Device employing a substrate of a material that exhibits the pyroelectric effect

An article is manufactured from a substrate of a material that exhibits the pyroelectric effect by depositing a film containing a selected material in a first state over a front surface of the substrate. At least one electrode is formed on the film, portions of the film being exposed around the electrode. The exposed portions of the film are subjected to a treatment such that they are converted from the first state to a second state, in which the material has a resistivity that lies within a desired range of values.

BACKGROUND OF THE INVENTION 
This invention relates to a method of manufacturing a device employing a 
substrate of a material that exhibits the pyroelectric effect. 
An important use of lithium niobate (LiNbO.sub.3) is in fabrication of 
electrooptic devices. In a typical electrooptic device, optical waveguides 
are formed in a monocrystalline substrate of LiNbO.sub.3 by diffusion of 
titanium into the crystal structure, and electrodes are deposited over a 
front surface of the substrate. Depending on the cut of the crystal, a 
buffer layer of SiO.sub.2 may be interposed between the substrate and the 
electrodes. Also, a buffer layer may be deposited on the back surface of 
the substrate. A potential difference is established between the 
electrodes, and the electric field that is thereby created in the 
LiNbO.sub.3 substrate influences the coupling of optical energy between 
the two waveguides. 
A common use for an electrooptic device is as an optical switch. An optical 
switch may be used in an optical time domain reflectometer (OTDR) to 
direct light emitted from a light source into a fiber under test and 
direct reflected and back-scattered light from the fiber under test to a 
detector, depending on the field existing in the substrate. It is 
desirable that the field that affects the condition of the switch depend 
only on the potential difference between the electrodes. 
LiNbO.sub.3 exhibits the pyroelectric effect. When a crystal of LiNbO.sub.3 
undergoes a change in temperature, the pyroelectric effect causes a change 
in the spontaneous polarization of the material, and this produces a 
proportional electric field in the material along its Z-axis. Therefore, 
when the temperature of an optical switch based on LiNbO.sub.3 changes, 
the behavior of optical modes propagating through the waveguides is 
influenced not only by the potential difference established between the 
electrodes but also by the pyroelectric field. 
The pyroelectric effect is discussed in C. H. Bulmer, W. K. Burns and S. C. 
Hiser "Pyroelectric Effects in LiNbO.sub.3 channel-waveguide devices", 
Appl. Phys. Lett., Vol. 48 (16), 1036 (1986), which confirms that the 
pyroelectric effect results in the performance of an electrooptic device 
based on LiNbO.sub.3 being highly dependent on temperature. 
The pyroelectric effect is discussed further in P. Skeath, C. H. Bulmer, S. 
C. Hiser and W. K. Burns, "Novel electrostatic mechanism in the thermal 
instability of z-cut LiNbO.sub.3 interferometers", Appl. Phys. Lett., Vol. 
49 (19), 1221 (1986), in which several possible methods to reduce the 
thermal instability of an electrooptic device are discussed. However, 
Skeath et al does not report on the efficacy of any of these methods. 
The pyroelectric effect is self-extinguishing, since the pyroelectric field 
will result in charge being attracted to the Z faces of the substrate, and 
this charge will produce an electric field opposite in direction to the 
pyroelectric field. Given sufficient time (on the order of an hour in 
typical room air), an equilibrium state will be reached in which the 
accumulated surface charge produces a field that exactly cancels the 
pyroelectric field. 
An integrated optic device, such as an optical switch, generally has 
conductive electrodes over some parts of its surface but not others. This 
inevitably causes the equilibrating process to proceed at a faster pace in 
the material directly underneath electrodes than in adjacent areas. There 
are two mechanisms for this. First, if the electrodes are connected to an 
external driver circuit, then a finite impedance will exist between them 
and "ground", where ground is any conductive material in the vicinity of 
the crystal (such as a metal package, or a block upon which the crystal 
sits). This finite impedance provides a path along which charge can move. 
Secondly, the sharp edges of the electrodes facilitate the ionization of 
surrounding air. As these mechanisms act, the surface charge density 
becomes different between regions with and without metal. Therefore, the 
electric field in the crystal will be different in the two regions. A plot 
of the field would show complicated fringing shapes at electrode edges. 
