Method of monolithically fabricating a microspectrometer with integrated detector

The present invention relates to microfabricated spectrometers including methods of making and using same. Microspectrometers can be formed in a single chip in which detectors and light sources can be monolithically integrated. The microspectrometer can be integrated into a sensor system to measure the optical and physical properties of solids and fluids.

BACKGROUND OF THE INVENTION 
A monochrometer is an optical instrument that can select a narrow band of 
wavelengths of light from a source which contains a broader spectrum. 
Spectrometers are the combination of a monochrometer and a detector such 
that the output of a spectrometer is an electrical signal which is 
proportional to the intensity of light in the selected narrow band. 
Monochrometers and spectrometers are used in many important commercial and 
defense applications, some of which include chemical analysis by optical 
absorption, emission line characterization, thin film thickness analysis, 
and optical characterization of mirrors and filters. 
The optical properties of an unknown material can reveal important 
information leading to a determination of its composition or physical 
properties. For instance, spectral analysis of optical emission lines are 
used to determine the atomic species of gaseous material. A second example 
is the routine use of optical spectra by the semiconductor industry to 
determine the thickness of multilayer thin films. These measurements are 
made with instruments incorporating optical spectrometers. A typical 
spectrometer is a precision instrument that usually consists of an 
entrance slit, a prism or grating, a couple of mirrors or lenses, and an 
exit slit. Lenses would normally be used to focus the light into the 
entrance slit and from the exit slit onto a detector. To scan through the 
spectrum, the grating or prism is rotated mechanically. The grating or 
prism separates the light into its spectral components and these are 
selected by the exit slit and measured with an optical detector. 
The conventional optical spectrometer is a large, expensive, precision 
instrument. Its quality is characterized by its ability to separate 
spectral components or in other words, by its resolution. Analytical 
equipment that incorporates optical spectrometers are by nature expensive 
and therefore relegated to applications that can justify the expense. 
While current spectrometers perform their function well, broader 
application of optical measurement techniques would be achieved with a 
small and less expensive alternative. 
SUMMARY OF THE INVENTION 
The present invention relates to a miniature optical spectrometer and 
methods for manufacturing and using such an instrument. The process takes 
advantage of microfabrication techniques to produce a microspectrometer 
that incorporates a wavelength selective micromechanical component and an 
optical detector. Microspectrometers offer significant advantages over 
existing instruments including significantly smaller size, lower cost, 
faster data acquisition rate, and much greater reliability. Because of 
these advantages, much broader application of optical measurement 
techniques can be achieved. The microspectrometer can also be built as a 
multisensor to measure fluid composition, pressure, mass loading 
transients and microscale turbulent properties of fluids. In these 
applications variations in the incoming optical signal from a light source 
are measured and correlated with the selected property or physical 
characteristic of the fluid being analyzed. 
The microspectrometer consists of a mechanical bridge structure which is 
fabricated on a substrate. The bridge contains a region near its center in 
which an optical mirror is placed. The mirror is designed to be reflective 
over a broad range of wavelengths and is fabricated using standard optical 
thin film deposition techniques or techniques used in conventional 
microfabrication technology. The bridge extends over the substrate 
material upon which a second mirror with the same spectral response has 
been fabricated. The mirror on the bridge and the mirror on the substrate 
are separated by air, an inert gas, a fluid, or a vacuum in the gap. The 
combination of the two mirrors and the gap create a miniature Fabry-Perot 
cavity. Providing an optical cavity where two mirrors are positioned 
adjacent to one another creates a spacing or gap such that at least one of 
the mirrors become transmissive over a narrow band of wavelengths. The 
band over which the mirrors become transmissive depends upon the spacing 
and the refractive index of the material, if any, located within the gap. 
The Fabry-Perot cavity therefore acts as an interference filter which 
permits the transmission of a narrow band of wavelengths as determined by 
the quality of the mirrors and the width of the gap. If the gap width is 
varied, the center frequency for the transmitted light also varies. Moving 
the bridge relative to the substrate varies the gap between the bridge and 
the substrate, thus changing the frequency of the transmitted light. 
In a further enhancement, a detector can be placed between the lower mirror 
and the substrate. The detector would be a photosensitive structure with 
sensitivity in the spectral region transmitted by the mirrors. It could be 
configured into a photoconductive or photovoltaic sensor with its output 
proportional to the intensity of the light transmitted by the Fabry-Perot 
cavity. Certain preferred embodiments employ a charge coupled device (CCD) 
as a detector. 
A preferred embodiment of the spectrometer includes, a means of moving the 
bridge relative to the substrate. One technique would be to incorporate 
electrostatic force plates. They can be fabricated in a transparent 
conductive material and be part of the lower mirror structure or, can be 
separate and to the sides of the lower mirror structure. In the latter 
case, the bridge length must be sufficient to accommodate the force 
plates. If an electric field is applied between the force plates and the 
bridge, a resultant force is produced in the bridge which pulls the bridge 
toward the substrate. This force is roughly proportional to the square of 
the applied electric field. These force plates can be used to move the 
bridge in a controlled manner over a range equal to about 1/3 of the total 
gap between the force plate and the bridge. Motion beyond this point 
results in unstable behavior which tends to pull the bridge down to the 
force plates suddenly. To be safe, the motion of the bridge should be 
restricted to a value less than 1/3 of the gap for static DC operation. If 
an AC field is applied to the force plates through a series capacitor, it 
is not necessary to restrict the range of motion to 1/3 of the gap 
spacing, thereby permitting larger controlled motions of the bridge. In a 
dynamic sense, the bridge can be made to resonate at one of its resonant 
frequencies by applying a time varying electric field with a frequency 
equivalent to that of the resonant frequency of the bridge. By making use 
of resonance, the bridge could be operated over greater excursions with a 
lower applied field. 
The position of the bridge relative to the substrate or in other words, the 
gap spacing controls the wavelength of the light transmitted into the 
detector. It is therefore important to monitor the bridge to substrate 
spacing. This can be accomplished by using a capacitive detection 
technique. A set of electrodes is placed under the bridge and the 
capacitance between the electrodes and the bridge is measured. It is 
inversely proportional to the gap spacing. This measurement can be made 
using a number of electronic techniques that include electronic bridge 
circuits, oscillators and switched capacitor circuits. 
In use, a light source consisting of a range of wavelengths whose 
distribution and amplitudes are to be determined is introduced to the 
spectrophotometer from the top of the bridge. The bridge is excited into 
resonance by the application of an electric field. The selected wavelength 
of the Fabry-Perot cavity varies in time synchronously with the bridge 
motion. The position of the bridge is monitored with the position 
detectors. This output along with the output from the detector provides 
all the information needed to determine the spectral distribution. 
This bridge positioning and detection subsystem also has non-optical sensor 
applications. As discussed in a later section, it has all of the hardware 
required for a microscale force balance system. With modified electronics, 
and use of a diaphragm bridge, the microspectrometer can be extended into 
a multisensor capable of measuring local mechanical and electric forces in 
the media which is being optically monitored.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a preferred embodiment of the invention including a simplified 
cross-section of a visible microspectrometer 10. To make a spectrometer, 
two important elements are required; a light detector 12 (the p.sup.+ 
/n-junction diode) and a wavelength selective element including an upper 
mirror, gap and lower mirror. In this proposal, a silicon photodiode is 
the preferred light detector for a spectrometer designed to function over 
the visible spectrum. Other choices of detector would extend the range 
into the infrared or ultraviolet. The photodiode can be fabricated in or 
on a silicon substrate 16 by doping the n-substrate with boron to create a 
p-n junction. The choice of silicon as the substrate material allows the 
incorporation of a sense amplifier and drive electronics on the same chip. 
