An improved microcantilever sensor is fabricated with at least one microcantilever attached to a piezoelectric transducer. The microcantilever is partially surface treated with a compound selective substance having substantially exclusive affinity for a targeted compound in a monitored atmosphere. The microcantilever sensor is also provided with a frequency detection means and a bending detection means. The frequency detection means is capable of detecting changes in the resonance frequency of the vibrated microcantilever in the monitored atmosphere. The bending detection means is capable of detecting changes in the bending of the vibrated microcantilever in the monitored atmosphere coactively with the frequency detection means. The piezoelectric transducer is excited by an oscillator means which provides a signal driving the transducer at a resonance frequency inducing a predetermined order of resonance on the partially treated microcantilever. Upon insertion into a monitored atmosphere, molecules of the targeted chemical attach to the treated regions of the microcantilever resulting in a change in oscillating mass as well as a change in microcantilever spring constant thereby influencing the resonant frequency of the microcantilever oscillation. Furthermore, the molecular attachment of the target chemical to the treated regions induce areas of mechanical strain in the microcantilever consistent with the treated regions thereby influencing microcantilever bending. The rate at which the treated microcantilever accumulates the target chemical is a function of the target chemical concentration. Consequently, the extent of microcantilever oscillation frequency change and bending is related to the concentration of target chemical within the monitored atmosphere.

FIELD OF THE INVENTION 
The present invention relates to instruments for measuring the vapor 
concentration of a predetermined chemical or compound dispersed within a 
monitored atmosphere. 
BACKGROUND 
A pressing need exists in many industries, disciplines and governmental 
interests for a highly sensitive and selective chemical vapor detector. To 
qualify, such a detector must have such diverse characteristics as being 
small, rugged, inexpensive, selective, reversible and extremely sensitive. 
The prior art is substantially represented by two sensor principles. One is 
the Surface Acoustic Wave (SAW) device and the other is the chemically 
sensitive Field Effect Transistor (Chem FET). Although these devices are 
reasonably inexpensive to produce, the respective sensitivity to the 
nanogram per mm.sup.2 range is less than desired. 
Spectroscopic approaches to this technical objective such as 
surface-enhanced Raman scattering (SERS) offer nanogram to picogram 
sensitivity but inherently require complex optical support and other 
equipment and all the consequential expense. 
Chromatographic methods of vapor concentration measurement also require 
bulky, expensive, fragile hardware and specialized consumables. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to teach the 
construction of a small, selective, inexpensive and highly sensitive vapor 
concentration detector. 
It is another object of the invention to provide a vapor detection sensor 
that is sensitive in the sub-picogram range. 
It is another object of the invention to provide a means for detecting the 
change in resonant frequency of a vibrating spring and interpreting the 
change in frequency as a indication of the presence of a desired vapor 
phase chemical. 
It is another object of the invention to provide a means for detecting the 
bending of a vibrating spring and interpreting the bending as a indication 
of the presence of a desired vapor phase chemical. 
It is another object of the invention to provide a means for detecting both 
the change in resonant frequency and the bending of a vibrating spring and 
interpreting both the change in frequency and the bending as a indication 
of the presence of a desired vapor phase chemical. 
Further and other objects of the present invention will become apparent 
from the description contained herein. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, the foregoing and 
other objects are achieved by the exploitation of physical principles 
primarily comprising (a.) the relationship between resonant frequency and 
spring constant of an oscillating spring; and (b.) the relationship 
between sorption induced mechanical strain of an oscillating spring and 
the resultant bending of said spring. 
An oscillator means provides an excitation signal to drive a piezoelectric 
vibrated microcantilever bar at a resonance frequency with a predetermined 
order of resonance. The piezoelectric vibrated microcantilever bar, which 
is the spring in this system, is treated to create regions having 
substantially exclusive affinity for the targeted compound. In some 
embodiments, the treated regions have an affinity for dissimilar vapor 
phase chemicals and sorptive properties. As the treated microcantilever 
element is vibrated in the monitored atmosphere, vapor phase molecules of 
the target compound attach to the treated regions of the microcantilever 
introducing sorption induced stresses to the microcantilever depending 
upon the relative treatment of the respective regions. The alteration of 
the microcantilever spring constant coupled with the change in 
microcantilever mass caused by sorption results in a related resonance 
frequency change. The mechanical strain established in the respective 
treated regions induces bending in the microcantilever. The change of 
frequency is independently measured by the reflected beam from a laser 
diode focused on and reflected from the microcantilever. The reflected 
laser beam is received by a photodiode detector serving as the signal 
source for a frequency counting circuit. The magnitude of the 
microcantilever frequency change is therefore proportional to the 
concentration of the targeted chemical or compound in the monitored 
atmosphere. 
