Hermetically sealed radiation imager

A hermetically packaged radiation imager includes a moisture barrier cover disposed over the imaging array and a hermetic seal structure disposed around the periphery of the moisture barder cover to seal the cover to the underlying substrate. The hermetic seal structure comprises a solder seal disposed in contact with the moisture barrier cover and a dielectric material layer disposed between the solder seal and conductive lines extending from the imager army across the substrate surface. The hermetic seal structure further includes a primer layer that is disposed between the dielectric material layer and the solder seal to provide a foundation to which the solder seal adheres. The dielectric material layer is deposited in an atomic layer epitaxy technique, thus providing a thin layer having high structural integrity.

BACKGROUND OF THE INVENTION 
This invention relates generally to radiation imagers and in particular to 
solid state imaging array that are hermetically-sealed from the ambient 
environment. 
Solid state radiation imaging systems are widely used for medical and 
industrial purposes. Commonly in such systems the radiation desired to be 
detected is absorbed in a scintillator material and the light photons 
generated by that absorption are then detected by an imaging array, such 
as a photodiode array. Both the scintillator material and the 
silicon-based semiconductive devices (such as photodiodes and thin film 
transistors (TFTs) used for addressing the photodiodes) in the imaging 
array are subject to degradation due to exposure to moisture, such as 
humidity in the ambient air. For example, cesium iodide, a common 
scintillator material, is hygroscopic, that is, it absorbs moisture from 
the atmosphere, which results in degradation of its luminescent 
properties; photodiodes and TFTs exposed to moisture suffer increased 
electrical leakage in various states of operation, which in turn results 
in higher noise in the array. 
Imaging arrays are typically disposed on a glass substrate and have 
conductive lines extending to the edge of the substrate to allow 
connection to external electronic circuits. Currently the imaging array is 
packaged by placing a moisture barrier cover, such as an aluminum plate, 
over the array, and coupling the aluminum plate to the glass substrate 
with an epoxy seal that runs along the perimeter of the plate. Although 
the epoxy material provides good structural strength, it is an 
insufficient barrier to the passage of moisture into the imaging array 
package, especially in environments having high ambient humidity. 
It is thus an object of this invention to provide a radiation imager that 
is effectively hermetically sealed from the ambient environment. 
SUMMARY OF THE INVENTION 
A hermetically packaged radiation imager in accordance with this invention 
includes a substrate having an imager array disposed thereon, with 
conductive lines extending across an array boundary portion of the surface 
of the substrate, a moisture barrier cover disposed over the imaging 
array, and a hermetic seal structure disposed on the boundary portion of 
the substrate surface so as to hermetically couple the moisture barrier 
cover to the substrate. The hermetic seal structure comprises a solder 
seal disposed in contact with the moisture barrier cover and a dielectric 
material layer disposed between the solder seal and the conductive lines 
extending across boundary portion of the substrate surface. 
The dielectric material layer typically comprises a high integrity 
inorganic dielectric material such as titanium oxide or silicon nitride. 
The dielectric material layer is relatively thin, with a thickness less 
than 2 .mu.m and typically between about 0.1 microns and 0.2 microns, and 
is deposited to have a substantially uniform thickness over the conductive 
lines and has high structural integrity, that is, it is substantially 
pin-hole free. 
The hermetic seal structure typically further includes a primer layer that 
is disposed between the dielectric material layer and the solder seal. The 
primer layer typically comprises a metallic material having a thickness in 
the range between 0.01 .mu.m and 0.5 .mu.m and that is disposed to provide 
a foundation to which the solder seal adheres. 
The dielectric material layer is deposited in an atomic layer epitaxy 
technique, thus providing a thin layer having high structural integrity. 
The solder material that comprises the solder seal is applied in a liquid 
state so as to flow and wet the surface of the moisture barrier cover and 
the primer layer, thus providing an effective (that is, high integrity) 
hermetic seal.

