U.S. patent application number 11/806159 was filed with the patent office on 2008-12-04 for substrate temperature accuracy and temperature control flexibility in a molecular beam epitaxy system.
Invention is credited to Stefan P. Svensson.
Application Number | 20080295764 11/806159 |
Document ID | / |
Family ID | 40086725 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080295764 |
Kind Code |
A1 |
Svensson; Stefan P. |
December 4, 2008 |
Substrate temperature accuracy and temperature control flexibility
in a molecular beam epitaxy system
Abstract
A control system and method for controlling temperatures while
performing a MBE deposition process, wherein the control system
comprises a MBE growth structure; a heater adapted to provide heat
for the MBE deposition process on the MBE growth structure; and a
control computer adapted to receive a plurality of dynamic feedback
control signals derived from the MBE growth structure; switch among
a plurality of control modes corresponding with the plurality of
dynamic feedback control signals; and send an output power signal
to the heater to control the heating for the MBE deposition process
based on a combination of the plurality of control modes. In one
embodiment, the plurality of dynamic feedback control signals
comprises thermocouple signals and pyrometer signals.
Inventors: |
Svensson; Stefan P.;
(Columbia, MD) |
Correspondence
Address: |
U S ARMY RESEARCH LABORATORY;ATTN AMSRL CS CC IP
2800 POWDER MILL RD
ADELPHI
MD
207831197
US
|
Family ID: |
40086725 |
Appl. No.: |
11/806159 |
Filed: |
May 30, 2007 |
Current U.S.
Class: |
117/86 ;
117/202 |
Current CPC
Class: |
Y10T 117/1008 20150115;
C30B 23/002 20130101; C30B 29/40 20130101 |
Class at
Publication: |
117/86 ;
117/202 |
International
Class: |
C30B 25/16 20060101
C30B025/16 |
Goverment Interests
GOVERNMENT INTEREST
[0001] The embodiments described herein may be manufactured, used,
and/or licensed by or for the United States Government without the
payments of royalties thereon.
Claims
1. A computer-implemented method of controlling temperatures while
performing a molecular beam epitaxy (MBE) deposition process, said
method comprising: providing a heating sequence for said MBE
deposition process on a MBE growth structure; receiving, in a
control computer, a plurality of dynamic feedback control signals
derived from said MBE growth structure; switching, in said control
computer, among a plurality of control modes corresponding with
said plurality of dynamic feedback control signals; and sending an
output power signal from said control computer to said MBE growth
structure to control said heating for said MBE deposition process
based on a combination of said plurality of control modes.
2. The method of claim 1, wherein said plurality of dynamic
feedback control signals comprises thermocouple signals and
pyrometer signals.
3. The method of claim 2, further comprising: receiving data
related to material properties of said MBE growth structure;
selecting temperature values based on said material properties of
said MBE growth structure; heating said MBE growth structure based
on the selected temperature values; growing crystals on the heated
MBE growth structure; and cooling said MBE growth structure.
4. The method of claim 2, further comprising performing a
thermocouple calibration sequence on a thermocouple and a pyrometer
monitoring said MBE growth structure.
5. The method of claim 2, further comprising setting proportional
integrating derivative (PID) control parameters in said control
computer to set a level of said output power signal.
6. The method of claim 1, further comprising: establishing output
power signal levels in said control computer; directly sending said
output power signal levels from said control computer to a power
supply unit; and controlling temperatures in said MBE deposition
process based on said output power signal levels.
7. A program storage device readable by computer, tangibly
embodying a program of instructions executable by said computer to
perform a method of controlling temperatures while performing a
molecular beam epitaxy (MBE) deposition process, said method
comprising: providing a heating sequence for said MBE deposition
process on a MBE growth structure; receiving, in a control
computer, a plurality of dynamic feedback control signals derived
from said MBE growth structure; switching, in said control
computer, among a plurality of control modes corresponding with
said plurality of dynamic feedback control signals; and sending an
output power signal from said control computer to said MBE growth
structure to control said heating for said MBE deposition process
based on a combination of said plurality of control modes.
8. The program storage device of claim 7, wherein said plurality of
dynamic feedback control signals comprises thermocouple signals and
pyrometer signals.
9. The program storage device of claim 8, wherein said method
further comprises: receiving data related to material properties of
said MBE growth structure; selecting temperature values based on
said material properties of said MBE growth structure; heating said
MBE growth structure based on the selected temperature values;
growing crystals on the heated MBE growth structure; and cooling
said MBE growth structure.
10. The program storage device of claim 8, wherein said method
further comprises performing a thermocouple calibration sequence on
a thermocouple and a pyrometer monitoring said MBE growth
structure.
11. The program storage device of claim 8, wherein said method
further comprises setting proportional integrating derivative (PID)
control parameters in said control computer to set a level of said
output power signal.
12. The program storage device of claim 7, wherein said method
further comprises: establishing output power signal levels in said
control computer; directly sending said output power signal levels
from said control computer to a power supply unit; and controlling
temperatures in said MBE deposition process based on said output
power signal levels.
