An IR source in the form of a micro-hotplate device
including a CMOS metal layer made of at least one layer of embedded on a
dielectric membrane supported by a silicon substrate. The device is formed in a
CMOS process followed by a back etching step. The IR source also can be in the
form of an array of small membranes —closely packed as a result of the use of
the deep reactive ion etching technique and having better mechanical stability
due to the small size of each membrane while maintaining the same total IR
emission level. SOI technology can be used to allow high ambient temperature
and allow the integration of a temperature sensor, preferably in the form of a
diode or a bipolar transistor right below the IR source.
Description
RELATIONSHIP TO EARLIER APPLICATIONS
This application is a continuation-in-part of U.S.
application Ser. No. 12/691,104 filed on Jan. 21, 2010, which is hereby
incorporated by reference in its entirety.
FIELD OF THE INVENTION
This invention relates to a thermal Infra-Red (IR) source
using a micro-hotplate fabricated on a microchip. The invention also relates to
integrating the IR source with an IR detector to make an NDIR sensor.
BACKGROUND TO THE INVENTION
It is known to fabricate a thermal IR source on a silicon
substrate consisting of a micro-heater formed within a thin membrane layer
(made of electrically insulating layers) that is formed by etching part of the
substrate. Such devices can be used to provide heat (e.g. 600° C.) with low
power consumption (typically from a few mW to hundreds of mW) for use as
infra-red sources/emitters.
For Example, Parameswaran et. al. “Micro-machined thermal
emitter from a commercial CMOS process,” IEEE EDL 1991 reports a polysilicon
heater for IR applications made in CMOS technology, with a front side etch to
suspend the heater and hence reduce power consumption.
Similarly, D. Bauer et. Al. “Design and fabrication of a
thermal infrared emitter” Sens & Act A 1996, also describes an IR source
using a suspended polysilicon heater although the device is not envisaged to be
fabricated in a CMOS process. Moreover, wafer bonding is used to encapsulate
the heater in vacuum (which adds extra fabrication steps and increases the
manufacturing cost).
Patent U.S.
Pat. No. 5,285,131 by Muller et al. and patent US2008/0272389 by Rogne et. al
both describe similar devices using a polysilicon heater.
San et. al. “A silicon micromachined infrared emitter
based on SOI wafer” (Proc of SPIE 2007) describe an IR emitter fabricated from
an SOI substrate using polysilicon as the heater and DRIE to form the membrane.
The use of polysilicon in all these designs reduces the
stability of the device as polysilicon resistance drifts in time at high
temperatures above 400° C.
Yuasa et. al “Single Crystal Silicon Micromachined Pulsed
Infrared Light Source” Transducers 1997, describe an infrared emitter using a
suspended boron doped single crystal silicon heater. The paper does not
envisage the device to be fabricated within a CMOS process.
Watanabe, in patent EP2056337 describes a suspended
silicon filament as an IR source. The device is vacuum sealed by bonding a
second substrate. This device is not envisaged to be fabricated in a CMOS
process, and the construction of the device also does not lend itself to be
fabricated in a CMOS process.
Cole et. al. “Monolithic Two-Dimensional Arrays of
Micromachined Microstructures for Infrared Applications” (proc of IEEE 1998)
describe an IR source on top of CMOS processed device. These IR sources consist
of a suspended micro-heater fabricated after considerable post-CMOS processing.
These extra processing steps add to the fabrication cost of the device.
Hildenbrand et. al. “Micromachined Mid-Infrared Emitter
for Fast Transient Temperature Operation for Optical Gas Sensing Systems”, IEEE
Sensor 2008 Conference, reports on a platinum heater on suspended membrane for
IR applications. Platinum is however not CMOS compatible and its use in CMOS
foundries is prohibited, as it acts as a deep dopant and can contaminate other
CMOS process steps.
Similarly Ji et. Al. “A MEMS IR Thermal Source For NDIR
Gas Sensors” (IEEE 2006) and Barritault et. al “Mid-IR source based on a
free-standing microhotplate for autonomous CO2 sensing in indoor applications”
(Sensors & Actuators A 2011) describe a micromachined IR source based on a
platinum heater. Weber et. al. “Improved design for fast modulating IR sources”
describe suspended as well as closed membrane designs for IR sources, both
using a platinum heater and a membrane consisting of Silicon oxide and silicon
nitride layers.
Spannhake et. Al. “High-temperature MEMS Heater
Platforms: Long-term Performance of Metal and Semiconductor Heater Materials”
(Sensors 2006) describes micro-hotplate based on either platinum or antimony
doped Tin oxide heaters.
