-carbon powders with a 0.1-g weight were put in

0.92 and activation of boron and phosphorus atoms implanted with

the quartz tubes, in which the packing density was controlled as15-2cm . The CHT heating system would be0.09 with an electrical conductivity of 55 S/m, which wasapplicable to fabrication of semiconductor devices such as thin

determined by the previous work as the best value for heating.13) Ara dose of 1x104-7)

film transistors (TFTs) and solar cells , although many heating
technologies have been developed such as rapid thermal annealing,laser annealing8,9), plasma jet annealing10), and rapid thermal
annealing.11) It gives an advantage of no electrical wiring to theCHT lamp. It allows a simple heating system and application toheat treatment in severe environment for example high humidity
atmosphere. Moreover, a capability of CHT lamp with highly
thermal proof structure is realized because the junction between
lamp materials and the electrode metals has always a serious
problem of thermally stress-induced damage.
In this paper, we first report characterization of the electric field
distribution in the cavity in the steady state during microwave

Fig. 1 Schematic image of a CHT heating system

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inert gas was filled in the quartz tube at 1400 Pa. The edges of themicrowave absorption by the CHT. The calculation of electric field

quartz tubes were closed by thermal welding. Two quartz rods

distribution in the cavity was also conducted in the case of nowere jointed at the edges of the CHT by thermal welding to hold

CHT.

the CHT. A CHT was used in a heating system, which consists of amagnetron for generating 2.45-GHz microwave with an electrical
power source, an isolator for blocking the reflectance of

2-3. Solar cell fabrication
4-inch-diameter 500-μm-thick 17-Ωcm n-type silicon substrates

microwave backward the magnetron, a power monitor, ancoated with 100-nm-thick thermally grown SiO2 layers wereimpedance matching box for reducing microwave reflectance, aprepared. The ion implantation of boron atoms was conducted withwave-guide tube, and a cavity made by Al metals. The Microwave

a dose of 2x1015 cm-2 at 25 keV to the top silicon surface of thegenerated by the magnetron was introduced to the cavity by thesubstrates. Phosphorus atoms with 2x1015 cm-2 at 75 keV was also

wave guide tube via the isolator, power monitor, and impedance

implanted to the rear surface. The boron and phosphorus atoms

matching box. The cavity had a structure with a cylindrically upperwith a dose of 1.0x1015 cm-2 were effectively incorporated in thepart with a diameter of 300 mm and a spherically lower part totop and rear silicon surface regions. The samples were cut to 4

promote interferingly multiple reflection of the microwave in thesectored pieces. They were placed in the heating system just below

cavity and to have the CHT effectively absorb the microwave.

the CHT and moved at 0.12 mm/s. The CHT temperature wasThose works resulted in a reflected power less than 4 W in the casecontrolled by PID feedback method at 1000, 1100, and 1200oC.

of an input power of 500 W. In contrast, the power monitor gave

After heating, the thermally grown SiO2 layer was removed using

the same value of the input and reflected power when the cavityhydro-fluoric acid. To estimate the sheet resistivity and effective

had no CHT. The cavity completely reflected the microwave

minority carrier lifetime τeff, we measured the 9.35 GHzpower. The cavity had a small window port for observing lightmicrowave transmittance of the samples in dark field andemission from CHT heated by microwave. A thermometer

635-nm-light illumination.14) Comb-type Al electrodes weredetecting 900-nm wavelength radiation light CHINO IR-FAS wasformed on the top surface and the rear sur-face was entirely coatedused to monitor the temperature of the CHT in real time. Thewith Al electrodes by vacuum evaporation. 100-nm-thick AlOxodetection lower limit of the thermometer was about 550 C. Thelayers was subsequently deposited on the top surfaces as

analog voltage signal of the thermometer is monitored in real timeanti-reflection layer. The samples were heated in H2O vaper at 0.8

by a PID circuit controller which was installed with a plan ofMPa at 230oC for 3 h to decrease defect states in the doped

temperature increasing and decreasing ratios, a target temperature

regions.15) The electrical current density as a function of applied

and its duration in its memory in advance. The controller sent avoltage (J–V) was measured under illumination of air mass (AM)signal to the power source of the magnetron to increase or decrease

1.5 light at 0.1 W/cm2.

the microwave power for coinciding the CHT temperature with theinitial temperature plan. A mechanically moving stage formed byquartz pipes was also installed in the cavity to move a sample justbelow the CHT.

3.

Results and discussion
Figure 2 shows calculated cross-sectional electric field

distributions in the cavity (a) with no CHT, (b) with 55-S/cm-CHTplaced at the central region of the cavity, and (c) with the2-2. Numerical calculation
The electric field distribution in the cavity in the steady state

10000-S/cm-CHT placed at the same position as (b) when the 200

during microwave irradiation was numerically analyzed by acavity and CHT surface outlines were traced by sold black curves.

simulator constructed with the three-dimensional finite element

The calculation resulted in the standing wave of the electric field in

moment method combined with the Cholesky decomposition. The

every case because of the three-dimensional Fresnel interference

grid system of the cavity and waveguide was formed with theeffect observed in the cavity. The numerical calculation resulted inequilateral-triangle elements with the sides ranging from 1 to 4 mm

existence of a high electric field intensity ranging from 1000 tofor the numerical calculation. They were assumed as perfect

