# Patent

## Title
Method of forming a phosphorus doped optical core using a PECVD process

## Abstract

Embodiments of the present invention provide a highly uniform low cost production worthy solution for manufacturing low propagation loss optical waveguides on a substrate. In one embodiment, the present invention provides a method of forming a PSG optical waveguide on an undercladding layer of a substrate that includes forming at least one silicate glass optical core on said undercladding layer using a plasma enhanced chemical vapor deposition process including a silicon source gas, an oxygen source gas, and a phosphorus source gas, wherein the oxygen source gas and silicon source gas have a ratio of oxygen atoms to silicon atoms greater than 20:1.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 10/279,366, now U.S. Pat. No. 7,080,528, entitled “METHOD OF FORMING A PHOSPHORUS DOPED OPTICAL CORE USING A PECVD PROCESS,” filed Oct. 23, 2002 by Hichem M'Saad et al., the entire disclosure of which is incorporated herein by reference for all purposes.


BACKGROUND
The present invention relates to the manufacture of integrated systems, and more particularly to a method of forming optical cores. Optical cores manufactured according to the present invention are useful in a variety of applications, and are particularly useful in integrated optical systems.
Integrated optical systems are an emerging technology that offer solutions to some previously unsolvable technological problems. Thus, integrated optical systems are becoming an increasingly important technology. Generally, optical systems utilize pulses of light rather than electric current to carry out such functions as data transmission, data routing, or other forms of data communication or data processing. One important structure commonly utilized in optical systems is an optical waveguide.
Optical waveguides are used to confine and direct light between the various components of an optical system. For example, optical waveguides may be used to carry Dense Wavelength Division Multiplexed (DWDM) light, which is used to increase the number of wavelengths in a single waveguide to achieve a higher aggregate bandwidth. FIG. 1 is a cross-sectional view of an optical fiber waveguide. The general structure of an optical waveguide 100 comprises two principal components: a core 103 and a cladding layer 102. The core 103 is the inner part of the fiber through which light is guided. It is surrounded by the cladding layer 102, which generally has a lower refractive index so that a light ray 105 in the core 103 that strikes the core/cladding boundary at a glancing angle is confined within the core 103 by total internal reflection. The confinement angle θc represents an upper limit for the angle at which the light ray 105 can strike the boundary and be confined within the core 103.
As more signals are incorporated into an optical waveguide channel, there is an increasing demand for components to route, switch, drop, and guide these light signals to their final destination. Many photonic components make this technology possible. These components include filters, modulators, amplifiers, couplers, multiplexers, optical cross-connects, Arrayed Waveguide Gratings (AWG), power splitters, and star couplers to name only a few. However, as optical technology matures, it is desirable to monolithically integrate various photonic components onto a single structure such as, for example, a silicon or glass substrate. Generally, processes for manufacturing an optical core on a substrate must result in structures that have good optical qualities. Additionally, such manufacturing processes must result in optical cores that are highly uniform across the substrate.
Attempts to integrate optical waveguides and photonic components onto a single chip have faced many challenges. For example, one optical waveguide core characteristic that is important to the performance of an optical system is propagation loss, which is also referred to as attenuation. Attenuation refers to the loss of light energy as a pulse of light propagates down a waveguide channel. The two primary mechanisms of propagation loss are absorption and scattering. Absorption is caused by the interaction of the propagating light with impurities in the waveguide channel. For example, electrons in the impurities may absorb the light energy and undergo transitions. The electrons may then give up the absorbed energy by emitting light at other wavelengths or in the form of vibrational energy (i.e., heat or phonons). Thus, the manufacturing processes used for building the optical waveguide cores must minimize the introduction of impurities that lead to propagation loss from absorption.
The second primary mechanism, scattering, results from geometric imperfections in the fiber that cause light to be redirected out of the fiber, thus leading to an additional loss of light energy. Accordingly, optical waveguide core uniformity is of paramount importance. Therefore, for satisfactory optical waveguides, the manufacturing process has the additional critical requirement of providing integrated optical waveguide core structures that are highly uniform across the substrate structure upon which they are built. Furthermore, the manufacturing process must maintain such uniformity across multiple process runs.
