Method of making a pyroelectric film sensing device

A substrate free single crystal pyroelectric film particularly suited for use in rapid thermal response sensors is made from a single crystal substrate by a method including the steps of: PA1 (A) etching a pattern into the substrate; PA1 (B) epitaxially growing a highly oriented superconducting material into the etched pattern to fill the etched pattern, PA1 (C) epitaxially growing a highly oriented crystalline film of a pyroelectric material over the entire surface of the substrate, and PA1 (D) dissolving away the highly oriented superconducting material.

FIELD OF INVENTION 
This invention relates in general to sensing devices for use in rapid 
thermal sensors including a highly oriented pyroelectric film and to their 
method of manufacture and in particular, to a sensing device for use in 
rapid thermal sensors including a substrate free highly oriented 
pyroelectric film obtained from a single crystal substrate and to its 
method of manufacture. 
BACKGROUND OF THE INVENTION 
Sensing devices for use in rapid thermal response sensors including 
pyroelectric thick and thin layers have been subject to the influence of 
the substrate material on which they were deposited. Their ability to 
respond to thermal changes has depended on the thermal conductivity of the 
substrate. If the substrate were free standing, the heat loss to the 
surrounding ambient would be at a minimum and the optical response would 
be greatly increased. Films that have been attached to substrates have 
also been restricted in their ability to respond to mechanical 
deformation. In this sense, a free standing film would act as a deformable 
membrane and could be used for sensor applications. A free standing film 
that is to be used as an infrared sensor would not need any cooling to 
remove heat that accumulates upon absorption of radiation. 
Substrate free or free standing films of pyroelectric materials have been 
made heretofore for use as infrared sensors. These films, however, were 
grown on glass substrates and as a result were polycrystalline. 
SUMMARY OF THE INVENTION 
The general object of this invention is to improve performance in devices 
such as infrared sensors, capacitors, laser detectors, thermal imaging, 
intruder alarms, fire alarms, pollution monitoring and gas analysis, 
radiometers and vidicon arrays. A more particular object of the invention 
is to provide a method whereby a substrate free highly oriented 
pyroelectric film can be obtained from a single crystal substrate. Another 
object of the invention is to provide a free standing pyroelectric film 
that is to be used as an infrared sensor and that will not need any 
cooling to remove heat that accumulates upon absorption of radiation. 
It has now been found that the aforementioned objects can be attained and a 
substrate free highly oriented pyroelectric film suitable for use as a 
sensing device in rapid thermal response sensors obtained from a single 
crystal substrate by a method including the steps of: 
(A) etching a pattern into the single crystal substrate, 
(B) epitaxially growing a highly oriented superconducting material into the 
etched pattern to fill the etched pattern, 
(C) epitaxially growing a highly oriented film of a pyroelectric material 
over the entire surface of the substrate, and 
(D) dissolving away the highly oriented superconducting material. 
In step (A), the single crystal substrate is preferably a material such as 
Si, MgO, or InSb. The substrate layer is etch patterned to give 
depressions that are in the order of 1 micron in depth and up to several 
microns in width. 
In step (B), the Si, MgO or InSb single crystal substrate is used to 
epitaxially grow a highly oriented superconducting material such as 123 
superconductor or Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 into the depression of 
step (A) to fill those depressions. The method of growth can be laser 
ablation, metal oxide chemical vapor deposition, molecular beam epitaxy, 
sol-gel spin etc. 
In step (C), a layer of pyroelectric material such as PbTiO.sub.3, 
SrTiO.sub.3, BaTiO.sub.3, Pb(TiZr)O.sub.3 or LaLiTi.sub.2 O.sub.6 is 
epitaxially grown over the entire surface of the substrate. 
In step (D), the layer of highly oriented superconducting material is 
removed by dissolving with dilute nitric acid. The result is a highly 
oriented pyroelectric film that is not connected to the substrate thus 
forming a bridge structure. The thermal and mechanical response of the 
pyroelectric is not subject to the influence of the substrate. The film in 
this form creates a deformable membrane. 
Electrical contacts can be deposited on the substrate before the growth of 
highly oriented superconducting material. The contact material can be 
polycrystalline silicon, Pt, Ti, SiO.sub.2, SiN etc. A particularly good 
contact layer is the compound RuO.sub.2 since it grows epitaxially with 
the substrate and the pyroelectric. The layer of highly oriented 
superconducting material is removed by dissolving it in dilute HNO.sub.3, 
leaving a pyroelectric film that is free standing and yet it is possible 
to make electrical measurements between the top and bottom. The contact 
layer should not be soluble in the HNO.sub.3 or any other solvent used to 
dissolve the layer of highly oriented superconducting material. 
A stack of alternating pyroelectric layers and air spaces can be made if 
the contact layer can be epitaxially grown on the pyroelectric layer. 
Although MgO is not a conductor, it can be used to epitaxially grow spacer 
layers between the pyroelectric layers. Devices such as these can be used 
to generate large surface area structures for gas analysis and absorption 
and collimation of radiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 to FIG. 5, a single crystal substrate, 10 is etch 
patterned in its top surface to give depressions, 12. The depressions, 12, 
are then filled with an epitaxially grown superconducting layer, 14 and a 
pyroelectric layer, 16 then grown over the entire top surface of the 
substrate, 10. The superconducting layer, 14 is then removed by 
dissolution in dilute nitric acid leaving a pyroelectric layer, 16 that is 
freestanding in the sense that it is only connected with portions,of the 
substrate, 10 and an air space, 18. If desired, electrical contact, 20 can 
be deposited on the substrate, 12 before the growth of the superconducting 
layer, 14. A stack of alternating pyroelectric layers 16, 16', and 16" and 
air spaces 18, 18', and 18" can be made if the contact layer, 20 can be 
epitaxially grown on the pyroelectric layer, 16. Although MgO is not a 
conductor, it can be used to epitaxially grow spacer layers between the 
pyroelectric layers, 16, 16', and 16" and air spaces 18, 18', and 18" can 
be made if the contact layer, 20 can be epitaxially grown on the 
pyroelectric layer, 16. Devices such as these can be used to generate 
large surfaces area structures for gas analysis and absorption and 
collimation of radiation. 
The method outlined in the foregoing paragraph is carried out to obtain a 
free standing highly oriented pyroelectric layer using MgO as the single 
crystal substrate, YBa.sub.2 Cu.sub.3 O.sub.7 as the superconducting 
layer, 14 and Pb (TiZr)O.sub.3 as the pyroelectric layer, 16. 
I wish it to be understood that I do not desire to be limited to the exact 
details of construction shown and described for obvious modifications will 
occur to a person skilled in the art.