Planar lightwave circuit active device metallization process

Details Planar lightwave circuit active device metallization process Planar lightwave circuit active device metallization process

2001-08-27

2004-03-23

Zarabian, Amir (Department: 2822)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Including integrally formed optical element

C438S031000, C438S618000, C438S689000, C385S129000, C385S130000, C385S131000

Reexamination Certificate

active

06709882

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to active-type planar lightwave circuits. More particularly, the present invention relates to a method for depositing precise metallization areas for refractive index control in active-type of planar lightwave circuits.
BACKGROUND OF THE INVENTION
Planar lightwave circuits comprise fundamental building blocks for the modern fiber optic communications infrastructure. Planar lightwave circuits (PLCs) are generally devices configured to transmit light in a manner analogous to the transmission of electrical currents in printed circuit boards and integrated circuit devices. Examples include arrayed waveguide grating devices, integrated wavelength multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, and the like.
PLCs generally involve the provisioning of a series of embedded optical waveguides upon a semiconductor substrate, with the optical waveguides fabricated from a silica glass. Planar lightwave circuits are constructed using the advanced tools and technologies developed by the semiconductor industry. Modern semiconductor electronics fabrication technology can aggressively address the increasing need for integration currently being used to make PLCs. By using manufacturing techniques closely related to those employed for silicon integrated circuits, a variety of optical elements can be placed and interconnected on the surface of a silicon wafer or similar substrate. This technology has only recently emerged and is advancing rapidly with leverage from the more mature tools of the semiconductor-processing industry.
PLCs are constructed with a number of waveguides precisely fabricated and laid out across a silicon wafer. A conventional optical waveguide comprises an un-doped silica bottom clad layer, with at least one waveguide core formed thereon, and a cladding layer covering the waveguide core, wherein a certain amount of at least one dopant is added to both the waveguide core and the cladding layer so that the refractive index of the waveguide core is higher than that of the cladding layer. Fabrication of conventional optical waveguides involves the formation of a silica layer as the bottom clad (BC), usually grown by thermal oxidation, or flame hydrolysis deposition (FHD), upon a silicon semiconductor wafer. The core layer is a doped silica layer, which is deposited by either plasma-enhanced chemical vapor deposition (PECVD) or FHD. An annealing procedure then is applied to this core layer (heated above 1000 C.). The waveguide pattern is subsequently defined by photolithography on the core layer, and reactive ion etching (RIE) is used to remove the excess doped silica to form one or more waveguide cores. A top cladding layer is then formed through a subsequent deposition process. Finally, the wafer is cut into multiple planar lightwave circuit dies and packaged according to their particular applications.
Prior art
FIG. 1
shows a cross-section view of two planar optical waveguides of a conventional PLC. As depicted in
FIG. 1
, the planar optical waveguides include two doped SiO
2
glass cores
10
a
-
10
b
formed over a SiO
2
bottom cladding layer
12
which is on a silicon substrate
13
. A SiO
2
top cladding layer
11
covers both the cores
10
a-b
and the bottom cladding layer
12
. As described above, the refractive index of the cores
10
a-b
is higher than that of top cladding layer
11
and the bottom clad
12
. Consequently, optical signals are confined axially within cores
10
a-b
and propagate lengthwise through cores
10
a-b.
PLC devices having multiple cores comprise the basic building blocks of active type optical devices. One such increasingly important optical device is an optical switch. As optical communications networks become more complex and carry more data traffic, optical switches play an increasingly important role. Optical switches play an increasingly important role as today's optical networks become more complex and carry more capacity. Optical switches can be deployed in applications such as network protection and restoration and dynamically reconfigurable add/drop modules. Although several switching technologies are available (e.g., opto-mechanical type switches, liquid crystal, etc.), PLC based thermo-optic switches, where light is guided in planar waveguides, is emerging as reliable technology of choice.
PLC based thermo-optic switches utilize materials such as silica or polymers which exhibit the “thermo-optic effect”, wherein their refractive indices change as their temperature is changed. This thermo-optic coefficient could either be positive like silica (approximately 10
−5
/° C.) or negative like polyimide (approximately 10
−4
/° C.). This type of switch is fast enough for protection and restoration purposes, compact, and well suited for integration with other PLC components, such as arrayed waveguide gratings, to form more complicated modules like an optical add/drop multiplexers. These thermo-optic effect based devices have been used in a variety of systems, such as, for example, optical switches, variable optical attenuators (VOAs), dynamic gain flattening filters (DGFF), and integrated devices such as a VMUX (VOA plus MUX).
Prior art
FIG. 2
shows a diagram of a Mach-Zehnder thermo-optic switch. As depicted in
FIG. 2
, a first waveguide (core
10
a
) and a second waveguide (core
10
b
) are used to implement input ports (e.g., Pin
1
and Pin
2
) and output ports (e.g., Pout
1
and Pout
2
) as shown. The first and second waveguides pass through a first coupling region
21
and a second coupling region
22
. A resistive thin film heater is deposited above each waveguide between the two coupling regions
21
-
22
(e.g., heater
1
and heater
2
).
The heaters are used to selectively heat one waveguide core or the other to change its refractive index, and thereby modulate an accumulated phase difference of light propagating through the two waveguide cores
10
a-b
. When light is launched into one of the input ports, it is split into the two cores
10
a-b
by the first coupler
21
with equal optical power and ½ phase difference. As light travels through the waveguide cores
10
a-b
, the phase difference can be altered using a temperature difference between the two cores, as controlled by the two heaters. After passing through the second coupler
22
, the two beams recombine either constructively or destructively at either of the two output ports, depending upon the exact phase difference between the two cores
10
a-b
. The exact phase difference is controlled by precisely controlling the current/voltage applied to the heaters. This modulation of temperature achieves the purpose of switching the light between the two output ports. The electrical power needed to switch each path is typically on the order of a few hundred milliwatts. Switches can also be cascaded for added extinction ratio without sacrificing much on insertion loss. The same technique can be used in VOA devices and other types of thermo-optic active PLC devices.
There exists problems with the prior art metallization process for the above described types of thermo-optic PLC devices. Different metals are chosen for heater and interconnect due to the different conductivity requirements for each layer. One must be capable of etching these differing metals without adversely affecting the other. These metals must be in contact with each other, which drives the requirement for highly selective etch processes. Ideally, those processes in which one layer etches readily and the other not at all. Adhesion of the two materials must be maintained. Many prior art thermo-optic PLC devices utilize chrome as the heater material and gold as the interconnect material. A chrome wet etch process requires special disposal due to the heavy metal content. Thus, the chrome wet etch process is thus overly expensive and time-consuming. Both gold and chrome can be dry etched using chlorine. The etchers designed to handle the corrosive chlorine gas can be co

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