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Lab-PC+ and Controlling of Miniature Alloying Furnace Unit
for Ohmic Contac Annealing
by
Hasan Efeoglu, Cevdet Coskun, Tevhit Karacali*, Sakir Aydogan,Y Kemal Yogurtcu
Faculty of Science and Arts, *Faculty of Engineering
Ataturk University
Category:
Semiconductor
Products Used:
Lab-PC+
NI-DAQ™
The Challenge:
Construction of a miniaturised furnace for alloying process and using a semiconductor research laboratory for
ohmic contact thermal treatment to get a much controllable process cycles from sample to sample
The Solution:
To develop a signal conditioner unit and a decoder unit for the functions of analogue instrument. Use of LabPC+ with NI-DAQ for data acquisition. Development of a software for temperature measurement, drive power
unit with digital PID using Q-Basic 4.5
Abstract
Ohmic contact formation to semiconductors is an essential process during the device production. In a research
program, repeatability and simplicity of ohmic contact process are the key factors. Conventional furnaces for
thermal treatment are not practical especially for short processes. Miniaturised alloying furnace with less
contamination and well definable heating cycle is well suited for annealing treatment with less capital spent.
That unit is commercially available, however a flexible one can be built and very well controlled using a
Lab-PC+ with digital PID. A practical design and control algorithm with NI-DAQ functions presented in
this study.
Introduction
In assessment of semiconductor materials and devices, measurements such as resistivity, Hall mobility,
magnetoresistance, DLTS, I/V, C/V are traditionally used. These type of measurements require reliable and
repeatable ohmic contacts to be made to the sample. The easiest way in obtaining a contact which satisfies the
ohmic contact definition is the melting on to one surface of the semiconductor slice a bead of metal loaded.
Such as In on GaAs. Also thermal diffusion of a suitable dopand from a preloaded metal (Au:Sb %2 Sb for nsilicon, Au:Ge, %12.5 Ge for n-GaAs, e.g.) after metalisation in vacuum is possible in getting high quality
ohmic contacts. The construction of a miniature furnace with small mass for short and repeatable annealing
process (down to 30 sec) is the main object of this study.
Miniature Alloying Furnace Design And its Electronic
The constructed alloying furnace cross section drawing is given in Figure 1. Aluminium base plate was coated
with TiN because of its standing against to corrosion. This coating also provides a clean construction. Optical
observation of sample under the thermal treatment was made possible by using a pair of Pyrex tube with one end
closed. The outer pyrex undetachable, for sample locating on top of the inner pyrex. Between two pyrexs a dry
nitrogen gas flows during the process. Specially made ceramic disk with embedded heater element for this
application is located inside of inner pyrex. That construction is isolated the heater to get in contact with sample.
Just above of heating element a 100Ω platinum sensor is located for temperature measurement. Locating Pt
sensor very close to heater element was provided us a fast feedback in controlling the temperature.
100Ω Platin
Resistance
Sample
Pyrex enclosure
Braze Fixing
ring
O-Ring
Outlet for nitrogen
The heart of the electronic control is the
controlling power triac by phase using a
TCA 785 integrated circuit. The
triggering angle of triac can be adjusted
continuously between 0o and 180o with
the aid of external voltage supplied from
DAC0 output of Lab-PC+ with 12 bit
resolution. The output of Lab-PC+ card
was 0-5V and that voltage conditioned to
give 10-0V as shown in Figure 2.
Heater inside
ceramic disc
TiN coated
Al Base
Inlet for nitrogen
Connection for power and
temperature measurement
100Ω Pt resistance from Sensing Device
Limited (SDL) is used as a temperature
sensing element. Constant current at
Figure 1. Drawing of Miniature Alloying Furnace.
1 mA is used to measure temperature
dependent resistance. Four wire
connection is chosen to get correct reading. The voltage appeared across Pt temperature detector conditioned
by a instrumentation amplifier as given in Figure 2c.
Linearisation of Voltage Corresponds to Temperature
Resistance correspond to temperature of sensor is obtained from the data sheet of 100Ω Pt supplied from SDL
(1). As usual, there was no linear connection between resistance and temperature. A numerical linearisation
process is carried out as described elsewhere (2).