Another important parameter relating to optical cores is stress, which in one form is related to the difference in the coefficient of thermal expansion between the optical core and surrounding structures. Stress can produce a density change in the optical core structure. Typically, stress in an optical core is anisotropic. Anisotropic stress causes the permittivity and refractive index of core to be functions of direction, which causes the light to deleteriously propagate faster in one direction than in other directions. For example, the refractive index along the direction of the stress can be different from the refractive index perpendicular to it, causing light polarized along the direction of the stress to propagate at a different rate than light polarized perpendicular to the direction of the stress. This phenomena is sometimes referred to as birefringence, and can lead to distortion of the optical signals in a fiber. Problems associated with stress become compounded as optical cores are manufactured on larger wafer sizes. In particular, stress on larger wafers (e.g., 200 mm and up) can lead to bowing, which interferes with subsequent lithography during patterning of the core.
Another parameter critical to manufacturing integrated optical cores is refractive index uniformity. Uniformity in optical waveguide applications requires an extremely tight film homogeneity across the substrate and between wafer to wafer. Prior art integrated circuit manufacturing processes have previously been used primarily for processing electrical devices on a semiconductor substrate, where the principal concern is the electrical properties of materials, usually characterized by the dielectric constant and breakdown voltage. However, for optical applications refractive index tolerances can be constrained, for example, to within the range of ±0.0002, which is substantially more stringent (e.g., about 2 orders of magnitude more stringent) than for semiconductor electronic fabrication processes.
At present, known integrated optical core manufacturing techniques have a variety of shortcomings. For instance, one prior art technique uses flame hydrolysis for depositing the optical core. However, flame hydrolysis processes are costly and lead to poor uniformity on large substrates. Another prior art technique uses a low pressure chemical vapor deposition process (“LPCVD”). However, known LPCVD techniques suffer from low deposition rates that result in increased production costs. Furthermore, prior art plasma enhanced chemical vapor deposition (“PECVD”) processes suffer from high propagation losses, high stresses, or poor uniformity.
Thus, there is a need for an improved method of manufacturing an integrated optical core to allow for the efficient integration of optical systems on a single structure.
SUMMARY
Embodiments of the present invention provide a highly uniform low cost production worthy solution for manufacturing low propagation loss optical waveguides on a substrate. In one embodiment, the present invention provides a method of forming an optical waveguide on an undercladding layer of a substrate that includes forming at least one silicate glass optical core on said undercladding layer using a plasma enhanced chemical vapor deposition process including a silicon source gas, an oxygen source gas, and a phosphorus source gas, wherein the oxygen source gas and silicon source gas have a ratio of oxygen atoms to silicon atoms greater than 20:1.
In another embodiment, a termination step is provided after the deposition step wherein all source gases, except the oxygen source gas, are evacuated from the deposition chamber after the optical core is formed.
In one embodiment, optical cores having a refractive index of less than 1.5 are formed. The contrast between the refractive index of the core and the refractive index of the cladding layer is less than 2%. In one embodiment, constrasts in the range of about 0.6%-1.0% may be formed.
In another embodiment, an anneal step is added to further improve the optical properties of the core.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of an optical fiber waveguide;
FIG. 2 shows a cross-section of a fiber and a graph illustrating the energy distribution of the light energy λE in the core;
FIG. 3 is a cross-sectional view of an optical waveguide having a core, an undercladding layer, and an uppercladding layer according to one embodiment of the present invention;
FIG. 4 illustrates a method of forming a PSG optical core using a PECVD process according to one embodiment of the present invention;
FIGS. 5A-5E illustrate a first method of forming an optical waveguide core according to one embodiment of the present invention;
FIG. 6A-E illustrate a second method of forming an optical waveguide core according to another embodiment of the present invention;
FIG. 7 illustrates an optical waveguide structure according to one embodiment of the present invention;
FIGS. 8A-B illustrate vertical and cross-sectional views of one suitable CVD system for practicing embodiments of the present invention;
FIG. 9 shows experimental data illustrating the relationship between phosphorous concentration and PH3 flow for temperatures of 400° C. and 550° C.
FIG. 10 shows experimental data illustrating the relationship between PH3 flow and refractive index for temperatures of 400° C. and 550° C.
FIG. 11 illustrates an anneal step according to one embodiment of the invention.
FIG. 12 illustrates the relationship of phosphorous content and steam anneal on the refractive index of a 7 μm thick PSG film at 400° C. and 550° C. according to one embodiment of the present invention.
FIG. 13 illustrates the relationship of phosphorous content and steam anneal on the refractive index range of a 7 μm thick PSG film deposited at 400° C. and 550° C. according to one embodiment of the present invention.
FIG. 14 illustrates the effect of multiple steam anneal cycles on the refractive index range of a 7 μm thick PSG film embodiments of the present invention.
FIG. 15 illustrates the effect of phosphorous content and steam anneal on stress.
FIG. 16 illustrates the effect of multiple anneal cycles on stress. As the phosphorous content is increased the PSG film becomes less compressive.
FIG. 17 illustrates the effect of phosphorous content and steam anneal on the change in thickness of PSG films.
FIG. 18 illustrates a PECVD process recipe for depositing a PSG optical core according to another embodiment of the present invention.