Additionally, the deflection, or bending, of the microcantilever element 
will vary in relation to sorption induced stresses. As the microcantilever 
element is vibrated in the monitored atmosphere, vapor phase molecules 
attach to the treated regions of the microcantilever thereby altering the 
mass loading and often inducing surface stresses to the microcantilever. 
The sorption induced stresses establish regions of differential mechanical 
strain consistent with the treated regions along the length of the 
microcantilever thereby inducing bending and a change in resonance 
frequency of the microcantilever. The bending magnitude and resonance 
frequency change are measured in a similar manner as the aforementioned 
method for independently measuring the resonance frequency change, however 
a center-crossing photodiode is employed for the detector. The resonance 
frequency change detection method and the bending detection method may be 
coupled to optimize detection of the overall microcantilever sensor 
assembly.

For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above-described drawings. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
While there has been shown and described what is at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications can be made 
therein without departing from the scope of the invention defined by the 
appended claims. 
FREQUENCY CHANGE EMBODIMENT 
To illustrate the basic invention operating principles of the resonance 
frequency change detection embodiment, reference is given to FIG. 1 
wherein element 10 represents a piezoelectric transducer supporting the 
attached end of a treated microcantilever 12 fabricated of quartz or 
silicon, for example. Responsive to a master oscillator 14 drive signal 
16, the microcantilever is driven by the piezoelectric transducer at a 
non-loaded resonance frequency. A laser beam 18 emitted by laser diode 19 
is reflected from the underside of microcantilever 12. The sweep of such 
reflection 20 is detected by an optical detector 22 such as a photodiode. 
As the reflected beam 20 sweeps back and forth across the detector 22, it 
produces a repetitive signal 24 with a frequency proportional to the 
oscillation frequency 16 of the microcantilever 12. Photodiode signal 24 
is amplified 26 and the sweep pulses counted over a predetermined interval 
by a counting circuit 28. The interval count is the substance of signal 30 
issued by counter 28. Simultaneously, the drive signal 16 is monitored by 
a second counting circuit 32 to produce the drive signal frequency count 
34. The values of signals 30 and 34 are compared by a differentiating 
circuit 36 to produce a resultant signal 38. 
The small differences between signals 30 and 34 are proportionally related 
to changes in the oscillating mass and spring constant of microcantilever 
12 due to an accumulation of target chemicals or compounds on the 
microcantilever. Such accumulations are induced by the chemically 
selective treated regions of microcantilever 12. These chemically 
selective treated regions provide sensitivity and selectivity. Selectivity 
will depend on how uniquely a specific vapor or class of vapors interact 
with the treated regions. 
BENDING DETECTION EMBODIMENT 
To illustrate the basic invention operating principles of the bending 
detection embodiment, reference is given to FIG. 2 wherein the mode of 
fabrication, excitation and fabrication of the microcantilever is the same 
as discussed in the frequency change embodiment shown in FIG. 1. A laser 
beam 18 emitted by laser diode 19 is reflected from the underside of 
microcantilever 12. The sweep of such reflection 20 is detected by an 
optical detector 27 such as a displacement detector having a first cell 23 
and a second cell 29, commonly known as a bicell. As the reflected beam 20 
sweeps back and forth across the detector 27, it produces repetitive 
detection signals 24 and 29 each having a frequency proportional to the 
oscillation frequency 16 of the microcantilever 12 and a magnitude 
proportional to the extent of microcantilever 12 bending. Bicell detection 
signals 24 and 25 are amplified by linear amplifiers 31 and 33, 
respectively and subsequently compared by a differentiating circuit 37 to 
produce a resultant signal 39 which is linearly proportional to the 
bending of the microcantilever 12. 