13. A control system for controlling temperatures while performing
a molecular beam epitaxy (MBE) deposition process, said control
system comprising: a MBE growth structure; a heater adapted to
provide heat for said MBE deposition process on said MBE growth
structure; and a control computer adapted to: receive a plurality
of dynamic feedback control signals derived from said MBE growth
structure; switch among a plurality of control modes corresponding
with said plurality of dynamic feedback control signals; and send
an output power signal to said heater to control said heating for
said MBE deposition process based on a combination of said
plurality of control modes.
14. The control system of claim 13, wherein said plurality of
dynamic feedback control signals comprises thermocouple signals and
pyrometer signals.
15. The control system of claim 13, wherein said control computer
is further adapted to: receive data related to material properties
of said MBE growth structure; select temperature values based on
said material properties of said MBE growth structure; and send
power signals to said heater to allow heating of said MBE growth
structure based on the selected temperature values, wherein
crystals are grown on the heated MBE growth structure.
16. The control system of claim 15, further comprising a
thermocouple and a pyrometer adapted to monitor said MBE growth
structure, wherein said control computer is further adapted to
perform a thermocouple calibration sequence on said thermocouple
and said pyrometer.
17. The control system of claim 15, wherein said control computer
is further adapted to set proportional integrating derivative (PID)
control parameters to set a level of said output power signal.
18. The control system of claim 14, further comprising a power
supply unit, wherein said control computer is further adapted to:
establish output power signal levels; directly send said output
power signal levels to said power supply unit; and control
temperatures in said MBE deposition process based on said output
power signal levels.
19. The control system of claim 14, wherein said MBE growth
structure comprises a substrate wafer.
20. The control system of claim 15, further comprising a
thermocouple and a non-contact temperature monitor adapted to
monitor said MBE growth structure, wherein said control computer is
further adapted to perform a thermocouple calibration sequence on
said thermocouple and said non-contact temperature monitor.
Description
BACKGROUND
[0002] 1. Field of the Invention
[0003] The embodiments herein generally relate to thermometry, and,
more particularly, to methods for controlling growth temperatures
in a molecular beam epitaxy system.
[0004] 2. Description of the Related Art
[0005] Molecular Beam Epitaxy (MBE) is one of a family of methods
used to grow single-crystal films on single crystal substrates
(epitaxy). In an MBE system, a substrate, on which the crystalline
film is to be grown, and several material sources are contained in
an ultra-high vacuum chamber. The sources are typically furnaces,
each containing a specific element to be deposited on the
substrate. A furnace uses an open crucible in which the evaporant
is placed. Moreover, resistive heating wires and heat shields
generally surround the crucible. Generally, the evaporant is heated
to a temperature that produces a material flux of desirable
magnitude from the crucible opening (a molecular beam) and which is
aimed towards the substrate crystal, where the molecules are
allowed to condense. The beam is turned on and off by a mechanical
shutter blade in front of the crucible opening. High stability of
the flux, and therefore growth rate, is accomplished by high
stability of the furnace temperatures.
[0006] In addition to stable source temperatures, a stable process
relies on a well-controlled substrate temperature. The substrate is
warmed by a resistively heated element placed behind it. Because
the substrate should be allowed to rotate around its azimuthal axis
during deposition to ensure good layer uniformity, there are
generally inadequate solutions to mechanically contact the
substrate for measurement of the temperature. Usually, a
thermocouple is placed somewhere behind the substrate in the
vicinity of the back side and the heater element. A substantial
difference between the real temperature of the substrate front side
and the thermocouple reading is therefore commonly observed. To
achieve a stable temperature, a proportional integrating derivative
(PID) control unit is typically used, which sets the output level
from a power supply to the substrate heater, depending on the
thermocouple reading. In many systems, the temperature setpoints
are set by a digital control computer, which also opens and closes
the mechanical shutters in front of the evaporation sources for
predetermined times, thus producing specific film thicknesses.
[0007] The difference between the thermocouple reading and the true
temperature is generally determined empirically. The most common
way of obtaining more reliable temperature readings is to use an
optical pyrometer. Typically, it is first calibrated by observing
some type of phase transformation that is known to take place at a
well-defined temperature. The pyrometer is then adjusted by setting
a value for the apparent emissivity of the substrate so that the
instrument reads the desired value.
[0008] It is possible to use the pyrometer signal directly as input
in the feed back loop. This could be accomplished by hard wiring
the pyrometer to the PID controller, or by feeding it to the
control computer and letting it send appropriate signals to the PID
controller. However, in either case, the use of the pyrometer is
limited to temperature ranges above several hundred degrees,
typically above 400.degree. C. Since the substrate is at room
temperature at the start of the process, pyrometers are generally
not useful during the initial warm up phase. Also, during the
deposition phase some materials may require deposition at
temperatures below 400.degree. C. In addition, some films and some
holders of small substrates are prone to let stray light from the
evaporations sources enter the pyrometer, which can produce
erroneous readings. In these cases thermocouple control, or
constant power output are the preferred choices.
[0009] Finally, deposition of layers that aim to produce structures
with optical interference properties, such as a Bragg mirror, can
be used. During deposition of such films the signal reaching a
pyrometer as well as the temperature observed by the thermocouple
exhibit strong oscillations, making both generally unreliable and
unsuitable for feedback control. Accordingly, there remains a need
to improve the MBE deposition sequence via more accurate
temperature control.