Conventional spectrometers use prisms or gratings as the wavelength 
selective element. In the microspectrometer, the wavelength selective 
element is essentially a Fabry-Perot cavity which consists of an upper 
mirror 14, an air gap 13, and a lower mirror 18. In a simple embodiment, 
the upper mirror might consist of 3 quarter wave layers of silicon and 
silicon dioxide. The air gap normal spacing would be half-wave and the 
lower mirror would consist of quarter wave layers of silicon and silicon 
dioxide on the p.sup.+ emitter of the photodiode. Including the 
substrate, this filter is referred to herein as a seven (7) layer filter. 
The upper mirror is supported by a bridge structure as shown in the top 
view of FIG. 2. The bridge structure is attached to the substrate at its 
bases. In its center a multilayer interference mirror has been fabricated. 
The spring sections of the bridge structure minimize stresses on the 
mirror. Below the bridge at its center is a silicon photodiode which is 
used as the detector. On either side of the diode, an electrostatic force 
plate has been placed. The bridge can be driven by any number of 
techniques including electrostatics, thermal and piezoelectric effects. 
The perceived optimum configuration would be one in which the bridge was 
caused to oscillate at its fundamental frequency. The bridge could span 
electrodes 22 and 24 on either side of the photodiode (as shown in FIG. 2) 
or could use the diode's emitter contact 26 as one plate of a variable 
capacitor. The other plate would be the bridge itself. A circuit is 
fabricated on the same chip which would employ this capacitor as an 
element in the feedback loop of an integrated oscillator. The position of 
the bridge relative to the diode would be proportional to the value of the 
capacitor just described and would therefore be known at all times. Once 
the spectrometer was calibrated, this position would be directly related 
to the wavelength of the light selected by the spectrometer. 
The spectrometer described above is a miniature version of a Fabry-Perot 
scanning interferometer. Previous spectrometers have been constructed 
using conventional machining techniques and very high quality optical 
surfaces. Earlier methods of fabrication could not provide the substantial 
reduction in size provided by the methods set forth herein. For example, 
single devices can range in surface area from about 10 square microns to 1 
square centimeter and preferably between 100 square microns and 1000 
square microns. In addition, ordinary optical flat of 1/4 wavelength is 
not sufficient for precise applications. For high precision measurements, 
1/20 to 1/100 wavelength is required. The most significant advantage of 
the Fabry-Perot interferometer relative to prism and grating spectrometers 
is that the resolving power can exceed 1 million or between 10 and 100 
times that of a prism or grating. Thus, the advantages of miniaturization 
include reduced vibration sensitivity, improved durability, reduced cost 
and size as well as higher scan speed. 
In the schematic diagram shown in FIG. 1, an p.sup.+ emitter layer is 
diffused into an n-type silicon substrate to create a photodiode 12. The 
p.sup.+ layer itself becomes part of the lower interference mirror 18, 
which includes a quarter wave SiO.sub.2 layer and a quarter wave silicon 
layer. An air gap width of half the center wavelength must be created. 
Above this a second interference mirror 14 consisting of quarter wave 
silicon and silicon dioxide layers must be created. Other material pairs 
can be used, where one film has a high index of refraction, such as 
silicon, and the other a low index material, such as silicon dioxide. An 
example of such a high/low index of refraction pair of materials is zinc 
sulfide and magnesium fluoride. This pair is widely used in commercial 
optical interference filters. In conventional interference filter designs, 
the center layer would also be a low index material. In this filter, that 
material is air which effectively has an index of 1.0. 
The number of layers in the mirrors determine their maximum reflectance. 
The greater the number of layers, the narrower the band width of the 
interference filter. In a preferred embodiment, a seven layer interference 
filter has been used. The performance of this filter can be analyzed using 
a matrix method to determine the optical transmission and reflection. The 
method used to create the curves shown in FIGS. 3A and 3B takes into 
consideration the index and absorption of all the layers. In this case, 
the indices and absorption coefficients of all the layers are indicated 
below along with the thicknesses of each layer. 
______________________________________ 
Layer Index Absorption Thickness 
______________________________________ 
Silicon 3.85 0.02 Substrate 
Silicon Dioxide 
1.45 0.0 1/4 wave 
Silicon 3.85 0.02 1/4 wave 
Air 1.0 0.0 1/2 wave 
Silicon 3.85 0.02 1/4 wave 
Silicon Dioxide 
1.45 0.0 1/4 wave 
Silicon 3.85 0.02 1/4 wave 
______________________________________ 
These numbers were used and the transmission of the filter was calculated 
as a function of the thickness of the air gap. The transmission represents 
the amount of light that enters the photodiode to be collected and 
converted to an electrical signal. A center wavelength of 0.5 microns was 
chosen. FIG. 3A shows the results of these calculations including a graph 
of the transmission versus the SiO.sub.2 gap spacing for Si/SiO.sub.2 
mirrors with 5 and 7 layers. Each peak is the response of the spectrometer 
to a monochromatic source of the indicated wavelength. The narrower peaks 
show the improvement in the resolution possible with additional filter 
layers (fwhm=6.25 nm @ lambda=0.5 microns). Note first the curve 
representing the transmission when the wavelength is set to 500 nm. In 
this case, the curve peaks at exactly 250 nm of separation between the 
upper mirror and the lower mirror or at precisely 1/2 the center frequency 
for the filter as expected. It should be noted that there are second order 
responses at large gap spacing and zeroth order peaks at small spacing. 
Two other curves show the results if the wavelength is set to 400 nm and 
600 nm. In these cases the peak moves in the direction expected, but the 
spacing of the air gap does not correspond to half the optical wavelength. 
In fact, the shift in the position of the peak is slightly greater than 
might be expected. The fact that the second order peak corresponding to 
400 nm is approaching the primary peak corresponding to 600 nm suggests 
that the dynamic range of the spectrometer will need to be limited in 
order to avoid spurious results. FIG. 3B shows the seven layer 
spectrometers response to near infrared light. These spectrometers will 
begin to develop interference with the second order fringes at about 1/2 
the minimum wavelength leading to a dynamic wavelength range of 2. 
Based on these results, the full width half maximum of the transmitted 
output is approximately 1/12th of the spacing between the peaks. This 
indicates a resolution limit for this spectrometer of approximately 16 nm. 
This result is not as good as that available from conventional 
spectrometers which would typically have a resolution exceeding 2 nm. In 
FIG. 3A, the results for a five layer spectrometer are shown for 
comparison. 
The resolving power of a Fabry-Perot spectrometer can be expressed as 
##EQU1## 
where R is the reflectivity of the mirrors and N=2nd/.lambda..sub.0 with n 
the index of refraction and d the spacing between the mirrors and 
.lambda..sub.0 is the center wavelength. This analysis indicates that a 
seven layer mirror centered at 0.5 .mu.m will have a reflectivity of 
approximately 99%. Use of the formula above would result in an estimate of 
the resolving power, RP=310. By definition RP=.lambda./d.lambda. and the 
predicted resolution at 0.5 .mu.m is 16 nm. A typical 30 layer 
interference mirror would have a reflection exceeding 0.999 and in this 
application provides a resolving power in excess of 3000, and a resolution 
of approximately 1.6 nm. 