Waveguides located in various positions with respect to electrode edges 
will be subjected to various field strengths, so waveguide coupling will 
be disturbed as though a complex external field had been applied. 
The equilibrating process can be made essentially instantaneous if the Z 
faces of the crystal are covered by conductive films and the films are 
electrically connected during the temperature change, since the conductive 
films contain mobile charge carriers that will respond immediately to the 
pyroelectric field by redistributing between the surfaces so as to cancel 
the pyroelectric field. 
In I. Sawaki, H. Nakajima, M. Seino and K. Asama, "Thermally stabilized 
z-cut Ti: LiNbO.sub.3 waveguide switch", Proceedings, Optical Fiber 
Communications Conference, 1987, it is proposed that a semi-insulating 
film of indium tin oxide (ITO) be provided over the front surface of the 
substrate of an electrooptic device, covering the electrodes and the 
exposed surface of the substrate. The ITO film reduces the resistance 
between the electrodes of the device by at least three orders of magnitude 
and results in a more uniform distribution of pyroelectric surface charge 
over the device. A disadvantage of the structure disclosed by Sawaki et al 
arises from the difficulty of ensuring a continuous film of ITO over the 
front surface of the substrate, since the electrodes cause step coverage 
problems and the small spacing between the electrodes makes it difficult 
to deposit material between the electrodes. 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the invention there is provided a 
method of manufacturing an article, comprising providing a substrate of a 
material that exhibits the pyroelectric effect, depositing a film 
containing a selected material in a first state over a front surface of 
the substrate, and subjecting portions of the film to a predetermined 
treatment such that they are converted from the first state to a second 
state in which the material has a resistivity that lies within a desired 
range of values. 
In accordance with a second aspect of the invention there is provided an 
article of manufacture comprising a substrate of a material that exhibits 
the pyroelectric effect and has a front surface, a film containing a 
selected material over the front surface of the substrate, the selected 
material having a first state, in which its resistivity lies within a 
desired range of values, and a second state, in which its resistivity lies 
outside that range of values, and at least one electrode overlying the 
film while leaving portions of the film exposed, the portions of the film 
that lie beneath the electrode being in the second state and the portions 
that are exposed being in the first state.

DETAILED DESCRIPTION 
FIG. 1A illustrates a substrate 2 of z-cut monocrystalline LiNbO.sub.3. 
Substrate 2 is one of several identical substrates that are formed from a 
monocrystalline wafer of z-cut LiNbO.sub.3. Using conventional techniques, 
two diffused titanium waveguides 4 are formed in substrate 2 beneath its 
front surface 8. Formation of diffused Ti waveguides in LiNbO.sub.3 
typically involves treatment at temperatures on the order of 1,000.degree. 
C. After formation of the waveguides, a buffer layer 10 of SiO.sub.2 is 
formed over front surface 8 by chemical vapor deposition. When layer 10 is 
formed on front surface 8, a buffer layer 12 is formed over the back 
surface 14 of substrate 2. However, layer 12 is not necessary to the 
operation of the illustrated device. 
A thin film 20 of Ti, typically having a thickness in the range from about 
100 to 200 .ANG., is deposited over the front surface of the die 21 
composed of substrate 2 and buffer layers 10, 12, and a continuous layer 
22 of Al is deposited over film 20. A continuous layer 30 of Al is 
deposited over buffer layer 12. As shown in FIG. 1B, layer 22 is patterned 
using standard photolithographic techniques to define two discrete 
electrodes 24 and a ring 26 that extends around the periphery of film 20. 
In the state shown in FIG. 1B, film 20 is highly conductive, so that the 
electrical resistance between electrodes 24 is negligible. 
The structure shown in FIG. 1B is baked in an atmosphere of oxygen and 
nitrogen. During the baking operation, in which temperatures in excess of 
about 250.degree. C. are attained, exposed portions 28 of metallic film 20 
are converted to a high resistivity state. After the baking operation, the 
wafer is diced, and a stripe 32 of conductive paint is applied to the side 
of die 21 to connect ring 26 to layer 30. The completed optical switch is 
shown in FIG. 2. 
