Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from solid or gel state – Using heat
Reexamination Certificate
1999-05-11
2004-08-03
Vinh, Lan (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from solid or gel state
Using heat
C117S013000, C117S034000, C117S213000, C117S948000
Reexamination Certificate
active
06770132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for annealing lithium niobate and fabrication processes involving the annealing of lithium niobate devices. More particularly, the present invention relates to a method for annealing in a pressurized oxygen gas atmosphere bulk lithium niobate and optical waveguides and modulators having lithium niobate substrates.
2. Background
Optoelectronic components can be fabricated on several types of substrates including polymers, glass, semiconductors (i.e., gallium arsenide (GaAs) and indium phosphide (InP)) and inorganic materials (i.e., lithium niobate (LiNbO
3
) and lithium tantalate (LiTaO
3
)). Characteristically, an electro-optic material is one in which the index of refraction changes with the application of an electric field. One of the most important components in optoelectronic systems is the modulator. Three competing technologies in this realm are: direct modulation of a semiconductor laser, semiconductor electro-absorption modulators, and the lithium niobate modulator. Currently, lithium niobate modulators are the modulation devices of choice for many systems because they yield high performance, are a relatively mature technology and other modulation schemes impose limits not faced with lithium niobate modulators.
Lithium niobate has proven to be a suitable medium for components such as amplitude modulators, phase modulators, optical switches, polarization transformers, tunable filters and wavelength-selective optical add/drop filters. Lithium niobate has also been used as the host for solid state lasers using rare earth ions, e.g., erbium. Most current telecommunication and cable television system applications for LiNbO
3
modulators involve discrete components for the optical transmitter subsystem. This configuration couples continuous wave lasers, typically diode-pumped YAG or erbium fiber oscillators, with lithium niobate external modulators and various wavelength and power stabilization components.
Lithium niobate is a popular nonlinear optical crystal for several reasons including its large electro-optic coefficients, the ease with which high quality optical waveguides are fabricated and its amenability to conventional integrated circuit processing techniques. High quality optical waveguides are those that possess low loss and high power handling capabilities. Additionally, LiNbO
3
is a hard material, thus easily polished for fiber coupling which makes its use in optical network systems relatively uncomplicated. It is also a relatively inexpensive crystal, due in part to its long history of use in surface-acoustic-wave (SAW) filters for radio frequencies. By comparison, lithium tantalate LiTaO
3
is essentially interchangeable with lithium niobate as far as modulator characteristics are concerned, but the use of LiTaO
3
is often cost prohibitive because it is not as widely commercially used.
For example, one type of modulator incorporating the use of LiNbO
3
is a Mach-Zehnder modulator. In a Mach-Zehnder modulator an incoming optical beam is split equally at a Y junction into two parallel waveguides, which then recombine at another Y junction after some distance. Electrodes are used to apply an electric field in the region of the optical waveguides. The electric field causes a relative phase shift between the two branches. If the phase shift in both arms is the same, then the two optical signals will recombine constructively at the second Y branch and no optical power will be lost. If there is a phase difference between the two arms, then there will be destructive interference and some optical power will be lost. The resulting destructive and constructive interference causes the output optical intensity to vary between a minimum and a maximum value.
In other electro-optic applications optical waveguide directional couplers can also be used as electro-optic modulators. In this type of modulator two waveguides are placed on the lithium niobate substrate in very close proximity to one another. The wave from one guide can “leak” into the other guide. By applying an electric field to the lithium niobate, the amount of “leakage” can be controlled, thus modulating the transfer of power from one guide to the other. Currently, different commercial application requirements favor either directional couplers or Mach-Zehnder modulators.
High-speed communications has led to a demand for the external modulator configuration. The most common approach utilizes a low-noise, high power diode-pumped laser whose signal is sent to the LiNbO
3
modulator via optical fiber. The modulator applies either analog or digital information to the optical carrier.
When using lithium niobate in the fabrication of optical waveguides and optical modulators it is desired to avoid having a niobium-rich, lithium-poor and/or oxygen poor composition. When bulk lithium niobate has such niobium rich-compositions, and is then processed at high temperatures (T>300 degrees Celsius), growth of the LiNb
3
O
8
phase in the crystal may occur. This phase is undesirable because it is not optically transparent and leads to high losses in optical waveguides and optical modulators.
Such niobium-rich compositions can occur in two different manners during fabrication of optical waveguides and optical modulators. First, typical ion exchange procedures result in the replacement of lithium atoms in the crystal lattice with a diffusing ion leading to a lithium niobate composition relatively rich in niobium. Second, standard high temperature (temperatures in excess of 300 degrees Celsius) processing of lithium niobate can cause Li
2
O out-diffusion, and result in niobium-rich, and lithium and oxygen poor compositions.
To eliminate the undesired LiNb
3
O
8
phase from forming in the crystal, high temperature processing, such as the stress relieving anneal process, is usually performed in a wet atmosphere of inert carrier gas, such as nitrogen (N
2
) or argon (Ar
2
), or in a wet atmosphere of oxygen (O
2
). This type of anneal process involves bubbling the inert carrier gas or oxygen gas through water (H
2
O). The wet atmosphere is beneficial because the H
2
O attacks the LiNb
3
O
8
phase and breaks it down into LiNbO
3
. A typical wet atmosphere anneal operation is performed at a temperature of about 350 degrees Celsius for a period of 5 to 6 hours in a wet, flowing environment. The present inventor has discovered that a drawback of this type of high temperature processing is that the H
2
O gives off undesirable protons which are attracted by the lithium niobate and result in an inadvertent proton exchanged surface layer occurring. These protons remain in the modulators after fabrication and flow relatively freely among the waveguides, the waveguides and the electrodes and in the lithium niobate crystal and the buffer layer interface. Undesirable free flowing protons can adversely affect a modulator's DC-bias stability because they act as charge carriers and are driven by the applied electric fields, causing the response of the final product to drift over time when a bias is applied.
Additionally, high temperature processing (>300 degrees Celsius) leads to oxygen out-diffusion of lithium niobate structures. This out-diffusion tends to form a surface layer on a lithium niobate substrate or a lithium niobate waveguide that is oxygen poor in composition. This oxygen poor region acts as a surface optical waveguide and causes undesirable leakage of light out of the lithium niobate substrate.
It is therefore highly advantageous to devise a new high temperature annealing process that inhibits the formation of the undesirable LiNb
3
O
8
phase in the crystal and outdiffusion of O
2
without the process introducing significant numbers of free flowing protons that will affect the modulator's DC-bias stability.
BRIEF DESCRIPTION OF THE INVENTION
In one aspect of the invention, a method for pressurized annealing of lithium niobate or lithium tantalate structures, such as optical modulators and optical wave guides, comprise
California Institute of Technology
Ritchie David B.
Thelen Reid & Priest LLP
Vinh Lan
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