Method for manufacturing a miniaturized solid state mass spectrograph

A method for forming a solid state mass spectrograph for analyzing a sample gas is provided in which a plurality of cavities are formed in a substrate, preferably, a semiconductor. Each of these cavities forms a chamber into which a different component of the mass spectrograph is provided. A plurality of orifices are formed between each of the cavities, forming an interconnecting passageway between each of the chambers. A dielectric layer is provided inside the cavities to serve as a separator between the substrate and electrodes to be later deposited in the cavity. An ionizer is provided in one of the cavities and an ion detector is provided in another of the cavities. The formed substrate is provided in a circuit board which contains interfacing and controlling electronics for the mass spectrograph. Preferably, the substrate is formed in two halves and the chambers are formed in a corresponding arrangement in each of the substrate halves. The substrate halves are then bonded together after the components are provided therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a gas-detection sensor and more particularly to a 
solid state mass spectrograph which is micro-machined on a semiconductor 
substrate, and, even more particularly, to a method for manufacturing such 
a solid state mass spectrograph. 
2. Description of the Prior Art 
Various devices are currently available for determining the quantity and 
type of molecules present in a gas sample. One such device is the 
mass-spectrometer. 
Mass-spectrometers determine the quantity and type of molecules present in 
a gas sample by measuring their masses and intensity of ion signals. This 
is accomplished by ionizing a small sample and then using electric and/or 
magnetic fields to find a charge-to-mass ratio of the ion. Current 
mass-spectrometers are bulky, bench-top sized instruments. These 
mass-spectrometers are heavy (100 pounds) and expensive. Their big 
advantage is that they can be used for any species. 
Another device used to determine the quantity and type of molecules present 
in a gas sample is a chemical sensor. These can be purchased for a low 
cost, but these sensors must be calibrated to work in a specific 
environment and are sensitive to a limited number of chemicals. Therefore, 
multiple sensors are needed in complex environments. 
A need exists for a low-cost gas detection sensor that will work in any 
environment. U.S. patent application Ser. No. 08/124,873, filed Sep. 22, 
1993, hereby incorporated by reference, discloses a solid state 
mass-spectrograph which can be implemented on a semiconductor substrate. 
FIG. 1 illustrates a functional diagram of such a mass-spectrograph 1. 
This mass-spectrograph 1 is capable of simultaneously detecting a 
plurality of constituents in a sample gas. This sample gas enters the 
spectrograph 1 through dust filter 3 which keeps particulate from clogging 
the gas sampling path. This sample gas then moves through a sample orifice 
5 to a gas ionizer 7 where it is ionized by electron bombardment, 
energetic particles from nuclear decays, or in a radio frequency induced 
plasma. Ion optics 9 accelerate and focus the ions through a mass filter 
11. The mass filter 11 applies a strong electromagnetic field to the ion 
beam. Mass filters which utilize primarily magnetic fields appear to be 
best suited for the miniature mass-spectrograph since the required 
magnetic field of about 1 Tesla (10,000 gauss) is easily achieved in a 
compact, permanent magnet design. Ions of the sample gas that are 
accelerated to the same energy will describe circular paths when exposed 
in the mass-filter 11 to a homogenous magnetic field perpendicular to the 
ion's direction of travel. The radius of the arc of the path is dependent 
upon the ion's mass-to-charge ratio. The mass-filter 11 is preferably a 
Wien filter in which crossed electrostatic and magnetic fields produce a 
constant velocity-filtered ion beam 13 in which the ions are disbursed 
according to their mass/charge ratio in a dispersion plane which is in the 
plane of FIG. 1. 
A vacuum pump 15 creates a vacuum in the mass-filter 11 to provide a 
collision-free environment for the ions. This vacuum is needed in order to 
prevent error in the ion's trajectories due to these collisions. 
The mass-filtered ion beam is collected in a ion detector 17. Preferably, 
the ion detector 17 is a linear array of detector elements which makes 
possible the simultaneous detection of a plurality of the constituents of 
the sample gas. A microprocessor 19 analyses the detector output to 
determine the chemical makeup of the sampled gas using well-known 
algorithms which relate the velocity of the ions and their mass. The 
results of the analysis generated by the microprocessor 19 are provided to 
an output device 21 which can comprise an alarm, a local display, a 
transmitter and/or data storage. The display can take the form shown at 21 
in FIG. 1 in which the constituents of the sample gas are identified by 
the lines measured in atomic mass units (AMU). 
Preferably, mass-spectrograph 1 is implemented in a semiconductor chip 23 
as illustrated in FIG. 2. In the preferred spectrograph 1, chip 23 is 
about 20 mm long, 10 mm wide and 0.8 mm thick. Chip 23 comprises a 
substrate of semiconductor material formed in two halves 25a and 25b which 
are joined along longitudinally extending parting surfaces 27a and 27b. 