SUMMARY
[0010] In view of the foregoing, an embodiment herein provides a
computer-implemented method of controlling temperatures while
performing a MBE deposition process, and a program storage device
readable by computer, tangibly embodying a program of instructions
executable by the computer to perform a method of controlling
temperatures while performing a MBE deposition process, wherein the
method comprises providing a heating sequence for the MBE
deposition process on a MBE growth structure; receiving, in a
control computer, a plurality of dynamic feedback control signals
derived from the MBE growth structure; switching, in the control
computer, among a plurality of control modes corresponding with the
plurality of dynamic feedback control signals; and sending an
output power signal from the control computer to the MBE growth
structure to control the heating for the MBE deposition process
based on a combination of the plurality of control modes.
Preferably, the plurality of dynamic feedback control signals
comprises thermocouple signals and pyrometer signals. The method
may further comprise receiving data related to material properties
of the MBE growth structure; selecting temperature values based on
the material properties of the MBE growth structure; heating the
MBE growth structure based on the selected temperature values;
growing crystals on the heated MBE growth structure; and cooling
the MBE growth structure. Moreover, the method may further comprise
performing a thermocouple calibration sequence on a thermocouple
and a pyrometer monitoring the MBE growth structure. Furthermore,
the method may further comprise setting PID control parameters in
the control computer to set a level of the output power signal.
Additionally, the method may further comprise establishing output
power signal levels in the control computer; directly sending the
output power signal levels from the control computer to a power
supply unit; and controlling temperatures in the MBE deposition
process based on the output power signal levels.
[0011] Another embodiment provides a control system for controlling
temperatures while performing a MBE deposition process, wherein the
control system comprises a MBE growth structure; a heater adapted
to provide heat for the MBE deposition process on the MBE growth
structure; and a control computer adapted to receive a plurality of
dynamic feedback control signals derived from the MBE growth
structure; switch among a plurality of control modes corresponding
with the plurality of dynamic feedback control signals; and send an
output power signal to the heater to control the heating for the
MBE deposition process based on a combination of the plurality of
control modes. In one embodiment, the plurality of dynamic feedback
control signals comprises thermocouple signals and pyrometer
signals. Preferably, the control computer is further adapted to
receive data related to material properties of the MBE growth
structure; select temperature values based on the material
properties of the MBE growth structure; and send power signals to
the heater to allow heating of the MBE growth structure based on
the selected temperature values, wherein crystals are grown on the
heated MBE growth structure. Moreover, in one embodiment, the
control system may further comprise a thermocouple and a pyrometer
adapted to monitor the MBE growth structure, wherein the control
computer is further adapted to perform a thermocouple calibration
sequence on the thermocouple and the pyrometer. Preferably, the
control computer is further adapted to set PID control parameters
to set a level of the output power signal. Additionally, the
control system may further comprise a power supply unit, wherein
the control computer is further adapted to establish output power
signal levels; directly send the output power signal levels to the
power supply unit; and control temperatures in the MBE deposition
process based on the output power signal levels. Moreover, the MBE
growth structure may comprise a substrate wafer. In another
embodiment, the control system further comprises a thermocouple and
a non-contact temperature monitor adapted to monitor the MBE growth
structure, wherein the control computer is further adapted to
perform a thermocouple calibration sequence on the thermocouple and
the non-contact temperature monitor.
[0012] These and other aspects of the embodiments herein will be
better appreciated and understood when considered in conjunction
with the following description and the accompanying drawings. It
should be understood, however, that the following descriptions,
while indicating preferred embodiments and numerous specific
details thereof, are given by way of illustration and not of
limitation. Many changes and modifications may be made within the
scope of the embodiments herein without departing from the spirit
thereof, and the embodiments herein include all such
modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The embodiments herein will be better understood from the
following detailed description with reference to the drawings, in
which:
[0014] FIG. 1 illustrates a schematic diagram of a hardware
configuration according to an embodiment herein;
[0015] FIGS. 2 through 5 are flow diagrams illustrating methods
according to the embodiments herein;
[0016] FIG. 6 is a graphical representation illustrating
temperature results as a function of time for a film deposition
sequence according to an embodiment herein;
[0017] FIG. 7 is a flow diagram illustrating a preferred method of
an embodiment herein; and
[0018] FIG. 8 is a computer system diagram according to an
embodiment herein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] The embodiments herein and the various features and
advantageous details thereof are explained more fully with
reference to the non-limiting embodiments that are illustrated in
the accompanying drawings and detailed in the following
description. Descriptions of well-known components and processing
techniques are omitted so as to not unnecessarily obscure the
embodiments herein. The examples used herein are intended merely to
facilitate an understanding of ways in which the embodiments herein
may be practiced and to further enable those of skill in the art to
practice the embodiments herein. Accordingly, the examples should
not be construed as limiting the scope of the embodiments
herein.
[0020] As mentioned, there remains a need to improve the MBE
deposition sequence via more accurate temperature control. The
embodiments herein achieve this by providing a technique to feed a
constant power level to the substrate heater and to use more than
one feedback signal and more than one control mode during an MBE
deposition sequence. Referring now to the drawings, and more
particularly to FIGS. 1 through 8, where similar reference
characters denote corresponding features consistently throughout
the figures, there are shown preferred embodiments.