DETAILED DESCRIPTION
1. Formation of a PSG Optical Waveguide Core
Embodiments of the present invention provide plasma enhanced chemical vapor deposition (“PECVD”) process techniques for depositing phosphorous-doped silicate-glass (“PSG”) optical cores having properties suitable for optical applications.
As mentioned above, PECVD processes have previously been used primarily for processing electrical devices on a semiconductor substrate, where the principal concern is the electrical properties of materials. Furthermore, known PECVD processes used for optical applications suffer from undesirably high loss, high stress, or poor refractive index uniformity. In contrast, PECVD processes carried out according to embodiments of the present invention achieve improved optical characteristics such as uniformity, stress, and propagation loss.
Embodiments of the present invention include using a PECVD process including a silicon source gas, an oxygen source gas, and a phosphorous source gas to form a PSG optical core on a substrate. Exemplary silicon source gases may include SixH2x+2 (e.g., SiH4, Si2H6, or Si3H8), SixH2x (e.g., Si2H4), SiH2Cl2, tetraethylorthosilicate (“TEOS”), methylsilane (i.e., (CH3)SiH3), dimethylsilane (i.e., (CH3)2SiH2) or trimethylsilane (i.e., (CH3)3SiH). Additionally, exemplary oxygen source gases may include N2O, CO2, or CO.
Embodiments of the present invention utilize a phosphorous source gas as a dopant the PECVD deposition process to alter the optical properties of the core. First, phosphorous may be used to decrease stress in the optical core. Second, phosphorous will also cause slight increases in the refractive index of the optical core, which may be used at least in part to set the contrast between the core and the cladding (i.e., the difference in refractive index between the core and the cladding). The refractive index of an SiO2 based core will be increased by a phosphorus dopant because phosphorus results in a greater polarization of the light in the core than oxygen. An exemplary phosphorus containing source gas is PH3. In one embodiment, the phosphorous source gas comprises 5% PH3 diluted in 95% N2. Other embodiments may use argon, helium, or other inert gases to dilute the phosphorous source gas. In another embodiment, undiluted PH3 may be used (i.e., 100% PH3), in order to provide better chamber to chamber and system to system repeatability.
Embodiments of the present invention improve uniformity of a PSG optical core by carrying out the PECVD process in an oxygen rich environment. As previously mentioned, a particular concern with the fabrication of optical waveguides is that the processes utilized must result in highly uniform structures. The inventors have discovered that carrying out the PECVD process in an oxygen rich environment can result in PSG optical cores with refractive index uniformity levels that are satisfactory for optical applications. More particularly, improvements in refractive index uniformity have been found where the ratio of oxygen atoms to the silicon atoms in the source gases is greater than 20:1. Phosphorous may then be added to reduce stress and achieve changes in the refractive index.
Too little oxygen in the chamber will starve the formation of SiO2. Therefore, insufficient oxygen content may lead to variations in the refractive index because there is poor SiO2 uniformity across the layer (e.g., there may be silicon to silicon bonds in the SiO2 layer). When the ratio of oxygen to silicon atoms is greater than 20:1, there is sufficient oxygen to bond with all of the silicon atoms for form SiO2, and additionally, fluctuations in the flow of the oxygen source gas will not impact SiO2 formation.
For example, in one embodiment of the present invention a ratio of 36:1 is used (e.g., N2O:SiH4=36:1). It is to be understood that the ratio of source gases may change according the particular silicon and oxygen content. It should be noted that increasing the oxygen content utilized during the PECVD process will also impact the refractive index of the film. Accordingly, when the PECVD process is carried out in an oxygen rich environment, more phosphorous should be incorporated into the optical core to maintain the desired refractive index.