OPTIMIZED FREQUENCY/BENDING EMBODIMENT 
To illustrate the basic invention operating principles of the optimized 
detection embodiment which combines the resonance frequency change and 
bending detection techniques, reference is given to FIG. 3 wherein the 
mode of fabrication, excitation and fabrication of the microcantilever is 
the same as discussed in FIG. 1 and FIG. 2. A laser beam 18 emitted by 
laser diode 19 is reflected from the underside of microcantilever 12. The 
sweep of such reflection 20 is detected by an optical detector 27 such as 
a displacement detector having a first cell 23 and a second cell 29, 
commonly known as a bicell. As the reflected beam 20 sweeps back and forth 
across the detector 27, it produces repetitive detection signals 24 and 25 
each having a frequency proportional to the oscillation frequency 16 of 
the microcantilever 12 and a magnitude proportional to the extent of 
microcantilever 12 bending. Bicell detection signals 24 and 25 are 
amplified by linear amplifiers 31 and 33, respectively and subsequently 
compared by a differentiating circuit 37 to produce a resultant signal 39 
which is linearly proportional to the bending of the microcantilever 12. 
Additionally, detection signal 24 is amplified 26 and the sweep pulses 
counted over a predetermined interval by a counting circuit 28. The 
interval count is the substance of signal 30 issued by counter 28. 
Simultaneously, the drive signal 16 is monitored by a second counting 
circuit 32 to produce the drive signal frequency count 34. The values of 
signals 30 and 34 are compared by a differentiating circuit 36 to produce 
a resultant signal 38. Proportionalities of resultant signal 38 are as 
previously discussed. 
USE OF SURFACE TREATMENTS 
Sensitivity of each embodiment will depend on the total change in mass and 
change in spring constant due to the absorbed vapor, and thus on the 
responsiveness of the treated region as well as the treatment thickness. 
The response time of the system will be dependant upon the treatment 
thickness and the rate of gas diffusion into the treated region. 
Any number of methods may be used to apply these selective treatments to 
the surface of the microcantilever including deposition from solutions 
using applicators such as microsyringes, Q-tips, brushes, and application 
by spin casting, dipping, air-brush spraying, Langmuir-Blodgett (L-B) fill 
transfer, plasma deposition, sputtering, evaporation, sublimation and 
self-assembled monlayers (SAMs). 
Sorption can be reversible or irreversible based on treatment chemistry. A 
representative reversible example is: water absorbed into a gelatin film. 
A representative irreversible example is an amalgam of mercury on gold. If 
rapid response and recovery are desired, films which are only a few 
monolayers thick are preferred. Thicker films may be used to increase 
sensitivity or dynamic range. 
MECHANICS OF ANALYTE/MICROCANTILEVER INTERACTION 
The sorption induced stresses of the microcantilever establish regions of 
increased mechanical strain. The increased mechanical strain alters the 
spring constant of the microcantilever and creates a point of inflection 
experienced during microcantilever vibration at a resonance frequency. The 
harmonic order of resonance established in the microcantilever during use 
is affected by the location of this strain which is affected by the 
location of treated and nontreated regions on the microcantilever. The 
sorption induced stresses of the microcantilever establish regions of 
increased mechanical strain. The increased mechanical strain alters the 
spring constant of the microcantilever and creates a point of inflection 
experienced during microcantilever vibration at a resonance frequency. 
For a microcantilever 12 having a density .rho., an area A, a Young's 
modulus E, and an area moment of Inertia I, the equation of motion for 
vibration perpendicular to the major axis (long axis) is given by: 
##EQU1## 
The frequency of vibration for the microcantilever 12, .omega..sub.n for 
the n.sup.th harmonic, is given by: 
##EQU2## 
The values of K.sub.n l are: 
##EQU3## 
where K is the wave vector and l is the length of the microcantilever. 
The moment of inertia I is given by: 
##EQU4## 
where w is the width and t is the thickness of the beam. The beam can be 
approximated as a spring of a spring constant k: 
##EQU5## 
The resonance frequency of the microcantilevers is given by: 
##EQU6## 
where the effective mass M.sup.* =0.24M, where M is the mass of the 
microcantilever. 
The above relationship illustrates that resonance is inversely proportional 
to the square root of the mass. Consequently, if a mass of material is 
added to the surface, the resonance frequency will change. For a uniformly 
deposited mass change, .DELTA.m, due to adsorption: 
##EQU7## 
where v.sub.1 and v.sub.2 are the resonance frequency before and after 
adsorption. 