[0021] An exemplary hardware embodiment is shown in FIG. 1. A
substrate wafer 1 of arbitrary shape is rotated around its azimutal
axis during deposition. The substrate wafer 1 is held in place by a
wafer holder 17. Behind the substrate wafer 1 is a resistive heater
2 and a thermocouple 3, none of which are in intimate contact with
the substrate wafer 1 or its holder 17. The heating is accomplished
by an electric power supply 4, which receives information about the
desired output power level from a hardware controller 5, for
example a PID controller, such as that available from Eurotherm,
Va., USA is preferably used. Such a hardware controller 5 is used
in conjunction with a control computer 6 that can send set points
and PID values to the hardware controller 5. Accordingly, the
controller 5 is connected to the computer 6. Additionally, a
control cable 16 may be connected from the computer 6 directly to
the power supply 4. An optical pyrometer 7 is positioned so that it
can capture radiation from the substrate wafer 1. A pyrometer 7 is
just one example of an optical, or even more generic, a non-contact
temperature monitor that is assumed to able to read the "true"
temperature of the substrate wafer 1. The embodiments herein
include a feature for self-calibration of the pyrometer (or any
other optical or otherwise non-contact temperature monitor) 7
versus the thermocouple 3 or constant power output as further
described below.
[0022] The signal flow 8 from the computer 6 to the controller 5
may comprise any of the following: PID parameters such as
proportional, derivative and integration constants, temperature
setpoints (T.sub.TC-setp), and output power values. The signal flow
9 to the computer 6 from the controller 5 may comprise actual
thermocouple temperatures (T.sub.TC-actual). The computer 6 also
inputs values taken from a temperature signal 10 from the pyrometer
7. The analog input 11 to the controller 5 from the system includes
voltage from the thermocouple 3. The output signal 12 from the
controller 5 to the power supply 4 is a percentage value of the
total available power. The output signal 13 from the power supply 4
comprises voltage and current. Finally, the pyrometer 7 converts
heat 14 radiated from the substrate wafer 1 to the temperature
signal 10. If the substrate wafer 1 is configured very small, heat
from areas 15 surrounding the actual wafer 1 or heat/light from
other objects reflected from areas 15 may also enter the pyrometer
7, which may cause erroneous readings, which is further described
below.
[0023] The embodiments herein use a feature of the hardware
controller 5 that allows bypass of the hardware PIED control and
allows the hardware controller 5 to output a constant power level
that can be set by the control computer 6. Preferably, a PID
control routine is implemented in software on the control computer
6, which can read both the signal 9 from the hardware controller 5
as well as the temperature signal 10 from the pyrometer 7. It is
possible, during a complex deposition sequence, to switch
dynamically between three different control modes: 1) thermocouple
feedback; 2) pyrometer feedback; and 3) constant power. In
addition, PID control parameters can be assigned dynamically during
deposition to optimize the temperature response of the substrate
wafer 1.
[0024] When the control cable 16 is added between the computer 6
and the power supply 4, output power levels can be sent directly to
the power supply 4 via the control cable 16, thereby bypassing the
controller 5. The functionality of the controller 5 and the
computer 6 differ from the conventional systems. Generally, more of
the decision-making and control work is moved from the low-level
hardware controller 5 to the computer 6. This enables dynamic
switching between the different control modes (described above)
during a process sequence.
[0025] Generally, in the conventional systems, the function of a
control computer is limited to reading the signal from the
pyrometer, and sending PID parameters and thermocouple set points
to the controller. A low-level controller accepts the PID
parameters and thermocouple set points, inputs the thermocouple
voltage and translates it into a temperature value,
T.sub.TC-actual, calculates the output percentage and sends the
value to the power supply.
[0026] Conversely, according to the embodiments herein, the action
of calculating the output percentage is moved up in the control
hierarchy to the control computer 6. The actual thermocouple
temperature is not only read in to the computer 6 from the
controller 5 for display purposes, but may be used in the output
power calculation routine. A second option is to use the signal 10
from the pyrometer 7 in this calculation. A third option is to
bypass the calculation and set a fixed output percentage via the
controller 5 or directly to the power supply 4 via the direct
connection afforded by the control cable 16. In the configuration
provided by the embodiments herein, the power level to be output by
the power supply 4 is set via the controller 5 in the form of a
percent level command. In the direct control configuration (i.e.,
using the control cable 16) the output power level can be set by
direct command from the control computer 6.
[0027] No hard rewiring is needed for the embodiments herein to
function. Moreover, the direct connection (i.e., using the control
cable 16) between the computer 6 and the power supply 4, and
comprising the addition of one serial communication control cable
16, does not interfere with the operation of the system 50. With
the hardware controller 5, only a reprogramming of the
communications protocol and pressing a front panel switch (not
shown) (from auto to manual) are required. It is possible that
these actions can be performed by the control computer 6. The same
configuration can also be used for control of an evaporation cell
(not shown). If so, the pyrometer 7 is eliminated and the substrate
wafer 1 is replaced by a crucible (not shown) containing the
evaporant.