The optical and dimensional properties of the layers vary with temperature 
so resolution will be degraded in applications which are not temperature 
controlled. This is conveniently addressed by incorporating a temperature 
sensor into the device. This measurement allows the optical signal to be 
temperature compensated with the appropriate signal processing. Many types 
of temperature sensors can be employed depending on the specific 
temperature range, sensitivity and linearity desired for a given 
application. Examples include transistor and diode structures, deposited 
thin film resistors and diffused or implanted resistors. 
Three fundamental steps are involved in the fabrication of a 
microspectrometer. These are detector fabrication, bridge/mechanical 
fabrication and interference mirror fabrication. The detector chosen 
depends among other things on the wavelength region desired. For the 
purposes of example, consider the design of a microspectrometer for the 
visible spectrum and a second for the near infrared. Silicon photodiodes 
can be the optimum choice for a visible spectrometer, owing to their 
wavelength sensitivity and the ease with which they can be fabricated and 
incorporated into the structure. Other detectors can be used including 
deposited photoconductors, phototransistors, and avalanche photodiodes. 
A number of different micromachining techniques can be used to fabricate 
the bridge structure. These include CVD deposition of polysilicon or 
silicon nitride. An alternative approach is to use electroplating to 
deposit the bridge structure. Nickel bridges can be used in one 
embodiment. Nickel plating requires a special plating container, equipped 
with temperature control and filtration. 
The final processing area relates to the fabrication of the mirror. This 
involves the deposition of optical quality layers. These layers must be 
deposited on the photodiode or detector surface, and in the hole at the 
center of the bridge structure. The preferred methods of deposition are by 
evaporation or by sputtering. Both processes are well characterized and 
understood by the industry. However, the optical properties of materials 
can vary as a function of the deposition techniques. In the optical 
coatings industry, extensive use of thickness monitoring equipment insures 
the correct optical thickness. Generally, a spectrometer is used to 
provide monochromatic light through a series of mirrors onto the sample 
surface and back to a detection system. The amplitude of the reflected or 
transmitted light from or through the sample is monitored. For a 
transmission sample, the transmission of the uncoated specimen will be 
high. As the deposition proceeds, the transmitted light amplitude 
decreases until it reaches a minimum at 1/4 wave. An operator can 
therefore monitor the deposition at the desired wavelength and optimize 
the coating for 1/4 wave or any desired multiple. 
In FIGS. 4A-4G a suggested process is outlined for the microspectrometer. 
In this process, a silicon photodiode 44 is first fabricated in the 
silicon substrate 40 in FIG. 4A. It must be oxidized, preferably with a 
quarter-wave of silicon dioxide 42, and then coated, preferably with a 
quarter-wave polysilicon layer 46 as shown in FIG. 4B. Metallization 48 
for the top contact of the photodiode is then deposited. This completes 
the bottom mirror of the interference filter. 
To create the bridge a nickel plating process can be used. In this process, 
a sacrificial layer 50 (FIG. 4C) is first deposited and patterned. Next, a 
plating base is deposited usually consisting of a nickel or gold thin 
film. Photoresist is spun on the wafer and patterned. Openings in the 
photoresist allow the plating base to be exposed to the plating solution 
during the plating process. The bridge structure 52 is defined with a hole 
53 or opening above the photodiode (FIG. 4D). After the bridge is plated 
the photo resist is removed. The filter layers 54, 56, 58 (FIGS. 4E, 4F, 
4G) which comprise the upper mirror are then deposited and patterned such 
that they remain attached to the bridge and fill the hole in the patterned 
nickel bridge. These layers will probably be evaporated or sputter 
deposited. Quarter-wave layers of silicon 54, 58 and silicon dioxide 56 
are preferred. Once the upper filter has been completed, the sacrificial 
layer can be etched away leaving an air gap 60 (FIG. 4G). This process 
must take place without detriment to the other layers in the 
microspectrometer structure. 
In certain applications it is desirable to incorporate signal conditioning 
electronics on the same substrate as the mechanical structure. In the case 
of the process described above, a silicon single crystal substrate was 
used as the starting material and therefore lends itself to the 
incorporation of on-chip electronics. The circuit can be fabricated prior 
to the fabrication of the micromechanical elements but would include the 
creation of the silicon photodiode. Also, circuitry employed in the 
analysis and comparison of measured spectra can also be integrated into 
the chip where appropriate. Circuit metallization must be compatible with 
the process used to create the micromechanical structure and will need to 
be protected from the etchant if aluminum is used for both the circuit 
metallization and the sacrificial layer. Alternatively, the circuit 
metallization could be used. Tungsten is another metal useful for circuit 
metallization. 
As suggested above, spectrometers are used in a scan mode to obtain 
absorption versus wavelength spectra. Absorption represents an interaction 
between light and the medium and can be highly specific. For example, 
infrared light includes an electric field which is oscillating at 
frequencies of 10.sup.12 -10.sup.14 Hz. If a vibrational mode of a 
molecule produces an alternating electric field, it can absorb incident 
radiation, but only at that vibrational frequency. Polyatomic molecules 
have many vibrational modes at infrared frequencies. Similar principles 
apply at other wavelengths but the nature of the atomic or molecular 
energy states changes. For example, higher frequency (visible and 
ultraviolet) interactions usually involve outer electron transitions while 
lower frequency microwave absorption typically involves rotational modes. 
Examination of absorption (or reflection) spectra and identification of 
the major absorption (or reflection) peaks often allows the user to 
identify the components in a medium. 
A different technique is usually employed for quantitative monitoring of a 
particular component of a sample being analyzed. In these applications, 
the scan mode is replaced by measurements made at a specific wavelength. 
The selected wavelength should be a characteristic absorption peak of the 
component and unaffected by interference from other components. Many 
factors affect absorption so the measured absorption is usually compared 
to a "baseline" measurement which is made in a nearby inactive part of the 
spectrum. Ratios of absorption peaks characteristic of two species in a 
mixture are also used. 
The microspectrometer is ideally suited for relative measurements of this 
type because the moving bridge can be treated as a two state device. Most 
of the measurement errors (source and sensor, drift, different pathlengths 
and temperature, etc.) are eliminated because the same components are used 
for high rate measurements of both sample and baseline absorption. In two 
stage or multistage operation the device operates at a plurality of 
wavelengths. For example, it can oscillate between two stages or it can be 
sequentially operated through more wavelengths that can be selected by the 
user. Thus the relative amounts of two or more constituents of the sample 
can be determined by rationing the absorption or intensity at wavelengths 
that are characteristic of each constituent. This device eliminates the 
need for two wavelengths or the use of two filters with one device. 
The small size and low unit cost of the microspectrometer makes array 
products practical. One implementation would utilize arrays of identical 
devices for pattern recognition, enhanced sensitivity and 
reliability-through-redundancy applications. Redundancy includes circuitry 
that places a second spectrometer element in the array on-line upon 
failure of another spectrometer element. The circuit can optionally 
identify failed components for the operator. 
The microspectrometer design of the present invention is capable of 
producing spectrometers with a total area of 30 .mu.m.times.30 .mu.m and 
smaller. With such a small device, an array of spectrometers as shown in 
FIG. 6B which are similar to current photodiode arrays can be produced. 