Additionally, the bending, of the microcantilever may change due to 
sorption induced differential stress. This stress may be large if the 
sorption on one region of the microcantilever varies with respect to 
another region. The microcantilever experiences bending under this 
differential stress, .DELTA.s: 
EQU .DELTA.s=s.sub.1 -s.sub.2 (8) 
where s.sub.1 and s.sub.2 are the respective sorption induced stress on the 
top and bottom surfaces of the microcantilever. 
The resultant bending, z, due to differential stress may be expressed: 
##EQU8## 
where t is the thickness, l is the length, v is the Poisson's ratio and E 
is the effective Young's modulus of the microcantilever. 
In many cases differential stress alters the spring constant of the 
microcantilever thereby providing another source of change in resonance 
frequency. Hence, the change in microcantilever resonance frequency can be 
due to the combination of changes in mass loading and spring constant. 
Often the resonance frequency of a microcantilever is given as stated in 
equation (6). However, upon introducing sorption induced surface stress, 
the spring constant k is expressed as: 
EQU k=k+.delta.k (10) 
where .delta.k is the surface contribution to the spring constant which may 
be expressed as: 
##EQU9## 
where s.sub.1 and s.sub.2 are the respective adsorption induced stresses 
on the top and bottom surfaces of the microcantilever and n is a 
proportionality constant. 
The effective mass of the microcantilever, M.sup.*, changes to: 
EQU M.sup.* =M.sup.* +0.24.delta.M (12) 
where .delta.M is the sorbed mass of the target chemical on the 
microcantilever treated region. 
Substituting equations (10) and (12) into equation (6) yields resonance 
frequency v.sub.1, representing the resonance frequency of the 
microcantilever due to the sorbed mass of the target chemical: 
##EQU10## 
Since both k and M change due to sorption, the resonance frequency after 
sorption may be written as: 
##EQU11## 
Therefore, as illustrated in FIGS. 12a, 12b, 12c and 12d, at least four 
distinct conditions can be observed due to sorption of molecules on a 
microcantilever where reference is given to equation 14 for the following 
discussion on the aforementioned conditions. In the first condition as 
shown in FIG. 12a, sorption induced alterations in spring constant are 
negligible and change in resonance frequency is due entirely to mass 
loading. In the second condition as shown in FIG. 12b, changes in both 
spring constant and mass loading are negligible. However, bending due to 
differential stresses established in the partially treated microcantilever 
is significant. Although the microcantilever deflects in this condition, 
there is little change in the observed resonance frequency. In the third 
condition as shown in FIG. 12c, sorption induced change in mass loading is 
negligible but the consequent change in spring constant significantly 
alters the resonance frequency. The fourth condition as shown in FIG. 12d 
combines the sorption induced changes in mass loading and spring constant 
in altering the resonance frequency of the microcantilever. 
FIG. 8 shows the displacement of various points along the length of a 
microcantilever having a length of 200 .mu.m for the first three orders of 
resonance, i.e., fundamental, second and third harmonics. For the first 
order resonance, maximum strain takes place at the base of the 
microcantilever where it increases rapidly from zero to a finite value. 
For second and third order resonances, changes in strain take place at 
different locations as indicated by the inflection points in the curves. 
As discussed above, sorption induced changes in stress can result in 
change in spring constant. If treated regions are placed on the 
microcantilever in regions of high strain, then sensitivity to sorption 
induced stress can be maximized. For example, sorption at the base of the 
microcantilever will exert a strong influence on first order resonance. 
FIGS. 9, 10 and 11 show three microcantilevers with adsorption sensitive 
areas specifically designed for sensitivity to the first three orders of 
resonance, respectively. FIG. 9 shows the preferred embodiment for a 
microcantilever optimized for response at the fundamental. FIG. 10 shows 
the preferred embodiment for a microcantilever optimized for response at 
the second harmonic. FIG. 11 shows the preferred embodiment for a 
microcantilever optimized for response at the third harmonic. 