The calculated linearisation equation using 0-600 oC range data set is given for Pt by
Tsamp. = 200 *V ACH 0 −
1
(0.0034234 *V ACH 0
− 0.33428
+ 7.1126E − 07)
(after simplification) Where Tsamp. is in Celsius. The VACH0 voltage corresponds to 17.85x0.001*RPlatin and the
17.85 is the gain of instrumentation amplifier, 0.001 is the constant current driven to Pt resistance and the
(0.001* RPlatin) is the voltage appeared at amplificator input. The errors given in Figure 3 were less than %1 in
that calculation. That direct calculation of temperature increased the system response to get stabilise the heater
power level at target temperature. A soft approach to target temperature was obtained using digital PID control.
The equation used for that control is given by
V DAC 0 = P * T samp.error + I * Tsamp.error * ∆t − D *
∆Tsamp
∆t
where P, I and D are the proportion, integration and derivation constants, respectively. Tsamp error is the difference
between desired temperature and last measured sample temperature. ∆t is the time between last two
measurements and ∆Tsamp is the difference between last two temperature measurements.
220K
Sync.
AC 25V 5A
to heater
+12 V
0.47µF
10K
1N4007 5
6
470µF
16V
1N4007
16
15
14
13
12
11
10
9
BTA16
150
820
2K2
-
LF351
+12V
20K
10K
1K
3K3
1K
+12V
3V9
1µF
+
LF351
0-5V
from DAC0
(power level
control)
47nF 150pF
(a)
1K
0.1µF
0.1µF
+
0.1µF
+
LF351
2.2µF
100K
1
1K
to
ACH0
10K
-
to
Pt
sensor
+
10K -
LF351
LF351
+
10K
10K -
From
Pt
sensor
LF351
+
1µF
10K
(b)
(c)
Communication with Lab-PC+ and Test
Measurements
A control programme written in QB4.5 was used
for the temperature measurement, power
percentage control with digital PID algorithm and
real time status presentation in graphical
environment. The developed program was used
the functions of NI-DAQ for DOS. As given in
Figure 4 analogue voltage corresponds to
temperature read from ACH0 input of LAB-PC+
card. The desired voltage to drive power, applied
from DAC0 output of same I/0 card. Measured
and applied voltage resolution was 12 bit and
range was ±5 V.
Error after Linearisation (oC)
Figure 2. Open Circuit of a) Power Control Unit, b) Constant Current Source , set to 1mA and
c) Voltage Reading Unit for Temperature.
3.0
1.5
0.0
-1.5
0
200
400
Set Temperature (oC)
600
-3.0
Figure 3. Errors after Linearisation for 100-550 oC
Temperature Range.
+12V
CURRENT
SOURCE
LAB
- +
PC+
1 mA
TEMP.
SENSE
POWER
UNIT
BUFFER
ACH0
1
DAC0
10
AGND
11
PC
Figure 4. A block diagram of Setup Used for Controling Aloying Furnace.
400
400
300
300
300
200
100
Temperature (oC)
400
Temperature (oC)
Temperature (oC)
PID parameters are software selectable. That gives a chance to one to select the suitable parameters for sensorheater pair. The feedback speed is one of the most important factor for short time annealing processes. The
miniaturisation provided to us a small mass for rapid heating and a compact construction provided a fast
feedback. With that system we were managed to carry out down to 30 sec annealing process. Three heating and
cooling cycles for 400 oC and 2 min time are given in Figure 5 with different PID parameters.
200
100
100
Anneal Temp. 400 oC
t= 2 min, P=0.7, I= 0.175, D= 2.0
Anneal Temp. 400 oC
t= 2 min, P=0.8, I= 0.175, D= 1.0
0
0
0
160
320
480 640
Annealing Time (sec)
200
Anneal Temp. 400 oC
t= 2 min, P=0.5, I= 0.175, D= 4.0
0
0
160 320
480
640
Annealing Time (sec)
0
160
320
480
640
Annealing Time (sec)
Figure 5. Three Heating Cycles with Different PID Sets Show System Designed with the Combination of Lab-PC+
can be Used for Thermal Annealing Processes. The Applied Power was 70% of Available Power and Heating Rate
was 2.7 oC/sec.
Conclusion
In conclusion the test measurements showed that the designed compact system can be used for annealing
processes in a semiconductor research. PID control algorithm and a fast responding of I/O card can heat the
sample to the process temperature without any overshot. Further reduction in mass of heater block can increase
the heating and cooling rate which that also shortens the total process time.
Acknowledgement
This instrumentation carried out for the project TBAG-1828 supported by Scientific and Technical Research
Council of Turkey (TUBITAK).