The two substrate halves 25a and 25b form at their parting surfaces 27a 
and 27b an elongated cavity 29. This cavity 29 has an inlet section 31, a 
gas ionizing section 33, a mass filter section 35, and a detector section 
37. A number of partitions 39 formed in the substrate extend across the 
cavity 29 forming chambers 41. These chambers 41 are interconnected by 
aligned apertures 43 in the partitions 39 in the half 25a which define the 
path of the gas through the cavity 29. Vacuum pump 15 is connected to each 
of the chambers 41 through lateral passages 45 formed in the confronting 
surfaces 27a and 27b. This arrangement provides differential pumping of 
the chambers 41 and makes it possible to achieve the pressures required in 
the mass filter and detector sections with a miniature vacuum pump. 
The inlet section 31 of the cavity 29 is provided with a dust filter 47 
which can be made of porous silicon or sintered metal. The inlet section 
31 includes several of the apertured partitions 39 and, therefore, several 
chambers 41. 
The miniaturization of mass spectrograph 1 creates various difficulties in 
the manufacture of such a device. Accordingly, there is a need for a 
method for making a miniaturized mass spectrograph. 
SUMMARY OF THE INVENTION 
A method for forming a solid state mass spectrograph for analyzing a sample 
gas is provided in which a plurality of cavities are formed in a 
substrate. Each of these cavities forms a chamber into which a different 
component of the mass spectrograph is provided. A plurality of orifices 
are formed between each of the cavities, forming an interconnecting 
passageway between each of the chambers. A dielectric layer is provided 
inside the cavities to serve as a separator between the substrate and 
electrodes to be later deposited in the cavity. An ionizer is provided in 
one of the cavities and an ion detector is provided in another of the 
cavities. The formed substrate is provided in or connected to a circuit 
board which contains interfacing and controlling electronics for the mass 
spectrograph. Preferably, the substrate is formed in two halves and the 
chambers are formed in a corresponding arrangement in each of the 
substrate halves. The substrate halves are then bonded together after the 
components are provided therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The key components of mass spectrograph 1 have been successfully 
miniaturized and fabricated in silicon through the combination of 
microelectronic device technology and micromachining. The dramatic size 
and weight reductions which result from this development enable a hand 
held chemical sensor to be fabricated with the full functionality of a 
laboratory mass spectrometer. 
The preferred manufacturing method utilizes bi-lithic integration wherein 
the components of mass spectrograph 1 are fabricated on two separate 
silicon wafers, shown in FIG. 2 at 25a and 25b, which are bonded together 
to form the complete device. Alternative techniques for incorporating the 
key silicon microelectronic components into structures fabricated using 
modern electronic packaging techniques and materials, e.g. LTCC, FOTOFORM 
glass, and LIGA, can also be used. 
The essential semiconductor components of mass spectrograph 1 are the 
electron emitter 49 for the ionizer 7 and the ion detector array 17. The 
other components utilize thin film insulators and conductor electrode 
patterns which can be formed on other materials as well as silicon. 
FIGS. 3a and 3b show the electron emitter 49 having a shallow p-n junction 
51 formed by an n++ shallow implant 53 provided on a p+ substrate 55. An 
n+ diffusion region 57 is provided in substrate 55. An opening 59 provided 
in said diffusion region 57 into which an optional implant formed of p+ 
boron and a n++ implant of, for example, antimony are placed. Electron 
emitter 49 emits electrons from its surface during breakdown in reverse 
bias. The emitted electrons are accelerated away from the silicon surface 
by a suitably biased gate 63, mounted on gate insulator 65, and a 
collector electrode provided on the top half of the ionizer chamber. 
FIG. 4 shows the detector array 17 having MOS capacitors 67 which are read 
by a MOS switch array 69 or a charge coupled device 69. The detector array 
17 is connected to an array of Faraday cups formed from a pair of Faraday 
cup electrodes 71 which collect the ion charge 73. 
The interior of the miniature mass spectrograph 1 showing the bi-lithic 
fabrication is shown in FIG. 2. Here the three dimensional geometry of the 
various parts of the mass spectrograph 1 are shown together with the 
location of the ionizer 7 and detector array 17. Preferably, the mass 
spectrograph 1 is fabricated from silicon. Alternatively, a hybrid 
approach in which the ionizer 7 and detector array 17 are mounted into a 
structure which is fabricated from another material containing the other 
non-electronic components of the device can be used. 