[0028] The core of the software program, which is run on computer 6
(of FIG. 1) is a continuously running routine that performs the PID
control as depicted in FIG. 2. Specifically, FIG. 2 shows a
constantly running loop, the purpose of which is to set the correct
output power using either of the three control modes. First, a
check is performed to see if constant power is used (100). If this
is the case, a jump occurs directly to the point where the output
power is set (110). If constant power is not to be used, the global
control parameters are read in to enable update in case they have
been changed by higher-level processes (101). Next, a check for
thermocouple or pyrometer mode is performed (102). If pyrometer
control is the chosen mode, the current pyrometer signal is read
(104), then the pyrometer control temperature is set equal to the
target temperature (106). If thermocouple control is to be used,
the current thermocouple reading is obtained (103). Subsequently,
the thermocouple temperature is translated to a real temperature,
which is defined as one given by the pyrometer (or any other
optical measurement technique) 7 (of FIG. 1) via the experimentally
determined relationship between the two (in this case a linear one)
(105). Once the new target and current real temperatures have been
determined, they are used in a PID calculation to determine the
needed output power at that instance in time (108). Next, the
calculated output power level is sent to the low level controller
(110) or, in the case when the control computer is direct connected
to the power supply, a corresponding number is sent to low level
controller (115). Finally, the process is halted for a short
system-dependent time interval (112) to allow the control event to
act before the cycle starts over.
[0029] A low system priority is provided so as not to interfere
with more time-critical processes, such as opening and closing
shutters (not shown). The software routine continuously monitors a
set of global variables which are a) the desired temperature
setpoint, (T.sub.setpoint); b) the control mode (equals control by
pyrometer 7 (of FIG. 1), control by thermocouple 3 (of FIG. 1) or
constant power output); and c) the PID values. Higher level
programs control the temperature by setting new values for the
global variables. Instantaneous changes in temperature are
accomplished by setting a new value for the temperature setpoint.
Gradual changes in temperature are accomplished by invoking
routines that execute a set of step-wise temperature changes with
specific time delays. More complex process sequences are built up
from combinations of such functions.
[0030] Another feature of the preferred embodiments is the ability
to switch dynamically between the three different control modes:
control by thermocouple 3, control by pyrometer 7, and constant
output power 13. With respect to FIG. 1, in the following examples,
it is assumed that the signal 10 from the pyrometer 7 represents
the true temperature. It is, however, possible to substitute the
signal 10 from the pyrometer 7 with a signal from any other type of
instrument that can provide reliable information about the
temperature of the sample.
[0031] Next, with respect to the deposition process for a
small-size, semiconductor sample with a small bandgap, the
following describes the outgassing and film growth on a sample of
such a small size that stray light may make the signal 10 from the
pyrometer 7 unreliable at lower temperatures. Furthermore, a
material is chosen, in this case GaSb, which requires a relatively
low deposition temperature, which is below the operating
temperature of the pyrometer 7. Although GaSb is chosen as an
example, the method can be applied to any other material with
similar characteristics.
[0032] A flowchart of the preparation and calibration process is
shown in FIG. 3 (with reference to FIG. 1). The first step in the
process is a decision point (125) at which it is determined if the
stray light to the pyrometer 7 (of FIG. 1) should be minimized. If
this is the case, the sample rotation is first set to a slow value
such that a significant number of pyrometer readings can be
obtained with a suitable azimuthal angular resolution (127). Next,
pyrometer signals are sampled for long enough time interval to
cover a 360 degree rotation of the sample. The minimum pyrometer
value in this data set is determined (129) and the rotation
continues until the same value is again read by the pyrometer 7 (of
FIG. 1), at which point the rotation is stopped (131). The position
of the wafer 1 (of FIG. 1) is now suitable and information about
the substrate material is sought (133). This could be in the form
of an interactive question to the operator or provided via higher
level control processes that call this routine. Based on the choice
of material, a suitable set of temperatures and control parameters
are called up from a memory component of computer 6 (135). Since
the process starts at room temperature, at which the pyrometer 7
(of FIG. 1) does not work, the initial control is set to
thermocouple mode (137).
[0033] The substrate wafer 1 is then ramped up to a target
temperature, T.sub.1 (139), which is higher than T.sub.2 and
T.sub.3, which are further explained below. A loop is then started,
in which the temperature signal 10 (of FIG. 1) is monitored to
determine if it is providing reliable information (141). The
criteria for this is that the thermocouple temperature should be
higher than some value T.sub.2, below which there is no reason to
even check for signal 10 (of FIG. 1) because the substrate wafer 1
is transparent. The next criterion is that the pyrometer reading
should be higher than T.sub.3, which is its minimum reliable
temperature (specific to the instrument). Finally, the time
derivative dT.sub.pyro/dt must be positive, or in other words the
temperature must be increasing. Typically, the pyrometer
temperature reading as a function of time shows initially a
decreasing behavior, when the substrate wafer 1 (of FIG. 1) is
transparent. The pyrometer temperature reading then reaches a
minimum at T.sub.3 when the substrate wafer 1 (of FIG. 1) starts to
become opaque and then exhibits an increase as the reading starts
being dominated by the radiation from the surface of the substrate
wafer 1 (of FIG. 1), which is then the real temperature). When
these criteria are met, the system 50 (of FIG. 1) switches to
pyrometer control (143). The wafer 1 (of FIG. 1) is then ramped up
(145) to a material-specific value, T.sub.4, above the temperature
at which the oxide desorbs and which is suitable for thermal
cleaning. This thermal cleaning is then allowed to continue for a
suitable, material-specific time (147). After this, a decision
point (149) determines if a calibration should be performed to
determine the relationship between the pyrometer readings and the
thermocouple or constant power values, or no calibration is needed
at all. If no calibration is needed a simple down-ramp to the
growth temperature under pyrometer control is started (153). If a
thermocouple calibration is selected this process is started (151),
similarly if a constant power calibration is selected this is
started (152). Both of these processes are further described in
FIG. 4. Finally, the crystal growth is started (155).