This array can be used in a three dimensional mode in that it will not 
only provide two dimensional image information but will provide spectral 
information as well. Such arrays would be extremely useful for medical 
applications, target recognition in military applications and for 
environmental monitoring. 
The basic design for the microspectrometer consists of a center plate 32 
with two sections of increased mass 34 supported by four springs 28 as 
shown in FIG. 6A. The springs are fabricated in nickel using a selective 
plating process and are designed to provide minimum resistance to motion 
normal to the wafer but are significantly stiffer for motion in the plane 
of the wafer. The center section 34, which can contain regions with 
additional mass that is used to reduce the resonant frequency, includes a 
hole 12 in which the filter is placed. The springs 28 are fastened to the 
substrate 30 at the outer edges of the device. The mass is an 
electroplated gold layer which can be selectively plated after the springs 
have been defined. As described previously, the upper filter would be 
deposited in a hole at the center of the device. The nickel springs might 
be approximately 2 .mu.m wide, approximately 30.mu. long (folded) and 0.5 
.mu.m thick. The gold layer is approximately 10 .mu.m thick and about 10 
.mu.m on a side. Since gold has a density of 19.3 gm/cm3, the total mass 
of the mirror support is therefore about 
EQU m=2*19.3 gm/cm3*10 .mu.m*(10 .mu.m).sup.2 *10.sup.-12 
cm3/.mu.m3,=3.86.times.10.sup.-8 gm. 
The springs are treated as four cantilevered beams as shown in FIG. 7. The 
deflection of a cantilever beam loaded at the end is defined as follows: 
##EQU2## 
where W is the applied load 
1 is the length of the cantilever, 
E is the Young's modules, and 
I is the moment of inertia. 
For a rectangular beam the moment of inertia is defined as follows: 
##EQU3## 
where h is the thickness and 
b is the width 
##EQU4## 
Each of the four equivalent sections behaves like a cantilever with the 
weight W being applied to its free end. The total deflection of the spring 
is 4v.sub.0 and the length used in the formula must be written as L/4 
where L is the total length of the folded spring, so 
##EQU5## 
Rearranging the equation into the form W=-k*4v.sub.0, gives the spring 
constant (k) with four springs supporting the mirror support mass as 
##EQU6## 
Substituting in reasonable values such as: b=2.0.times.10.sup.-4 cm, 
h=0.5.times.10.sup.-4 cm, 
L=30.times.10.sup.-4 cm, and 
E=2.07.times.10.sup.12 dynes/cm.sup.2 
results in 
k=7.66.times.10.sup.3 dynes/cm. 
Therefore the resonant frequency of each individual spectrometer will be 
EQU .omega.=(4k/m).sup.1/2 .about.8.91.times.10.sup.5 radians/second, 
so 
EQU f=.omega./2.pi..about.142 KHz. 
The magnitude of the electrostatic force between two parallel plates of a 
capacitor ignoring fringing fields can be expressed as 
##EQU7## 
where d is the spacing between the electrodes, 
.epsilon..sub.0 is the permitivity of free space, 
A is the are of one of the plates, and 
V is the applied voltage. 
The permitivity of free space .epsilon..sub.0 is expressed as 
8.85.times.10.sup.-7 dynes/volt.sup.2. Using the configuration shown in 
FIG. 8, the force exerted by the electrostatic field is counteracted by 
the spring such that 
##EQU8## 
where x is the distance the mirror support has moved away from its 
equilibrium position. 
This leads to the relationship between the voltage and the position 
EQU V=(2kx/.epsilon..sub.0 A).sup.1/2 *(d-x), 
which has been plotted in FIG. 9. For small x, the voltage required to hold 
the proof mass in position varies approximately as the square root of the 
distance. As the position increases, the voltage required to hold the 
proof mass increases monotonically but at a ever decreasing rate. At a 
point one third of the original distance, d, the slope (dV/dx) is zero. 
Further increases in the position require less holding voltage. Therefore, 
if the position were to increase beyond d/3, then at a fixed voltage, the 
bridge body would continue to be accelerated until the force plates of the 
capacitor met. Therefore, for voltages above the maximum value (v.sub.th) 
indicated on the curve, the system would be unstable. To operate the unit 
safely, the voltage should be restricted to a value well below V.sub.th. 
It can also be appropriate to incorporate stops in the mechanical design 
to prevent the electrostatic plates from collapsing together. As part of 
the design considerations, the initial spacing, d plays a crucial role in 
the performance of the device. 
When using an AC electric field a broader range of static operation of the 
microspectrometer can be obtained. For example if the force on the bridge 
due to the applied AC voltage be: 
EQU F=1/2.epsilon.AV.sup.2 /d.sup.2 =1/2.epsilon.AV.sup.2.sub.AC /d.sup.2 
*sin.sup.2 .omega.t 
where 
V.sub.AC =the magnitude of the applied AC voltage 
.omega.=the frequency of the applied AC voltage. 
To work properly, .omega. must be greater than the resonant frequency of 
the bridge and far from any other mechanical resonances of the bridge. The 
average force the is: 
##EQU9## 
We can now capacitively couple; As a result, the voltage on the bridge is 
now 
##EQU10## 
However, C.sub.bridge changes with the motion of the bridge. 
##EQU11## 
where d is the initial spacing 
X is the displacement. 
The voltage on the bridge is consequently given by the expression: 
##EQU12## 
and the average force is now, 
##EQU13## 
This force does not increase indefinitely as X grows to d but instead 
reaches a maximum of 
##EQU14## 
Therefore, using a series capacitor allows one to control the position of 
the bridge in a static manner by eliminating the instability that is found 
when a DC voltage is used to deflect the bridge. 
Another implementation addresses the order effects illustrated in FIGS. 3A 
and 3B. Order effects limit the dynamic range of a single device. However, 
the limitation can be removed by using an array of devices which have 
different center frequencies. 
When used to measure composition, the microspectrometer is normally mounted 
behind an optically transparent window in order to protect it from dust 
and corrosive fluids. It is possible to integrate this packaging function 
into the device itself. The result is a more complex structure, with 
greater electronic support requirements. However, the integrally sealed 
structure has greater capability: it can characterize physical and certain 
electrical properties of the media in contact with the device in addition 
to the compositional measurements described above. 
Optically, the sealed multisensor 34 is identical to the Fabry-Perot device 
described above. However, the moving bridge is replaced by a moving 
diaphragm 38 as shown in FIG. 5A. Planarity in the center section of the 
diaphragm can be maintained by a dual strike process, or by use of 
convolutions that stiffen the center relative to the peripheral region. 
Plating conditions must be carefully controlled in order to avoid 
excessive stress levels. Alternatively, similar materials such as a 
polysilicon diaphragm on a silicon substrate 39 can also be used. A dogleg 
32 is included for relief of thermal expansion mismatch. Practical 
implementation often requires that the outermost layer resist 
environmental damage from the fluid (chemical attack, erosion, surface 
fouling and scaling) without compromising optical performance. Deposited 
films of diamond, silicon carbide and boron nitride are examples of such 
layers. In applications where chemical attack is a problem, for example, 
elimination of film defects is a primary objective. Thus, diamond-like 
films would often be preferred rather than single crystal diamond films 
which can be susceptible to substrate defects. Note that environmental 
compatibility is bidirectional, that is, the fluid must not degrade the 
device and the device must not degrade the surrounding material. This 
issue is a particular concern in biological applications where the 
exterior layers of the device must meet biocompatibility criteria. 