As shown in FIG. 5a, the top 25a and bottom 25b parts of the bi-lithic 
structure 75 are bonded together and mounted with a circuit board 77 
containing the control and interface electronics. This board 77 is then 
inserted into the permanent bias magnet 79 as shown in FIG. 5b. The 
electronics circuits can also be monolithically integrated with the 
silicon mass spectrograph structure or can be connected in a hybrid manner 
with either a hybrid mass-spectrograph or all silicon mass-spectrograph 
structure. 
A cross-section of the all-silicon mass spectrograph 1 is shown in FIG. 6. 
The top 25a and bottom 25b silicon pieces are preferably bonded by indium 
bumps and/or epoxy, which is not shown. The first step in the fabrication 
of the all-silicon mass spectrograph 1 is the etching of alignment marks 
in the silicon substrate 25. This assures proper alignment of the etched 
geometries with the cubic structure of the silicon substrate 25. Once the 
alignment marks are etched, 40 .mu.m deep chambers 41 are etched in each 
half 25a and 25b of the silicon substrate 25. These chambers are etched 
using an anisotropic etchant such as a potassium hydroxide etching agent 
or ethylene diamine pyrocatechol (EDP). After the chambers are formed, the 
orifices between the chambers are formed by etching 10 .mu.m deep 
features. These orifices are also etched using the anisotropic etching 
agent. 
Once all the major etching is completed, an oxide growth and subsequent 
etching is performed to round out any sharp edges to assist in the 
metallization process. Another oxide growth forms dielectric 81 which 
separates the substrate halves 25a and 25b from the electrodes 83. An n+ 
diffusion layer 57 as described above and shown in FIGS. 3a and 3b is 
diffused in the substrate 25 to define the ionizer 7. The ionizer gate 
dielectric is then formed by depositing a layer of dielectric, such as 
nitride or oxide. An antimony implant is then provided to define the 
ionizer emitting junction. The optional boron p+ layer 61 can be implanted 
to better define the shallow p-n junction 51. 
Once the ionizer is formed, the ionizer and interconnect can be metallized 
by depositing a 500 Angstrom layer of chromium followed by depositing a 
5000 Angstrom layer of gold. Ionizer passivation is accomplished by 
depositing a 100 Angstrom layer of gold or other suitable material. 
A 5 .mu.m layer of indium can be evaporated on substrate halves 25a and 25b 
to form the indium bumps. The substrate halves 25a and 25b can then be 
bonded and encapsulated in a hermetic seal 85. 
The processes utilized are found in any microelectronic fabrication 
facility, except for the spray resist application necessary to uniformly 
coat the non planar geometry, and the photolithographic techniques used to 
define electron emitter and electrode structures at the bottom of 40 .mu.m 
chambers. 
The structures shown in FIG. 2, except for the ionizer 7 and ion detector 
17, can be fabricated by a variety of other means with the ionizer 7 and 
ion detector 17 inserted in a hybrid manner. Available techniques for this 
fabrication include mechanical approaches which form metallic or ceramic 
structures. The minimum feature sizes for mechanically formed geometries 
is around 25 .mu.m (0.001") which is only a factor of two larger than the 
10 .mu.m width of the ion optics aperture used in the all-silicon device. 
Thus it is feasible to fabricate a hybrid mass-spectrograph which is 
perhaps a few times larger than the all-silicon spectrograph 1, but is 
still many times smaller than a conventional laboratory mass spectrograph. 
Spark erosion or EDM techniques can be utilized to achieve the 25 .mu.m 
feature sizes at reasonable cost in metals. Dielectric insulating layers 
are required to isolate the electrodes in the ionizer, mass filter and 
Faraday cup areas from the metal. 
Fabrication of the mass spectrograph structure from dielectrics such as 
plastic or glass is attractive since a number of insulating layers can be 
eliminated. Because silicon is a low resistivity semiconductor, several 
dielectric layers are used in the all-silicon mass spectrograph to prevent 
grounding of the electrodes. LIGA can be used to form a mold for a plastic 
to serve as the dielectric with the required mechanical and vacuum 
properties. Alternatively, a UV sensitive glass such as FOTOFORM brand 
glass manufactured by Corning, Inc can also be used as the dielectric. 
LIGA and quasi-LIGA processes have been developed to produce very high 
aspect ratio (&gt;100:1) structures of micrometers width in photoresist or 
other plastic materials such as Plexiglas by photolithographic techniques 
using synchrotron radiation or short wave length UV. This is presently an 
expensive process, but once the precise mold is made many structures can 
be fabricated at low cost. Electrode and interconnect metallization can be 
defined by photolithography as in the all-silicon case. 
UV sensitive glasses are shaped using photolithographic techniques and can 
achieve feature sizes down to 25 .mu.m with masking, UV exposure, and 
etching techniques similar to those used in semiconductor processing. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting as to the scope of the invention which is to be given the 
full breadth of the appended claims in any and all equivalents thereof.