DETAILED DESCRIPTION OF THE INVENTION 
A radiation imager 100 comprises an imaging array 105 disposed on a 
substrate 110. Substrate 110 typically comprises glass or similar 
non-conductive material that provides the necessary structural strength 
for the array. Imaging array 105 typically comprises a photosensor array 
coupled to a scintillator; the photosensor array commonly comprises 
photodiodes and associated address lines and switching devices to enable 
each photodiode in the array to be coupled to readout electronics (not 
shown). A plurality of conductive lines 120 (a representative one of which 
is illustrated in the FIGURE) extend from imaging array 105 across a 
surface 112 of substrate 110 towards the edge of substrate 110 at which 
contact to external readout circuits can be made. 
Imaging array 105 is enclosed within a hermetically sealed chamber 130 that 
is bounded by substrate surface 112, a moisture barrier cover 140, and a 
hermetic seal structure 150. Moisture barrier cover 140 typically 
comprises a material that prevents the passage of moisture therethrough 
and that presents a low absorption cross section for the type of radiation 
which imager 100 is designed to detect. For example, for an x-ray imager, 
moisture barrier cover 140 is a plate that typically comprises aluminum 
and has a thickness in the range between about 5 mils (0.005 in) and 50 
mils (0.05 in). Moisture barrier 140 is disposed over imaging array 105 
and commonly is held in place by a bonding layer 145 that is disposed 
between barrier cover 140 and substrate 110; bonding layer 145 typically 
comprises epoxy or the like having a thickness (between moisture barrier 
cover 140 and substrate 110) in the range between about 10 mils (0.01 in) 
and 100 mils (0.1 in). 
In accordance with this invention, hermetic seal structure 150 comprises a 
dielectric material layer 160, a primer layer 170, and a solder seal 180. 
Dielectric material layer 160 is disposed over substrate surface and 
conductive lines 120 in a boundary portion 115 of the substrate surface, 
that is the area of the surface at which the seal is made between 
substrate 110 and moisture barrier cover 140 to form hermetically sealed 
chamber 130 that contains imaging array 105. Dielectric material layer is 
formed in the fabrication process after completion of imaging array 105 
and after bonding layer 145 is deposited and moisture barrier cover 140 is 
positioned over imaging array 105 and in contact with bonding layer 145. 
Inorganic material layer 160 is formed in an atomic layer epitaxy 
technique (also called atomic layer growth techniques), a technique that 
provides precise control of film thicknesses, a high degree of thickness 
uniformity, low deposition temperature (less than about 200.degree. C.), 
and coverage of areas that are not in the line-of-sight of the evaporation 
sources. 
In atomic layer epitaxy techniques, a single atomic or molecular layer of a 
precursor material is deposited at a time, and thus a single layer of a 
material or compound can be formed on a surface, including non-planar 
surfaces (such as substrate surface 112 with conducting lines 120 
thereon), while maintaining precise thickness control. In accordance with 
this invention, dielectric material layer 160 typically comprises an 
inorganic dielectric such as silicon oxide, titanium oxide, silicon 
nitride, or the like. In synthesizing a compound film, the surface to be 
coated is placed in an evacuated chamber connected to a supply of starting 
material for the film to be formed. The growth technique consists of 
alternatively injecting a precursor of each component of the film (either 
an element or a compound) under conditions that promote the deposition of 
a monolayer of the desired compound. For example, the precursor materials 
for forming a layer of titanium oxide consist of titanium tetrachloride 
and water vapor, which are alternately injected into the chamber 
containing imager 100, which is masked so that the deposition occurs only 
along boundary portion 115 of substrate 110. Excess raw material beyond 
that of a monolayer is removed after each delivery by evacuation of the 
chamber or sweeping with an inert gas such as nitrogen. 