[0034] The preparation of a GaSb wafer 1 includes a heating
sequence that takes the temperature up from room temperature, to a
value above which the native oxide desorbs, which for GaSb is
approximately 600.degree. C. The initial temperature ramp targets a
value, T.sub.1, which is chosen to be higher than the values
T.sub.2 and T.sub.3, as further described below. At room
temperature the pyrometer 7 cannot be used since the wafer 1 does
not appear opaque to the instrument, nor does it emit enough light
to produce a signal detectable by typical available instruments.
Thermocouple feedback is therefore used. The control computer 6
monitors the signals from both the thermocouple 3 and the pyrometer
7 and switches the input signal 11 when a set of criteria are met.
These criteria include: thermocouple signal 11 greater than
T.sub.2, (an empirically determined value that depends on the
heating efficiency of the particular system heater configuration
and the wafer material properties), signal 10 greater than T.sub.3
(the minimum reliable output temperature of the pyrometer), and the
time derivative of the signal 10 greater than zero (i.e. the
temperature is verified to be increasing).
[0035] Before the thermocouple-based upramp is started, the
apparent signal from the pyrometer 7 can be monitored as a function
of azimuthal orientation of the substrate holder 17. With the
substrate wafer 1 and holder 17 at room temperature, any elevated
temperature signal 10 from the pyrometer 7 is due to spurious stray
light from other hot objects in the growth chamber (not shown).
Because the substrate holder 17 normal is not necessarily perfectly
aligned with the axis of the pyrometer 7, and various hot cells
such as dimer-crackers for Group V cells are off-axis relative to
the pyrometer 7, there are azimuthal angels at which the reflected
stray light is minimized (or maximized). The heating sequence can
therefore be started with a short time sequence, during which the
azimuthal rotation speed is set to a low value, and the length of
the sequence is chosen so that at least one full turn is completed.
During this time the signal 10 is recorded. The minimum signal
value is found by the computer 6 and the azimuthal rotation is
continued. The control computer 6 then continues to monitor the
signal 10 but stops the rotation when the signal 10 again reaches
the minimum value. This sequence assures that the error in the
signal 10 is minimized. For larger wafers, uniform heating, which
is accomplished by continuous rotation, is more important, but
since the stray light from the holder 17 is then much lower, the
optimization feature can be bypassed.
[0036] After the oxide has desorbed, the temperature is further
raised a few ten degrees to T.sub.4, to clean the substrate by
thermal outgassing. It remains at this level for a few minutes
after which the temperature is lowered to a value suitable for
deposition and crystal growth, T.sub.growth. The temperatures,
T.sub.4 and T.sub.growth, are chosen empirically for each type of
substrate wafer 1 and the desired outcome of the process, using
separate optimizations. According to the embodiments, the down ramp
can be broken into stages that generate a relationship between the
signal 10 and the reading of the thermocouple 3 or the amount of
power sent to the heater 2. This calibration relationship can then
be used for control during the subsequent film growth if the signal
10 is deemed to be unreliable, either because of stray light, or
because the growth temperature might be below the operating range
of the pyrometer 7 (as shown in the last stages in FIG. 3). The
calibration sequence can also be repeated an arbitrary number of
times and at arbitrary interruption points during the film growth,
if it can be expected that the relationship has changed by the
presence of the deposited film. An example of this would be InGaAs
deposition on InP, which increases the absorption of the wafer/epi
film combination such that less heat is required to maintain a
certain surface temperature, which is registered by the
thermocouple 3 (of FIG. 1) as a lower reading.
[0037] The down-ramp is broken into steps--a minimum of two--and
preferably four. For each step the temperature is lowered under
pyrometer control until a predetermined value is reached. It is
then held at this level while the thermocouple temperature, or
power level, is monitored. When the time derivative of the
thermocouple signal is zero, the system 50 (of FIG. 1) is assumed
to be in equilibrium and the values from the two temperature
monitors are recorded. If instead the power output is recorded, a
time averaging is done as shown in FIG. 4.
[0038] In FIG. 4, the calibration of the thermocouple 3 (of FIG. 1)
begins with a step counter update (175). The counter counts the
number of steps that will be used in the calibration. The
temperature of the substrate wafer 1 (of FIG. 1) is then ramped
down to a preset value T.sub.pyro,j (177). The temperature of the
thermocouple 3 (of FIG. 1) is then monitored (179). The temperature
reading exhibits an initial fast drop, and after a short period of
time, slowly comes to a steady state when the wafer 1 (of FIG. 1)
and heater 2 (of FIG. 1) reach equilibrium. When the change in the
thermocouple reading as a function of time reaches a preset small
value, the system 50 (of FIG. 1) is considered to be in equilibrium
and the pair of thermocouple and pyrometer values are recorded
(181). This process is repeated n times (183). Once the entire set
of temperature pairs has been determined, a least square fit is
obtained (185). In the general case, any functional relationship
could be determined. The parameters that describe the functional
relationship, in the case of a straight line, the slope, k, and the
intercept, m, are stored as global variables (187). The control
mode is then switched to thermocouple (189) and a ramp started to
the growth temperature (191).