The time varying gap thickness is normally determined by force plate 
excitation, and measured by the capacitive detector. The integrally sealed 
version has an additional characteristic: gap thickness is also affected 
by pressure forces from the medium. This DC offset in bridge position is 
readily measured by the detector, and corrected by adjusting the DC level 
in the force plate circuit. Thus, the force plate DC signal is a measure 
of fluid or barometric pressure. Thus the system can be employed for 
optical, mechanical and temperature measurements of the medium. 
Also of importance is the ability to monitor fluid "noise" sources such as 
transient surface charges, density fluctuations and pressure pulses. These 
effects arise from turbulence, multiphase effects and characteristics of 
nearby equipment. In industrial processes, most process noise is low 
frequency (below 30 Hz). As a result, the noise attributable to pressure 
pulses can easily be separated from the higher frequency bridge 
excitation; it is essentially a low frequency signal superimposed on the 
DC position offset mentioned above. When the AC drive signal is set to 
zero it is possible to measure the force applied by the fluid to the 
diaphragm by applying a DC signal and measuring the AC signal induced by 
movement of the diaphragm. 
Surface charge effects can also be monitored depending on the fluid and the 
degree of electrical grounding. In many flow applications, immobilized 
static charge layers form in the fluid at the solid-fluid interface. These 
charges, in turn, attract charges that are less tightly bound. Therefore, 
they form and decay in close correlation with the incidence of turbulent 
eddys and secondary phases. This surface charge effect is related to the 
phenomena termed "streaming potential" and might be expected to occur only 
in ionic liquids. However, similar effects have been observed in some 
non-conductive fluids. Formation and decay of electrical charges at the 
diaphragm surface modulates diaphragm voltage. Thus, the sealed 
microspectrometer drive and position detector system described in greater 
detail below enables electrical surface charge effects in biological, and 
other fluids to be monitored either separately, or as part of a device 
which correlates them with optical properties. 
The resonant frequency of an element, f.sub.n, is a function of mass. 
Typically: 
##EQU15## 
where m* is the effective mass of the diaphragm plus the fluid in contact 
with it. This effect has been used in the past to make densitometers. The 
present invention extends this capability to smaller sizes. It also allows 
density fluctuations in multiphase fluids to be measured and correlated 
with transients in local optical properties. 
In summary, the sealed microspectrometer enables simultaneous high rate 
characterization of the composition, physical and electrical properties of 
moving fluids. Potential applications include atmospheric studies, wind 
tunnel instrumentation and chemically reacting systems. 
The major differences in fabrication between the sealed microspectrometer 
and the standard design involve the need for backside electrical contacts 
and for access to the sacrificial layer under the diaphragm. These 
requirements can be accommodated by implanting and driving p+ runners for 
each of the frontside structures in (100) silicon wafers. Late in the 
process, a "well" 35 is etched from the backside with the etch resistant 
p+ runners 33 being exposed as "diving boards" in the well 35. This is 
conveniently accomplished by using a standard piezoresistive pressure 
sensor process based on hot KOH. Lightly doped silicon and the aluminum 
sacrificial layer are rapidly etched in this process, leaving the p+ 
regions exposed at the bottom of the well. Shadow mask deposition of gold 
37, as shown in the bottom view of FIG. 5B, brings these contacts out to 
the back surface of the wafer for probe testing and mounting. As shown in 
FIG. 5B, an additional p+ runner 33 extends to the diaphragm base on the 
frontside of the substrate 39. Similar runners can also be used for the 
force plate and emitter connections. 
It should also be realized that other advantages are realized by placing a 
liquid in the gap. In such a case, the motion of the bridge would be 
significantly retarded by the liquid. This makes it possible to statically 
operate the device over gap ranges exceeding the one third limit 
previously mentioned. This can be implemented by pulsing the voltage 
applied to the force plates and monitoring the position. By varying the 
pulse width, duty cycle or height, the gap spacing could be maintained. 
Limitations on these techniques are established by considering the 
bandwidth of the mechanical system which consists of the bridge and 
substance within the gap, and the bandwidth of the electrical feedback 
circuit. Stability is achieved when the bandwidth of the electronics 
exceeds that of the mechanical system. 
The liquid in the gap could be either high or low index material. The 
device will operate as described previously with a low index liquid. With 
a high index liquid, the mirrors are adapted in the following way. Whereas 
with the low index liquid a high-low-high three layer mirror is used, with 
a high index material, a low-high-low index material, a low-high-low index 
mirror is preferred. This provides a low index layer to the outside 
environment and has the benefit of reducing the reflectivity of the 
spectrometer to incident light, thereby providing higher sensitivity. 
A microspectrometer is described herein which can be fabricated on silicon 
substrates using conventional silicon microelectronic fabrication 
techniques. A micromechanical bridge structure is used to support an 
optical interference mirror. The bridge, gap, and the underlying silicon 
detector form a complete interference filter. Initial modeling results 
demonstrate that when the gap between the bridge and silicon detector is 
varied, the center wavelength of the interference filter changes. The 
interference filter represents the wavelength selective component of the 
spectrometer and takes the place of a prism or grating in a conventional 
spectrometer. 
An electrostatic drive sets the bridge into oscillation at its fundamental 
frequency. This causes the upper mirror to move periodically closer and 
farther from the photodiode creating a time varying gap spacing. The 
wavelength of the light selected for transmission to the photodiode is 
therefore also time varying and with the same frequency as the bridge. The 
output of the photodiode will be a periodic spectrum of the incident 
light. 
In some applications, it may be more suitable to separate the wavelength 
selective element from the light source or detector. FIG. 10 shows a 
possible implementation in which both the light source and detector are 
remote. As shown, the Fabre Perot interferometer is fabricated as 
described previously in a silicon substrate. The silicon substrate is 
later etched in an anisotropic etchant from the back side to create a 
groove or pit into which an optical fiber could be inserted. 
Alternatively, the groove could be created with any shape if etching 
techniques including isotropic chemical etching, dry etching, plasma 
etching and ultrasonic machining. A second bulk silicon micromachined 
etchant has been bonded to the surface of initial silicon substrate. This 
part is added specifically for the purpose of aligning a second optical 
fiber to the Fabre Perot interferometer. Light from a remote source can 
now be coupled into the wavelength selective element, the output of which 
is transmitted to a remote detector. The benefits of this approach include 
the ability to monitor hostile environments by separating the 
micromachining device from the source. Additionally, it may be 
advantageous to cool the detector. Separation of the detector and the 
wavelength selective element would allow cooling of the detector without 
detriment to the micromechanical device. This could be especially 
important if control electronics are included on the same chip with the 
micromechanical device. 