When a temperature change causes a pyroelectric field to be generated 
between the faces of substrate 2, stripe 32 allows charge to be 
redistributed between films 20 and 30 to produce a counteracting field so 
that there is no net field in the substrate. This occurs within a time on 
the order of milliseconds. Therefore, the time required to achieve 
equilibrium between the pyroelectric field and the field due to surface 
charge is very short, and for most purposes no instability in performance 
of the switch due to change in temperature is observed. The high 
resistivity of portions 28 of film 20 prevents excessive power dissipation 
when voltage is applied between electrodes 22. 
Preferably, the baking operation is composed of at least one baking cycle, 
which comprises a heat-up phase, a constant temperature phase and a 
cool-down phase. The maximum temperature that is attained during the 
baking operation may be in the range from about 350.degree. C. to about 
400.degree. C., which is sufficiently low that the properties of the 
diffused waveguides are not affected significantly. 
In the preferred process, the heat-up phase lasts about 15 minutes and the 
constant temperature phase of the first baking cycle lasts about 30 
minutes. Cooling to 250.degree. C. is controlled to take place quite 
slowly, e.g. over a period of 70 minutes, because conversion of the 
metallic film to its high resistivity state continues at a significant 
rate until the temperature falls below about 250.degree. C.. The cooling 
may be accelerated after the initial portion of the cool-down phase. At 
the end of the cool-down phase, the resistivity of exposed portions 28 is 
measured. If the resistivity is in the desired range, which may be, for 
example, 1E7 to 1E11 ohms per square, the baking operation is 
discontinued. Otherwise, a second baking cycle is performed, and the 
length of the constant temperature phase of the second cycle depends on 
how close the measured value of the resistivity is to the desired range of 
values. 
By depositing the titanium film before the electrode metal, a continuous 
film is provided in the narrow gap between the electrodes and step 
coverage problems are avoided. 
The baking operation does not affect the portions of layer 20 that lie 
beneath electrodes 24 and ring 26. The nature of the change in the exposed 
portions of film 20 that takes place during the baking operation is not 
fully understood. The composition of exposed portions 28 of converted film 
20 was measured by electron scanning chemical analysis, in which material 
is progressively removed by sputtering and the composition of the exposed 
material is measured by auger analysis. It was found that for small 
depths, portions 28 contain titanium and oxygen in an approximately 
stoichiometric ratio, which suggests that portions 28 are titanium 
dioxide. For greater depths, Si, Ti and O.sub.2 were found, indicating 
diffusion between buffer layer 10 and film 20, and they were present in 
the stoichiometric ratios of SiO.sub.2 and TiO.sub.2. These analysis 
results imply that exposed portions 28 would be electrically insulating. 
However, measurement of the resistance between electrodes 24 indicates 
that although the resistivity of portions 28 is much higher than the 
resistivity of metallic Ti, the material of portions 28 still exhibits a 
finite conductivity. 
It will be appreciated that the invention is not restricted to the 
particular embodiment that has been described, and that variations may be 
made therein without departing from the scope of the invention as defined 
in the appended claims and equivalents thereof. For example, the invention 
is not restricted to application to an optical switch, but may also be 
applied to other electrooptic devices such as interferometers and 
modulators. Furthermore, the invention is not limited to use of titanium 
as the material of film 20, since other materials may be used in a similar 
process to achieve a similar result. The invention is not restricted to 
the particular process steps that have been described. Thus, layer 30 may 
be deposited after the wafer is diced, in which case layers of Al may be 
deposited simultaneously on the sides of the die so that the separate step 
of depositing paint stripe 32 is not required. Layer 30 need not be 
adhered directly to die 21, and its function may be provided by mounting 
die 21 on a metal plate. Treatments other than heating in an atmosphere 
containing oxygen may be used to modify the material of the film deposited 
on the front surface of the die to convert it to the state in which it has 
the desired resistivity. It is intended that the claims should not be 
interpreted as being limited to the treatment being performed in cycles, 
but to cover also methods in which the resistivity is measured throughout 
the treatment and the treatment is discontinued when the resistivity 
reaches the desired level.