6000 kV/m in the case of no CHT, as shown in Fig. 2(a). In

electric conductor which reflects completely the microwave. Thecontrast, the electric field intensity was very low in the case of theCHT grid was formed with 1-mm-side-equilateral-triangle

cavity with the CHT. The most of region of the cavity had theelements. The electrical conductivity of carbon was widely

electric field ranged from 1 to 6 kV/m, as shown in Fig. 2(b). It

changed from 0.1 to 10000 S/m to find an effective condition of

was reduced to at least 0.1% of the initial intensity of the cavity

W microwave was continuously introduced in the cavity. TheAM-FPD'19

was 5 kV/m when the electrical conductivity ranged between 10and 55 S/m. This means that the effective absorption of themicrowave power by CHT reduced the electric field intensity inthe cavity. The average electric field intensity gradually increased
as the electrical conductivity further increased. The calculation
supports experimental results of low microwave refection from thecavity and effective heating of the CHT.1,2)

Fig. 3 Average electric field intensity in the cavity space as afunction of the electrical conductivity of the CHT
Figure 4 shows (a) the sheet resistivity of the samples and (b) τeffin the case of 635 light illumination to the boron (solid circles) andphosphorus (open circles) implanted surfaces as functions of CHT
heating temperature. Although, the sheet resistivity of as-implanted
sample was high of 626 Ω/sq caused by 9.35-GHz-microwave
absorption by the low concentration of the electron carriers in thesilicon bulk, it decreased from 215 to 146 Ω/sq as the CHT heating
temperature increased from 1000 to 1200oC. This experimental
result shows that the activation in implanted region and carrier
generation were successfully achieved by CHT heating. τeff wasFig. 2 Calculated cross-sectional electric field distributions in
the cavity (a) with no CHT, (b) with 55-S/m-CHT placed at thecentral region of the cavity, and (c) with the 10000-S/m-CHT
when the 200 W microwave was continuously introduced in thecavity.

also increased by CHT heating, although it was low of 1.1x10-5
and 8.1x10-6 s in the cases of light illumination to the as-boron andas-phosphorus implanted surfaces. The 1200oC-CHT-heating
achieved 2.0x10-4 and 2.2x10-4 s in the cases of 635-nm-lightspace without CHT. On the other hand, the electric field intensity
distributed from 10 to 60 kV/m in the case of 10000-S/m-CHT, as
shown in Fig. 2(c). These calculated results indicate that the cavity
effectively accumulated the microwave power, and that themicrowave power in the cavity was almost completely absorbedby the 55-S/m-CHT. Absorption of microwave power depends onthe electrical conductivity. A CHT with a very high electricalconductivity (a high packing density of carbon particles) is notsuitable because of high reflectivity caused by a high extinction
coefficient. Figure 3 shows the average electric field intensity in thecavity space as a function of the electrical conductivity of the CHT.
Although the average electric field intensity was very high of 1173
KV/m in the case of no CHT, it was decreased by installing CHTs,
as shown in Fig. 3. The minimum average electric field intensity

Fig. 4 (a) Sheet resistivity of the samples and (b) τeff in the caseof 635 light illumination to the boron (solid circles) andphosphorus (open circles) implanted surfaces as functions ofCHT heating temperature. Arrows ① and ② represent τeff inthe cased of light illumination to the boron and phosphorus
implanted surfaces.

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illumination to the boron and phosphorus implanted surfaces. This

cm-2 for boron (p+) and phosphorus (n+) implanted surfaces. Viameans that defect states caused by ion implantation wereformation of Al metal electrodes and AlOx-antireflection layer

effectively reduced. Our numerical analysis resulted in that theresulted in solar cell characteristics with the short circuit current

1110-2

recombination defect densities were 1.3x10 and 9.2x10 cm forboron (p +) and phosphorus (n +) implanted surfaces.16)
Figure 5 shows characteristics of solar cells fabricated with CHT
heating at temperatures of 1000, 1100, and 1200oC. The AM 1.5

density, open circuit voltage, fill factor, and conversion efficiencyof 39 mA/cm2, 0.55 V, 0.69, and 15%, respectively, underillumination of air mass 1.5 at 0.1 W/cm2.

light illumination realized typical photovoltaic characteristics forevery solar cell sample. The short circuit current density and open
circuit voltage increased as the CHT heating temperature increased.
The 1200oC heating realized the best performance with the shortAcknowledgments
This work was partially supported by Japan Science and Technology
Agency ASTEP (No. AS3015022S).

circuit current density, open circuit voltage, fill factor, andconversion efficiency of 39 mA/cm2, 0.55 V, 0.69, and 15%,
respectively. These results indicate that the temperature-controlled
CHT heating has a capability of activating dopant-implanted
regions, reducing the defect states, realizing high performancesilicon solar cells.

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Fig. 5 Characteristics of solar cells fabricated with CHT heating
temperatures of 1000, 1100, and 1200oC.

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4.

Conclusions
Silicon solar cells were fabricated by the CHT heating to form

the pn junction in 500-μm-thick single crystalline n-type silicon
substrates. 1.0x1015 cm-2 boron and phosphorus ion implanted
regions were heated from 1000 to 1200oC by the PID feedback
CHT heating equipment which consisted of 2.45-GHz microwave

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three-dimensional finite element numerical simulation method
resulted in the most effective electrical conductivity of CHT range
from 10 to 55 S/m to absorb the microwave power. The sheet
resistivity decreased to 146 Ω/sq by 1200oC heating and thedensity of defect states also decreased to 1.3x1011 and 9.2x1010

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