In FIG. 10, both an input fiber 82 for incident light 81 and an output 
fiber 86 to couple the received light 87 to the detector are shown. It 
should be understood that in specific applications, the input, output or 
both fibers could be eliminated. Fabrication of the Fabre Perot 
interferometer with components bridge 84 and substrate 85 are fabricated 
as a separate element. This device could then be used in conjunction with 
a separate detector element. For instance, in infrared applications, a 
lead sulfide (PbS) detector may be appropriate. This detector could be 
fabricated in a piece of glass as is usual, and brought into close 
proximity to the Fabre Perot interferometer. In this way, the processing 
associated with the detected manufacturing would be separate and apart 
from the fabrication of the interferometer. The sealed microspectrometer 
can be used as a single element to detect and quantify the occurrence and 
frequency of optical, density, pressure and electric disturbances which 
occur in a fluid near the solid-fluid interface. Back etching of the 
silicon substrate 85 can be used to center the output fiber 86 or fiber 
bundle and mounting element 83 is used to center the input fiber 82 
relative to the cavity. Element 83 can be mounted or integrally fabricated 
onto the substrate 84. 
FIG. 11 is a schematic cross section showing an array 100 of sealed 
microspectrometer sensing elements 102 which are positioned to receive 
incident light 106 which is passing through fluid 104. Light 106 can be 
generated by a broad or narrow band light source which can be directed to 
the array by fiber optics or can be ambient light. The array 100 can be 
mounted in the wall 108 of a pipe or some other interface with a fluid. 
The fluid can be stationary or moving relative to the array. 
Implementation as an array provides information on the size or scale of 
the disturbances noted above. The array format also allows the user to 
actively interact with the interfacial fluid. For example, a transient 
voltage pulse in one or more diaphragms will alter the electric field and 
generate an acoustic signal in the adjacent fluid 104. Response to these 
artificial disturbances would be detected by other elements in the array. 
Additionally, one or more of the positions in the array could be occupied 
by light emitters 110. Emitted light would be scattered by inhomogeneities 
in the fluid, detected by the nearby sensing elements, and interpreted in 
terms of phase, angle, intensity and spectral distribution. 
The light emitters 100 could be monolithically incorporated on the 
substrate. These could, for instance, include Light Emitting Diodes 
(LEDs), Lasers, or broadband sources such as hot filaments. In the case of 
narrow band or line emitters, such as LEDs and surface emitting lasers, 
these are typically fabricated in III-V materials such as GaAs, GaInP, 
InP, GaAlAs, etc. Techniques for transferring these materials and devices 
onto silicon by deposition or thin film transfer are well known. Laser 
light sources would be particularly advantageous if used in conjunction 
with fluorescence and Raman scattering measurements. Broadband emission 
from thermal filaments can be used for reflectivity measurements. An 
optical signal can thus be produced in the chip and directed onto the 
sample under study. Fiber optics can be used to couple the source, sample 
and sensor. 
FIG. 12 schematically illustrates a single spectrometer that is shown with 
a temperature compensating circuit. A drive voltage is generated in the 
drive circuit 128. Its output is converted through an AC Voltage source 
126 to the bridge 122 and is used to control the position of the bridge 
122 and upper mirror 124. The capacitance between the bridge and the drive 
counter electrodes (123) on the substrate 120 is monitored by applying a 
small ac-signal. This signal is provided by the AC Voltage source 126. The 
AC current is measured in the AC current measurement circuit 130. This 
current is directly proportional to the gap spacing. Through the signal 
conditioning circuit 136 the AC-current is converted to a voltage which is 
proportional to the wavelength (.lambda.) of light being measured by the 
photodiode formed from the photodiode emitter 121 and the substrate 120. A 
temperature compensation circuit 134 which can typically employ a silicon 
transistor 135 is used to compensate both the position measurement and the 
output of the photodiode and its amplifier. The photodiode amplifier and 
signal conditioning 132 produces an output (A) that is directly 
proportional to the incident light energy in the photodiode. A data 
processor and memory can be employed in any of the following embodiments 
to control operation of the spectrometer or array and record spectra, 
images or other data being collected. 
FIG. 13 schematically illustrates a circuit and device similar to the one 
just described except that the bridge capacitance and force plates have 
been combined In this single capacitor system the force plate can be used 
to move the bridge and measure the gap. In this case, capacitance of the 
upper mirror 152 relative to the silicon photo diode 151 is monitored. In 
order to achieve this goal, the mirror must be at least partially 
conductive. The electrostatic drive voltage is applied by the drive 
circuit 156 between the partially conducting mirror 152 and the surface of 
the photodiode 151. This causes the gap to change as described previously. 
Again a small AC signal is superimposed in the drive voltage by AC source 
158 to provide capacitive displacement sensing. In a typical application, 
its frequency would be much greater than that of the drive voltage. The AC 
current measurement circuit 160, signal conditioning circuit 162, 
temperature measurement circuit 166 and the photodetector amplifier and 
conditioning circuit function as described previously. 
In FIG. 14 a CCD array 170 has replaced the usual single photodiode beneath 
the bridge 172. The output of the CCD array 170 is monitored using an 
array detector circuit 178. This circuit may, for instance, scan the array 
and produce composite video output. The signal may be further conditioned 
at 180 to compensate for temperature effects and to linearize the output 
as a function of wavelength. The drive and position detection circuit 176 
is similar to that described in the previous two figures. 
FIG. 15 illustrates an implementation of the basic concept in which an 
array of individual scanning elements 196a-d has been assembled. Each 
element has its own drive and detector circuit 194a-d. The output from 
these circuits is the input to an analog multiplexer 192. A second analog 
multiplexer 200 selects the corresponding photodetector signal from 
detectors 198a-d formed on chip 190. Both the position signal and 
photodetector signal are sent to signal conditioning circuitry 206 and 202 
which is meant to compensate for non-linearities in the output of the 
photodiodes with wavelength, temperature and other non-linearities. 
Temperature compensation is provided by circuit 204 as previously 
described. Information about which detector has been selected may be 
output by the multiplexer circuitry if it is selected by on-chip 
electronics or may be input on an external processor. 
Finally, hybrid packaging techniques could be used to assemble various 
elements discussed above. For instance, one employs a package which 
incorporated a silicon chip with drive and sense circuitry, a separate 
chip containing the interferometer and a last chip containing the 
detector. 
When built as a sealed multisensor, the microspectrometer offers 
simultaneous measurement of fluid composition, pressure, mass loading 
transients and microscale turbulent properties of fluids. 
A microspectrometer as described above will be extremely useful in many 
industrial applications. Optical spectrometers are currently used to 
determine the constituents of stack gases, for hazardous gas monitors in 
at-risk ambient air sites, for other chemical analysis, for flame 
analysis, in instruments used to determine film thickness, in both in 
vitro and in vivo measurements of biological fluids or tissue and in many 
other applications. Existing spectrometers are large and expensive, 
limiting their use. The disclosed microspectrometer provides a means to 
perform optical analysis at very low cost and in very small spaces that 
are otherwise difficult to access and accurately analyze. 
A preferred embodiment of the invention involves gas sensing which can be 
accomplished by several methods including spectral analysis. In this 
embodiment, a microgas sensor is provided in which the sample gasses are 
stimulated to emit photons at characteristic frequencies. These photons 
enter the spectrometer which produces an output proportional to the 
spectral intensity of the incoming light. Complete spectra can be obtained 
in fractions of a millisecond. These spectra are compared with known 
spectral characteristics to determine the constituent gasses. 
Existing instrumentation for gas spectral analysis tends to be large and 
complex. The miniaturized microspectrometer offers significant advantages 
over existing instruments including smaller size and weight, lower cost of 
fabrication, faster data acquisition and improved reliability. The optical 
properties of an unknown material can reveal important information leading 
to a determination of its composition or physical properties. For 
instance, many have used spectral analysis of optical emission lines to 
determine the atomic species of gaseous materials for many years. Optical 
spectra are also used routinely by the semiconductor industry to determine 
the thicknesses of multilayer thin films. 