[0039] The procedure for constant power versus pyrometer
calibration proceeds in a substantially similar manner as also
illustrated in FIG. 4. The calibration starts with a step counter
update (176). The counter counts the number of steps that will be
used in the calibration. The temperature of the substrate wafer 1
(of FIG. 1) is then ramped down to a preset value T.sub.pyro,j
(178). The output power is then averaged to obtain a reliable
number (180). The pair of thermocouple and power values are then
recorded (182). This process is repeated n times (184). Once the
entire set of temperature pairs has been determined, a least square
fit is obtained (186). In the general case, any functional
relationship could be determined. The parameters that describe the
functional relationship, in the case of a straight line, the slope,
k, and the intercept, m, are stored as global variables (188). The
pyrometer control mode is then maintained and a ramp started to the
growth temperature (190). Finally, the control mode is changed to
constant power mode (192).
[0040] When the whole set of steps have been completed, a
functional relationship is determined. This could be any function
that can be determined from the given number of observations.
Again, with respect to FIG. 1, the simplest relationship, which
works very well for pyrometer and thermocouple calibration, is a
linear one, in which case a least squares fit is performed on the
data set of the signal 10 and the values of the thermocouple 3.
T.sub.pyro=k*T.sub.TC+m
[0041] In the case of constant output power as a function of signal
10, a linear relationship also works surprisingly well. However, it
is not obvious that this would be the case in an arbitrary system,
in which case a more complex relationship may have to be found. The
values of the slope, k, and the intercept, m, are stored in global
variables in the control system, accessible by any other routine.
Similarly, more fitting parameters would be stored and accessed in
the case when more complex functions are used. As mentioned, the
pyrometer signal 10 is assumed to represent the true temperature.
However, since the deposition temperature of the GaSb wafer 1, in
this case, will be lower than the minimum reliable temperature of
the pyrometer 7 and some of the deposition sequence will be done
using sources that produce excessive amounts of stray light, this
process may use thermocouple feedback, or constant power. The
flowchart for the high-level process is shown in FIG. 5.
[0042] The growth of the wafer 1 (of FIG. 1) begins with the growth
of GaSb at a temperature of 490.degree. C. (201). Next, the
temperature is ramped down to 400.degree. C (203). The details of
this process are described below. Before growth is started at the
lower temperature, new PID values may be set to optimize the
stability of the temperature control at this level (205). Next, a
multilayer structure is grown (for example, the multilayer
structure may include alternating layers of InAs and GaSb) (207).
Once the multilayer sequence is completed, the PID values may be
changed back to numbers more appropriate for control of the
temperature at higher levels (209). Then, the temperature is ramped
up, in this case to a value of 500.degree. C. suitable for
annealing of the wafer 1 (211). Finally, the growth process is
terminated and the substrate wafer 1 (of FIG. 1) is cooled down
(213). The ramping events (203) and (211) comprise a sequence of
events aimed at invoking the correct control modes. An initial
check is performed of which one of the three modes (thermocouple,
pyrometer, or constant power) is to be used (215). If constant
power is chosen, the power level corresponding to the end target
temperature of the ramp is calculated and set. This includes a wait
time to ensure stability (220). If pyrometer control is used, the
target temperatures are set equal to pyrometer values (217). If
thermocouple control is used, the target temperatures are
translated to thermocouple values (216), in this case using the
linear relationship determined during the calibration sequence
describe in FIG. 4. Finally, the temperature set points are changed
stepwise to accomplish the ramp (218).
[0043] The two temperature changes include a check of control mode,
a calculation of the control setpoint, and a ramp, changing the
temperature to the new setpoint. In FIG. 5, two changes of PID set
points are also included that may be introduced to optimize
stability or agility at specific stages of the process. An example
of the real temperatures during a deposition sequence is shown in
FIG. 6. This demonstrates the result of first running the process
described in FIG. 3 and FIG. 4 and immediately afterwards running
the process in FIG. 5.
[0044] The upper curve in FIG. 6 shows the recorded pyrometer
temperature as a function of time and the lower one the
thermocouple values. From 0 seconds to approximately 1,200 seconds
the pyrometer signal represents transmitted light from the heater 2
(of FIG. 1) through the substrate wafer 1 (of FIG. 1) and is
consequently not used for control. At approximately 1,800 seconds
the control is handed over to the pyrometer 7 (of FIG. 1) and the
ramp to 600.degree. C. is continued. At approximately 3,000 seconds
the stepwise down ramp under pyrometer control is started and
thermocouple readings are recorded. This represents the calibration
of pyrometer versus thermocouple process. At approximately 4,500
seconds the control mode is changed to thermocouple control.
Shortly afterwards the growth is started by Ga and Be shutters
being opened. These reveal stray light from the ovens that is
visible as an increase in the pyrometer signal and may produce
incorrect temperature control if this were used for feedback. At
approximately 7,000 seconds the growth is interrupted and a down
ramp process begins. Between 8,000 and 12,500 seconds a multilayer
is deposited at the lower substrate temperature. Alternating levels
of stray light is again visible in the pyrometer signal 10 (of FIG.