Through the use of atomic layer epitaxy techniques, dielectric layer 160 is 
fabricated such that it has a relatively thin thickness, that is a 
thickness less than about 2 .mu.m, and that is typically in the range 
between 0.1 .mu.m and 0.2 .mu.m. The thickness of dielectric layer 160 is 
substantially uniform (that is, the thickness of the layer does not vary 
more than about 5% from its nominal thickness, and in particular the 
thickness of the dielectric layer is substantially uniform over conductive 
lines 120). Further, the atomic layer epitaxy techniques result in the 
deposition of a high integrity inorganic dielectric layer, that is, a 
layer that is structurally robust and substantially free of pinholes such 
that it provides an effective electric insulator between solder seal 180 
and underlying conductive lines 120. For example, in a solid state x-ray 
imager in accordance with this invention, dielectric layer 160 has a 
specific resistivity not less than about 10.sup.6 .OMEGA.-cm. Dielectric 
layer 160 is shaped to be disposed between solder seal 180 and conductive 
lines 120 and thus provide electrical insulation. As shown in the FIGURE, 
dielectric layer 160 typically extends across horizontal part of boundary 
region 115 of substrate 110; even though, as shown, dielectric layer 160 
does not extend over epoxy material of bonding layer 145, a hermetic seal 
is provided between epoxy material and solder seal 180; alternatively, 
dielectric layer 160 is disposed so that it further extends up the 
sidewall of bonding layer 145 to moisture barrier cover 140. 
Next in the fabrication of hermetic seal structure 150, primer layer 170 is 
deposited so that it conforms to the shape of dielectric material layer 
160 so as to provide a foundation to which the solder seal 180 can adhere 
(that is, primer layer is disposed between solder seal 180 and dielectric 
layer 160 in the finished device). Primer layer 170 thus comprises an 
adhesion promoter, such as a metallic material comprising two metals, for 
example titanium and gold. Primer layer 170 is deposited in an RF sputter, 
or alternatively, an evaporative process to a thickness in the range 
between about 500 .ANG. and 5000 .ANG.. 
Solder seal 180 is disposed over primer layer 170 and extends around the 
periphery of moisture barrier cover 140 so as to provide a seal that is 
impervious to moisture in the ambient air surrounding imager 100. Solder 
seal 180 is formed from solder material that has a low melting point (less 
than about 200.degree. C.), that is a solid at the anticipated operating 
temperature ranges of imager 100, and that forms a moisture impervious 
bond with moisture barrier cover 140 and primer layer 170. By way of 
example and not limitation, effective solder materials for solid state 
imager 100 as described above include tin, lead, indium, silver, bismuth, 
cadmium, or alloys or combinations of such materials. The solder material 
used is heated an deposited on imager 100 in a liquid state (typically one 
that is quite viscous to provide control of the spread of the solder 
material) so that the solder material wets the surface of moisture barrier 
cover 140 and the underlying primer layer 170. The solder material thus is 
deposited so that it conforms and bonds to the underlying surfaces and 
thus reduces gaps, voids, or interstices in coverage when the solder 
material hardens, thereby providing an effective moisture seal. For 
example, indium solder melts at about 100.degree. C. and can be applied to 
flow around the boundary region 115 so that it is in contact with both 
moisture barrier cover 140 and primer layer 170; when cooled, the indium 
solder material hardens into solder seal 180. Dielectric layer 160 is 
disposed between solder seal 180 and conductive lines 120 such that no 
electrical shorting occurs between solder seal 180 and the conductive 
lines. The solder material is typically deposited so that solder material 
does not spill over onto the top of cover 140. 
In operation, imager 100 can be placed in a variety of operating 
environments (such as might occur with a portable imager used for medical 
or industrial applications), including environments with high humidity, 
and not suffer degradation in performance due to high ambient moisture. 
Hermetic seal structure 150 provides an effective moisture-impervious seal 
between the substrate and the moisture barrier cover without otherwise 
affecting the operation of the imager. 
While only certain features of the invention have been illustrated and 
described herein, many modifications and changes will occur to those 
skilled in the art. It is, therefore, to be understood that the appended 
claims are intended to cover all such modifications and changes as fall 
within the true spirit of the invention.