This embodiment is based on the analysis of the unique spectra of optical 
energy emitted by excited gases. At room temperature, the atoms that 
comprise gasses are typically found with their electrons occupying all the 
lowest energy states. By providing energy to the electrons, they can be 
excited into higher energy states. Once excited, electrons will decay to 
the ground states emitting photons at characteristic wavelengths. Each 
species has its own characteristic spectra that can be used to distinguish 
it from other gas species. 
Gas analyzers are designed to isolate specific gas species and to quantify 
their abundance. The present system involves the construction and 
integration of a miniature pump, optical spectrometer and gas excitation 
chamber. The small volume pump continuously moves samples of gas into an 
excitation chamber where it is ionized using a high potential corona 
discharge. Radiation emitted from the ionized gas is directed toward a 
miniature optical spectrometer. The spectrometer, as described previously 
herein will decompose the light into its individual spectral components. 
This spectra is analyzed using signal processing algorithms. Integration 
of the signal processing with the micromechanical components is also 
provided. 
The complete device fits on a silicon chip no larger than a square 
centimeter. The pump gas excitation chamber can be fabricated using bulk 
micromachining techniques. The spectrometer is a surface micromachined 
device that, in this embodiment, is placed directly above the ionization 
chamber. 
The sensitivity of the system does not rely on the selectivity of a 
particular material, but rather on the emissive properties of the gas 
under examination. This allows the construction of a single instrument 
which can be adapted for a specific application. This is accomplished by 
an analysis of the spectral output of the proposed device. Yet at the same 
time, this instrument is fabricated using the batch fabrication techniques 
characteristic of solid state sensors. In addition, the device is capable 
of responding in as little a one millisecond depending on the relative 
intensity of the signal to be discriminated. 
In the present device, grooves are formed in the silicon to channel the 
gasses from the sample source into the excitation center. Channels in 
silicon are created using a number of etchants. Some etchants are 
selective to specific silicon planes. Most anisotropic silicon etches etch 
the (111) plane much more slowly than any other crystalline plane. After 
etching in an isotropic etchant, some of all of the surfaces exposed are 
(111) planes. This feature of anisotropic etching can be used to create 
well defined mechanical structures. 
A second method for creating micromechanical structures is to fabricate 
them on the surface of the wafer using deposited materials. In the present 
device, the spectrometer is fabricated using surface micromachining 
techniques. 
The present embodiment of a microgas analyzer 210 contains three key 
components (FIG. 16), a pump 212, an excitation chamber 216 and a 
microspectrometer 220. The pump and chamber are fabricated in a silicon 
wafer 224 using bulk micromachining techniques. The microspectrometer is 
fabricated using surface machining on a second transparent substrate 226 
such as a Pyrex wafer. The Pyrex wafer is subsequently bonded to the 
silicon wafer 224 using electrostatic or adhesive bonding such that the 
spectrometer is place above the excitation chamber as shown FIG. 16. 
The present gas sensor can employ electrostatic pumps, and more 
particularly, a peristaltic pump. One pump 212 consists of a compliant 
diaphragm formed over a smoothly etched channel 214. Electrodes in the 
channel 214 and on the diaphragm can be excited consecutively to cause the 
pump to push gas down the channel. Gas enters the excitation chamber 
through inlet 222 where it is excited using an alternating electric field. 
The corona created by this technique can be maintained at one atmosphere 
and is sufficiently energetic to excite the gas to emit at its 
characteristic frequencies. 
The gas is excited using a technique similar to the used to create ozone by 
a silent discharge in airfed ozonisers. The present ionizer consists of 
metal electrodes of very large area (for example 5 cm.times.1 cm) with 2 
mm separation between them. The lower electrode is covered with silicon 
dioxide (dielectric constant=3.8) 1 mm thick. The ions are formed by 
electron impact disassociation. The electrons are created by supplying 
alternating high voltage at the electrodes which helps ionizing the gas. 
The value of the desired breakdown voltage for air is given by the Paschen 
relation as follows: 
EQU V.sub.3 =30d+1.35 Kvolts 
In our case d=0.1 cm which gives the value of V.sub.3 as 4.35 KV. When this 
voltage is applied numerous low current discharges are produced which are 
distributed homogeneously over the electrodes and discharge small areas of 
the dielectric. These electric discharges act on the atoms and molecules 
in the gas and ionizes them. 
The values of dielectric and air capacitors for this particular example are 
calculated as 16.82 pF and 4.42 pF respectively. Assuming that a voltage 
source with peak voltage 6 KV and frequent 10 Khz, the power provided to 
the corona is approximately 2 Watts. The efficiency of the ionizer is 
determined by the amount of the power used by the electrons to create 
ions. In a ozonizer similar to the present system the efficiency was about 
95%. That means only 5% of the power is being given to the ions and is 
eventually dissipated as heat. In our case the dissipated power is 20 
mW/cm.sup.2 which is well within parameters of chip fabrication. 
The microspectrometer is a simple structure consisting of a lower mirror, a 
variable gap and an upper mirror and detector as described previously. The 
upper and lower mirrors consist of a series of high and low index 
materials. The thickness of each layer is set to one quarter of the center 
frequency for the spectral range to be analyzed. The gap is set to one 
half the center frequency. The upper mirror is supported on a 
micromechanical bridge that is fabricated on the Pyrex using surface 
micromachining techniques. In addition to supporting the upper mirror, a 
photoconductive detector is also deposited on the bridge and an actuator 
moves the upper mirror and detector closer and farther from the lower 
mirror, the wavelength transmitted through to the detector varies from 
short wavelengths to high wavelengths. Light which is emitted by the 
excited gas passes through the Pyrex and into the lower mirror of the 
spectrometer. Selected wavelengths are examined as the bridge moves from 
its lower position to its high position. The electrical resistance of the 
photoconductor is monitored and varies in accordance with the spectrum of 
the gas sample. 
Since the wavelength range of the spectrometer is limited, it may be 
desirable to have more than one device in each analyzer. Two or more 
spectrometers forming an array can each scan different spectral ranges. 
Conventional spectrometers use prisms or gratings as the wavelength 
selective element. In this system, the wavelength selective element is 
essentially a Fabry-Perot interference filter with one important 
difference. The center layer of the interference filter is an air gap that 
is created by fabricating a micromechanical bridge above the lower mirror. 
The two mirrors that are components of the interference filter are 
deposited both in a hole on the bridge and directly on the surface of the 
substrate. The bridge can be moved by any number of techniques including 
electrostatics as proposed here, or through thermal and piezoelectric 
effects. The preferred configuration is one in which the bridge was caused 
to oscillate at its fundamental frequency. 
A further embodiment is a device providing a monolithic sensing system. It 
is mounted in a two port device header similar to those used for 
differential silicon pressure sensors. The input port on the header can 
include a small filter to prevent dust from entering the analyzer. 
An ordinary optical flat of 1/4 wavelength is not sufficient for precise 
applications. For high precision measurements, an optical flat of 1/20 to 
1/100 wavelength is required. The most significant advantage of the 
Fabry-Perot interferometer relative to prism and grating spectrometers is 
that the resolving power can exceed 1 million or between 10 and 100 times 
that of a prism or grating. 