As already mentioned, Platinum is incompatible with CMOS
processes and so these devices cannot be fabricated in a CMOS process. This
increases the fabrication cost and means that circuitry cannot be fabricated
with the device.
Tu et. al, “Micromachined, silicon filament light source
for spectrophotometric microsystems” Applied Optics, 2002, presents design of a
light source employing single crystal silicon heaters on an SOI membrane.
Suspended filaments however, have less mechanical stability than a full
membrane.
U.S. Pat. No. 6,297,511 by Syllaios et. al. describes an
IR emitter made on a suspended membrane with a resistive heater which can be of
various materials such as titanium, tungsten, nickel, single crystal silicon or
polysilicon. U.S.
Pat. Nos. 5,500,569, 5,644,676, 5,827,438 by Bloomberg et. al. report on IR
sources with either polysilicon or metal (such as tungsten, tantalum,
titanium-tungsten alloy, molybdenum) heaters. However, these devices are not
envisaged to be fabricated using a CMOS process.
WO 02/080620 A1 by Pollien et. al. suggests using metal
silicides as the heater material in micro-hotplates. The silicide is mentioned
as having a polycrystalline structure from silicides of tantalum, zirconium,
tungsten, molybdenum, niobium and hafnium. The possible use of such devices as
IR sources is mentioned. However metal silicides are not standard materials
used in commercial CMOS processes. Advantages of manufacturing the
micro-hotplates by a standard CMOS process are given, however no mention is
made of how this can be achieved given that metal silicides is not a material
found in CMOS processes. In addition no mention of a CMOS process is made in
the claims of the patent.
It is also known to fabricate IR detectors in silicon
technology. Kim et. al. “A new uncooled thermal infrared detector using silicon
diode” Sens & Act A 89 (2001) 22-27 describes a diode for use as an IR
detector. U.S.
Pat. No. 6,597,051 describes a thermopile fabricated by micromachining for use
as an IR detector. Eminoglu et. al. “Low-cost uncooled infrared detectors in
CMOS process” describes an IR detector using diodes on a microbridge membrane
fabricated in a CMOS process Sens & Act A 109 (2003) 102-113. A. Graf et.
al. “Review of micromachined thermopilers for infrared detection,” Meas. Sci.
Technol. 18(2007) R59-R75) describes various thermopile based micro-machined IR
detectors reported in literature. It is also known to make NDIR sensors, for
example Fordl and Tille “A High-Precision NDIR CO2 gas sensor for automotive
applications” IEEE Sensors Journal vol 6 No.6 2006, and patent US2007/0102639
by Cutler et. al describe typical NDIR sensors consisting of a filament bulb as
an IR source, and a thermopile based IR detector. The two are placed at the
opposite ends of a small chamber where gas can enter through a semi permeable
membrane (which blocks dust and IR radiation from outside). Depending on the
concentration of the target gas, the amount of IR emission of a particular
wavelength is absorbed within the optical path, and using the measurement from
the IR detector can be used to determine the gas concentration. Most NDIR
sensors also have an optical filter to allow only a small range of wavelengths
to reach the IR detector so as to make it specific for the gas that absorbs
that wavelength.
Other patents, such as US2008/0239322 by Hodgkinson et.
al., U.S.
Pat. No. 7,244,939 by Stuttard et. al, US2008/0308733 by Doncaster
et. al., and U.S.
Pat. No. 7,541,587 by Cutler et al. describe similar devices.
In almost every case, the IR emitter and detector are two
different components but packaged together. An exception is U.S. Pat. No.
5,834,777 by Wong, where both the emitter and detector are on the same chip
with an optical path made on the chip. However in this case, because the
optical path is on the chip, it is a very small distance for the IR emission to
travel, and so the sensor has a low sensitivity.
STATEMENT OF THE INVENTION
In accordance with one aspect of the present invention
there is provided an IR source comprising a resistive heater made from a CMOS
metal on a dielectric membrane fabricated in a CMOS process followed by a back
etch. The CMOS metal may comprise at least one layer of tungsten.
According to one embodiment of the present invention,
there is provided a micro-hotplate fabricated using a CMOS process. The process
starts with a simple silicon wafer, or an SOI wafer which is processed using a
standard commercial CMOS or SOI process that uses tungsten as an interconnect
material for electronic devices. The tungsten interconnect metal is used to
form the micro-heater for the device. A Ti/TiN liner is used to improve the
stability of the metal. The CMOS processing step is followed by a back etching
step to form the membrane. This step can be either dry etching by DRIE or wet
anisotropic etching such as KOH or TMAH.