1) while the thermocouple level remains flat. At approximately
13,500 seconds the temperature is ramped up and held at a higher
annealing temperature (500.degree. C.) until shortly before 15,000
seconds, when the process is terminated by shutting off the power
of the heater 2 (of FIG. 1), resulting in a rapid temperature
drop.
[0045] FIG. 7, with reference to FIGS. 1 through 6, illustrates a
flow diagram of a computer-implemented method of controlling
temperatures while performing a MBE deposition process, wherein the
method comprises providing (301) a heating sequence for the MBE
deposition process on a MBE growth wafer 1; receiving (303), in a
control computer 6, a plurality of dynamic feedback control signals
10 derived from the MBE growth wafer 1; switching (305), in the
control computer 6, among a plurality of control modes
corresponding with the plurality of dynamic feedback control
signals 10; and sending (307) an output power signal 8 from the
control computer 6 to the MBE growth wafer 1 to control the heating
for the MBE deposition process based on a combination of the
plurality of control modes.
[0046] The embodiments herein can include both hardware and
software elements. In the software embodiment includes but is not
limited to firmware, resident software, microcode, etc.
Furthermore, the embodiments herein can take the form of a computer
program product accessible from a computer-usable or
computer-readable medium providing program code for use by or in
connection with a computer or any instruction execution system. For
the purposes of this description, a computer-usable or computer
readable medium can be any apparatus that can comprise, store,
communicate, propagate, or transport the program for use by or in
connection with the instruction execution system, apparatus, or
device.
[0047] The medium can be an electronic, magnetic, optical,
electromagnetic, infrared, or semiconductor system (or apparatus or
device) or a propagation medium. Examples of a computer-readable
medium include a semiconductor or solid state memory, magnetic
tape, a removable computer diskette, a random access memory (RAM),
a read-only memory (ROM), a rigid magnetic disk and an optical
disk. Current examples of optical disks include compact disk-read
only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD.
[0048] A data processing system suitable for storing and/or
executing program code will include at least one processor coupled
directly or indirectly to memory elements through a system bus. The
memory elements can include local memory employed during actual
execution of the program code, bulk storage, and cache memories
which provide temporary storage of at least some program code in
order to reduce the number of times code must be retrieved from
bulk storage during execution.
[0049] Input/output (I/O) devices (including but not limited to
keyboards, displays, pointing devices, etc.) can be coupled to the
system either directly or through intervening I/O controllers.
Network adapters may also be coupled to the system to enable the
data processing system to become coupled to other data processing
systems or remote printers or storage devices through intervening
private or public networks. Modems, cable modem and Ethernet cards
are just a few of the currently available types of network
adapters.
[0050] A representative hardware environment for practicing the
embodiments of the invention is depicted in FIG. 8. This schematic
drawing illustrates a hardware configuration of an information
handling/computer system in accordance with the embodiments of the
invention. The system comprises at least one processor or central
processing unit (CPU) 20. The CPUs 20 are interconnected via system
bus 22 to various devices such as a RAM 24, read-only memory (ROM)
26, and an input/output (I/O) adapter 28. The I/O adapter 28 can
connect to peripheral devices, such as disk units 21 and tape
drives 23, or other program storage devices that are readable by
the system. The system can read the inventive instructions on the
program storage devices and follow these instructions to execute
the methodology of the embodiments of the invention. The system
further includes a user interface adapter 29 that connects a
keyboard 25, mouse 27, speaker 34, microphone 32, and/or other user
interface devices such as a touch screen device (not shown) to the
bus 22 to gather user input. Additionally, a communication adapter
30 connects the bus 22 to a data processing network 35, and a
display adapter 31 connects the bus 22 to a display device 33 which
may be embodied as an output device such as a monitor, printer, or
transmitter, for example.
[0051] The embodiments herein provide more precise control of the
temperatures used in an MBE process. The substrate temperature,
which may be critical for the quality of the grown crystal, can be
set in an optimum way, even when artifacts are present in the
signals from various temperature reading sources. An optimum
feedback mode can be set dynamically during the deposition process
so as to minimize artifacts. An optical temperature monitor, such
as a pyrometer 7, is used as an indicator of true temperature (by
definition) and its values are related to corresponding readings of
a thermocouple 3 or the power level. The embodiments herein can be
used to control an MBE evaporation source by performing the PID
control in software rather than by a low level controller 5 or by
setting constant power levels, when thermocouple feedback is not
possible (e.g., during mechanical failures).
[0052] The foregoing description of the specific embodiments will
so fully reveal the general nature of the embodiments herein that
others can, by applying current knowledge, readily modify and/or
adapt for various applications such specific embodiments without
departing from the generic concept, and, therefore, such
adaptations and modifications should and are intended to be
comprehended within the meaning and range of equivalents of the
disclosed embodiments. It is to be understood that the phraseology
or terminology employed herein is for the purpose of description
and not of limitation. Therefore, while the embodiments herein have
been described in terms of preferred embodiments, those skilled in
the art will recognize that the embodiments herein can be practiced
with modification within the spirit and scope of the appended
claims.
* * * * *