A completely integrated device includes a photodetector. Thus another 
embodiment includes an additional silicon wafer 230 with a photodetector 
and intelligent circuit added to monitor and analyze the resulting data. 
The additional detector and circuit is shown in FIG. 17 and is fabricated 
by forming a cavity 240 and forming a detector 242 within the cavity 240. 
Additional circuitry 244 can be fabricated in the wafer, and can be 
connected directly to other components of the spectrometer and/or off the 
device to monitoring or control circuits. This detector wafer 230 can 
include a linear or planar detector array that can be aligned with an 
array of spectrometers and gas chambers. An example of the integrated 
device is described below in connection with FIG. 20. 
In the schematic diagram shown in FIG. 18, a lower interference mirror 250 
which includes a quarter wave Si layer, a quarter wave SiO.sub.2 layer and 
a quarter wave silicon layer is placed on the Pyrex substrate 256. An air 
gap width of half the center wavelength in formed. Above the gap, a second 
interference mirror 252 supported by bridge structure 254 consisting of 
quarter wave silicon and silicon oxide layers is formed. The choice of 
silicon and silicon oxide is for convenience but other materials as 
described herein can also be used. Other material pairs can also be used, 
where one film has high index of refraction, such as silicon, and the 
other a low index material, such as silicon dioxide. In conventional 
interference filter designs, the center layer would also be low index 
material. In this filter, that material is air which effectively has an 
index of 1.0. 
The reflectivity for a seven layer mirror centered at 0.5 .mu.m will have a 
reflectivity of approximately 99%. Use of earlier the formula results in 
an estimate of the resolving power, RP=310. By definition, 
RP=.lambda./d.lambda. and a predicted resolution at 0.5 .mu.m is 16 nm. A 
typical layered interference mirror has a reflection exceeding 0.999 and 
in this application would provide a resolving power in excess of 3000. 
A process for forming the ionization chamber and to monolithically 
fabricate the channels leading to and from the chamber is shown in FIGS. 
19A-19E. 
In FIG. 19A, a reaction chamber is formed in a silicon substrate 304 using 
the following process. Alignment marks are etched on both sides of the 
wafer 304, and a back etch is performed to create contact via 306 for the 
excitation electrode. Using the alignment marks on the side of the wafer 
opposite the contact via 306, a reaction chamber 308 and a flow tube or 
channel 305 running along the surface of the wafer 304 are formed using an 
anisotropic etch as shown in FIG. 19B. The wafer is oxidized 310 and a 
hole 315 is cut through oxide 310 to permit removal of the silicon through 
the hole 315 as shown in FIG. 19C. This results in a hole 316 through the 
silicon which is used to contact the electrode 318 that is formed in the 
excitation chamber (FIG. 19D). 
A critical step in chamber fabrication is the creation of an electrically 
insulating layer that can be about 1 mm thick in a preferred embodiment 
that is used to isolate one of the electrodes. To accomplish this, a glass 
slip 318 is coated with an appropriate metal layer 320 (Cr/Au) and bonded 
to the bottom of the ionization chamber 308 using a eutectic bonding 
technique. Electrical access to the lower electrode is accomplished 
through the aligned and etched holes 315, 316 created on the backside of 
the silicon wafer. 
As described previously, the aligned etched feature on the back of the 
wafer that permits electrical contact to the lower excitation electrode 
which is accomplished by direct wire bonding to layer 320 or back 
interconnect extending to a bonding pad on the back of the wafer. To 
accomplish this, double sided masking is required. To simplify the 
process, the gas injection channel depth and the ionization chamber depth 
can be equal in a preferred procedure, but aren't necessarily equal. If 
they are equal, both can be created simultaneously. Corner compensation 
features can be used to maintain the integrity of the edges of the deep 
etch features. 
In order to attach the lower electrode to the silicon substrate, and at the 
same time maintain isolation from the chamber gasses, eutectic bonding can 
be employed. This technique involves the deposition of a low temperature 
alloy, Au/Ge for instance, to the lower surface of the ionization chamber. 
The chamber in the present example is initially about 2 mm in depth. Even 
with such a large etch feature, photoresist patterning can be employed. 
After patterning, a Cr/Au coated 1 mm glass electrode is placed Cr/Au 
surface down into the pit. The silicon wafer is subsequently heated to 
affect a bond between the Au surface and the eutectic layer. Electrical 
connection to the electrode can be established by wire bonding directly to 
the exposed side. 
In the creation of the ionization chamber, the glass electrode bonding uses 
glass pieces that are patterned with metal and therefore are preferred to 
be in wafer form. After patterning they can be cut to size. They are 
capable of surviving the bonding temperatures (500 C) without softening. 
It is preferable if the glass has a thermal coefficient of expansion that 
matched silicon. An electrostatic pump is fabricated to provide fluid flow 
into 332 and out of 334 the chamber 308 through channel 305 as shown in 
FIG. 19E. The micropump can include two identical microvalves joined to 
opposite sides of a micromechanical membrane by microchannels. The 
microvalve dimensions can vary from 10 .mu.m-100 .mu.m in length, for 
example. The channels can be formed in several arrangements to direct the 
flow to and from the chamber. The electrodes used to drive the membrane or 
walls of the microchannel can be coated with silicon nitride or other 
material that is compatible with the fluid to be analyzed. In this 
embodiment, two membranes 330 are formed to provide paristalitic action on 
the input side of chamber 307 and a third membrane 330 is formed on the 
output side. 
The surface micro-machining process includes photolithography steps and 
chemical vapor deposition steps. To begin, a first PSG layer is deposited 
and etched to form the channel 305 or channels. Polysilicon is etched to 
define the lower electrodes of the valves and pumping membranes. This is 
followed by the deposition of another encapsulating layer of silicon 
nitride. A plasma etch is then used to cut through both nitride layers 
exposing the ends of the channel. The second sacrificial layer of PSG is 
then deposited and etched to define the valve and membrane spacer areas. A 
silicon nitride layer and a polysilicon layer are subsequently deposited. 
The polysilicon is patterned and etched to define the upper electrodes. A 
final layer of silicon nitride is deposited and patterned to open the 
sacrificial PSG and contact areas. Finally, the PSG is laterally etched in 
an acid solution to open the channels. 
As shown in FIG. 20 the three wafers are bonded together to form the gas 
analyzer device. Wafer 350 is an optically transparent material such as a 
Pyrex plate which contains a counter electrode 354 and the spectrometer 
352. It is bonded to the silicon wafer 370 after the channel and 
ionization chamber have been formed. The electrode on the plate 350 is 
aligned to the silicon wafer 370 prior to bonding. This is accomplished in 
a conventional aligner. Wafer bonding requires careful surface preparation 
and can be conducted at high temperatures (450 C) and high voltages 
(800V). Wafer 360 having the photodetector 362 is also bonded to plate 
350. Contacts to the detector can be made through the back of wafer 360 
through via 364 or interconnect hole 366. Electrical connections to the 
spectrometer and photodetector can include bonding pads and patterned 
metalization lines formed on the glass wafer 350. 
Equivalents 
Those skilled in the art will know, or be able to ascertain using no more 
than routine experimentation, many equivalents to the specific embodiment 
of the invention described herein. These and all other equivalents are 
intended to be encompassed by the following claims.