Additionally, dissimilar compositions, each composition having an affinity 
for a different vapor phase molecule, may be implemented to treat the 
microcantilever. The dissimilar treatments may be positioned on the 
microcantilever at different points of inflection along the length of the 
microcantilever. The corresponding response at each treated area along the 
length of the microcantilever may then be monitored to identify the 
presence of several different vapor phase molecules with a single 
microcantilever. 
EXAMPLE 
FIG. 7a shows the resonance frequency response for a silicon 
microcantilever treated with a gelatinous film. As the ambient humidity is 
increased, the effective spring constant increases thusly yielding a 
proportionate increase in resonance frequency. FIG. 7b shows the bending 
of the same microcantilever represented as error voltage. The variation in 
surface stress as a function of humidity produces changes in 
microcantilever bending proportionate to humidity. 
MULTIPLEXED MICROCANTILEVERS 
The invention embodiment illustrated in FIGS. 4, 5 and 6 is representative 
of a 16 sensor array of treated microcantilevers having treated regions 
70, each microcantilever having different sorption characteristics with 
respect to the treated regions. Within an open ended cylinder 42 of 0.5 
in. dia. by 0.5 in. height supported by a flanged base 44, a transverse 
partition 46 separates an outer, sensor volume 48 from a sealed, interior 
circuit volume 50. A single piezoelectric transducer 52 drives 16 
microcantilevers 54. However, each microcantilever surface is 
distinctively treated as described herein. 
Beneath the oscillating end of each microcantilever 54 is a laser diode 56 
and optical detector 58 for respectively emitting and receiving laser 
beams 60 reflected from respective microcantilevers 54. Integrated 
microprocessor circuitry within the circuit volume 50 receives the raw 
optical detector 58 signals for development of respective, frequency and 
bending differential signals at leads, 62. 
These several, distinctive signal leads 62 are connected with data 
processor terminals for preprogrammed analysis. Response patterns from 
several sensors are characteristic of the chemical or chemical combination 
present in the monitored vapor. Pattern recognition methods may be used 
for response to patterns that correspond to conditions of interest. 
Another significant advantage of an array system is that it can easily 
identify a number of vapor conditions that is far in excess of the number 
of sensors in the array. Furthermore as new conditions arise, it is 
feasible to make the instrument responsive to them by changing only the 
pattern recognition software. 
Another means for multiplexing the microcantilevers is to provide a 
microcantilever having several treated regions with each treated region 
having an affinity for a different vapor phase chemical. As sorption 
induced stresses and mass loading characteristics, consistant with the 
presence of a target chemical or group of chemicals, impact the resonance 
frequency of the microcantilever, the change in resonance frequency is 
independently detected at each of the treated regions along the 
microcantilever and respective detection signals are emitted. Each of the 
respective detection signals are compared with the drive oscillation 
signal to generate a differential signal proportional to the frequency 
difference between the drive oscillation signal and each of the respective 
detection signals. The differential signal is then related to a known 
spring constant corresponding to a known vapor phase chemical on each of 
the respective treated regions along the spring element. 
ADDITIONAL EMBODIMENTS 
From the foregoing disclosure, it will be appreciated that a 
microcantilever plate may be utilized in lieu of the microcantilevered 
bars 12 or 54 thereby raising the surface-to-volume ratio for greater 
sensitivity. 
Additionally, sensor frequency and bending may be measured by means other 
than the photodetection method previously described. By one such other 
method, a silicon or GaAs microcantilever is fabricated with 
piezoresistive properties. The electrical resistance of the 
microcantilever changes under beam flexure. The resonant frequency and 
bending may be monitored as a microcantilever resistance signal. 
Another sensor monitoring method relies upon capacitance synchronization 
using a parallel matched structure located a short distance from the 
moving structure. 
Another sensor monitoring method relies upon electron tunneling between the 
cantilever and a fixed surface located a short distance from the moving 
structure. 
Any of these alternative frequency and bending measuring methods would make 
the instrument more compact, durable, less expensive to manufacture and 
eliminate the need for separate optoelectric devices. 
The above described sensor can be further modified to operate under liquid 
either by vibrating the microcantilever directly or by setting the 
microcantilever into oscillation by mechanically moving the liquid 
surrounding the microcantilever and observing the changes in frequency and 
bending corresponding to maximum amplitude of either characteristic. 
While there has been shown and described what is at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications can be made 
therein without departing from the scope of the invention defined by the 
appended claims.