The membrane or the heater can be either circular or
rectangular shaped, the circular shape having an additional advantage of
reducing the mechanical stress. The heater can be of any shape such as meander,
spiral, ring, multiple rings etc. The device may also consist of one or more
metal heat spreading plates above the heater. The device may also have a metal
heat spreading plate formed from the top metal layer which is then exposed by
removing the passivation. A silicon heat spreading plate may also be fabricated
just below the heater to improve the temperature uniformity. This can be formed
either using the active silicon layer in an SOI process, or for a bulk process
by doping the silicon region during before bulk etching to leave a silicon
island unetched during the back etch. Alternately, a diode (i.e. thermodiode),
or a thermotransistor (npn or pnp with one junction shorted), or a resistive
track of silicon maybe be formed below the heater (or adjacent to the heater)
instead of the heat spreading plate, and can act as a temperature sensor. The
device may also have a resistive temperature sensor formed from one of the
metal layers. The heater itself can also be used as a temperature sensor—in
which case two extra tracks can optionally be connected to the heater to
improve the resistance measurement using a 4-wire measurement.
In another embodiment of the invention, the IR source
consists of an array of several membranes etched by DRIE packed together, each
with its own micro-heater made from tungsten. This improves redundancy in case
one of the devices fails. Another use of the array is to compensate for drift.
For example, in an array of two, only one maybe used regularly, and the other one
turned on only occasionally to calibrate the drift of the main heater.
Alternately, two or more micro-hotplates can be driven in a cycle so that only
one is on at any given time, and so increase the overall lifetime of the
device.
Another use of the array is to have an array of smaller
membranes instead of one large membrane. A large membrane is mechanically less
stable compared to a small membrane, but a small membrane device will have
lower IR emissions. By using an array of small membranes, the mechanical
stability of a small membrane can be achieved while having high levels of IR
emission. The use of DRIE to etch the membranes means that the membranes can be
packed very close together and very little extra space on the chip is required
when compared to a single large membrane. The micro-heaters can be electrically
connected either so that they are driven together at the same time, or driven
individually.
The micro-hotplates in the array can also be driven
independently at different temperatures. This results in a broader spectrum of
IR emission, and when used in an NDIR gas sensor system, can help improve the
selectivity if a number of detectors are used. Alternately, the optics in the
NDIR system can be designed so that the emission through each emitter in the
array passes through a different IR filter and onto different detectors. This
allows the capability of sensing more than one gas using a single NDIR sensor.
In another embodiment of the invention the micro-hotplate
is covered with a coating to improve the IR emission. This coating can be of
any type, such as carefully controlled layers of silicon oxide, silicon nitride
or polymers (e.g. polyimide). Alternately materials such as carbon black,
carbon nanotubes, metal oxides or graphene can be grown or deposited on the
micro-hotplate. These materials have high emissivity and therefore improve the
amount of IR emitted. Other materials having high emissivity can also be used.
Such materials can be deposited post-CMOS onto the heating area of the micro-hotplate
via techniques similar to inkjet or nano depositions or can be grown via CVD
across the entire wafer or only locally using the micro-hotplate as the source
of heat during growth. Several micro-hotplates can be connected together across
the silicon wafer to facilitate local growth.
In another embodiment of the invention, an IR filter is
combined with the IR source. This is by using back etching to form a thin
membrane consisting of silicon dioxide and/or silicon nitride on a silicon or
SOI chip or wafer. This membrane can act as an IR filter. This chip/wafer is
then combined with the IR source by the use of wafer bonding. The composition
of the membrane acting as the filter can be changed and other materials can be
deposited on the membrane to change the filtering properties as desired.
Alternatively the filter can be made by etching
selectively the CMOS metal layers above the silicon in a mesh shape or as dots.
The mesh size or the size of the dots and the distance between the dots are
adjusted to filter the desired emission at particular wavelengths and/or to
increase the emission at particular wavelengths. The etching of the metal
layers above the silicon may be done in the CMOS sequence, and therefore does
not come with additional cost.
This method can be combined with arrays by using an array
of filters wafer bonded onto an array of IR source. Each filter can have either
the same properties, or different properties to allow a different spectrum of
wavelengths.
Another important aspect is the packaging of the
micro-hotplates. Any standard packaging such as TO-5, TO-39 or TO-46 can be
used or they can be placed directly onto a PCB board, however the lids should
be open to have a cavity to allow the emission of IR. In addition, the
packaging can be done with IR reflecting surfaces below the chip as well as on
the sides of the chip to improve the direction of the emission. The packaging
may also include a filter in addition to, or in place of the filter waferbonded
to the IR source or that made of the CMOS metal layer.
It can also be packaged directly in an NDIR chamber.
Another possible packaging method is by flip chip, where a bump bond is applied
to the bond pads, and the chip is packaged upside down on a PCB or on a
package. An advantage of this method is that the IR is emitted through the
trench, and the side walls of the trench act as a reflector. This makes the
beam more directional. A reflecting material maybe deposited onto the trench
sidewalls to improve their reflectivity. Alternatively the back-etch can be
controlled using various wet and dry techniques to shape the walls of the
trench to enhance the reflectivity. Additional metal layers within the membrane
and the back plate of the packaging surface also act as reflectors.
Because according to this invention the IR source is made
in a CMOS process, circuitry can be integrated on the same chip with the IR
source. This can include the drive circuitry for the heater, circuitry for the
temperature sensor, as well as a temperature controller circuit and other
complex circuitry. The drive circuitry can be made to modulate the IR source
and drive it at various frequencies. For example a very simple circuit could be
made of only one MOSFET placed in series with the heater. By applying a
controlled potential on the gate of the MOSFET, the heater can be switched on
and off. The pulse width and the amplitude of the pulse on the gate control the
temperature of the micro-hotplate.
In another embodiment of the invention, an IR detector is
integrated on the same chip as the IR source. The IR detector consists of
either a thermopile or an array of thermodiodes or thermotransistors on a
membrane. If the detector is a thermopile it can consist of one or more
thermocouples connected in series with one junction inside the membrane and one
outside. The two thermocouple materials may consist of p or n doped single
crystal silicon, n or p doped polysilicon or a metal (such as tungsten). If
thermodiodes are used they may consist of a P+/N+ junction, or may have a p or
n type well or drift region between. The diodes can be connected as an array to
improve the sensitivity. Thermodiodes in particular have the advantage that
their temperature coefficient is constant for high temperatures up to 500° C.
Circuitry can be integrated on-chip to process the detector signal. Similarly
thermotransistors are made in CMOS technology using bipolar npn or pnp
structures with at least one junction shorted. The thermodiode or
thermotransistors or circuits based on these are preferable, as the process
control of active elements in CMOS such as diodes and transistors is better
than that of passive elements such as resistors.
To improve the performance, the IR detector may also have
an IR absorbing material such as carbon nanotubes, carbon black, graphene,
polyimide, a polymer, metal films, metal blacks, thin film stacks or other
materials with high IR absorption deposited on the top of the membrane. The IR
absorbing layer should be carried out post-CMOS and can be formed by CVD, local
growth or ink-jet deposition techniques.
Alternatively the IR absorption of the integrated IR
detector can be increased by etching selectively the CMOS metal layers above
the silicon in a mesh shape or as dots. The mesh size or the size of the dots
and the distance of the dots are adjusted to increase the optical signal at a
particular wavelength and/or to filter out signal at other wavelengths. The
etching of the metal layers above the silicon is done in the CMOS sequence, and
therefore does not come with additional cost.
The chip may be packaged to be used as an NDIR sensor
within a package such that there is a partition between the two devices and the
IR emission cannot travel directly from the source to the detector. Instead,
the IR emission has to travel a much longer path to reach the source via an IR
filter. This is achieved during both chip and package design. When designing
the chip, the dielectric oxide between the emitter and detector is filled with
vias and metal layers to block the transmission of IR within the dielectric
oxide. After this a partition is formed above the chip which can be done during
packaging, or earlier by wafer bonding with a patterned substrate on top.
Complex circuitry can be integrated on the chip for drive and signal processing
of both the IR source and detector on the chip.
The packaging to form such a sensor can be of different
types. One embodiment of the invention is to package the chip in a cylindrical
package with walls made from a reflective surface with a filled centre, so that
the IR radiation travels in a circular path (reflecting from the package walls)
from the emitter to the detector part of the chip. The optical path also has an
optical filter to allow only the wavelength of interest to reach the IR
detector. The package is covered with a particle filter to prevent air borne
particles from coming in the optical path.
Another embodiment is for the package to be rectangular
with the chip on one side, and a reflective surface on the far side of the
package allowing reflected IR to travel from the source to the detector.