The optical properties of the core may be further improved by carrying out the PECVD process at increased processing temperatures. For example, in one embodiment, processing temperatures of 550° C. or higher can be used during the PECVD process. Increased processing temperatures during the optical core deposition step are advantageous because high temperatures lead to a higher desorption and thus further reduce hydrogen content, which leads to reductions in propagation loss.
Energy is generally a function of the pressure used in the processing chamber, and the power supplied to the processing chamber. Some embodiments of the present invention form an optical core at a pressure of less than 10 Torr and an RF power of between about 1-2 Watts/cm2 (e.g., about 300-600 Watts for a 200 mm wafer). In one embodiment, an optical core is formed at a pressure of about 2.8 Torr, and RF power of about 1.5 Watts/cm2 (e.g., about 400 Watts for a 200 mm wafer).
Optical cores according to embodiments of the present invention may be formed by either a one-step or a multi-step deposition. A one-step deposition is when the total thickness of the core is deposited without interruption. On the other hand, a multi-step deposition is when the core is deposited layer by layer. For example, in a two-step deposition about half (or some other portion) of the thickness is originally deposited, and then the wafer is take out of the chamber during an in-situ dry clean of the chamber. The wafer is then returned to the chamber, and the deposition is completed. Additional steps may be used by sequentially depositing thinner portions of the core during each deposition step.
One problem that has been observed is that after the deposition is completed, optical imperfections are formed on the newly deposited material. Optical imperfections are also sometimes referred to as hazing. These optical imperfections appear in both doped and undoped oxides. Thus, one-step deposition is preferred for optical cores because a multi-step deposition results in an interface within the oxide that may include these optical imperfections. However, in some instances it may be desireable to utilize a two-step deposition for thicker cores. Embodiments of the present invention include a termination step that reduces these optical imperfections. Substantial improvements in hazing can be achieved by adding a termination step after the deposition step, wherein all source gases except the oxygen containing gas are evacuated from the deposition chamber. For example, one embodiment of the present invention includes a N2O plasma purge step lasting between about 5-10 seconds, during which all other source gases are evacuated.
Embodiments of the present invention may also use a post deposition anneal step to further improve the optical properties of the core. The anneal step may be carried out in a steam ambient for lowest possible stress. However, other ambients, such as molecular oxygen (O2) or nitrogen (N2) for example, may also be used. A post furnace anneal step can improve both the refractive index range, propagation loss, and stress. The inventors have also found that the effect of a post furnace anneal step on the refractive index of the core is independent of the temperature used during the deposition process. In some embodiments, the anneal step utilizes a temperature between 1000-1100° C. for between about 2-4 hours. In one exemplary embodiment, improvements in refractive index uniformity, propagation loss, and stress are found to occur after a furnace anneal of about 2 hours. One exemplary temperature that may be used for the anneal step is about 1050° C.
Embodiments of the present invention are particularly advantageous for forming low contrast optical cores. The contrast, Δn, of an optical core is given by the following equation:




  
    Δ
    ⁢
    
      
    
    ⁢
    n
  
  =
  
    
      
        n
        core
      
      -
      
        n
        cladding
      
    
    
      n
      core
    
  




Low contrast optical cores typically have a Δn<2%, and typically include optical cores with lower refractive indices (i.e., n<1.5). Low contrast cores are advantageous for a variety of applications including low insertion loss applications such as splitters, interleavers, or concentrators, which may utilize a contrast of about Δn=0.6%, for example. Low contrast cores may also be advantageous for arrayed waveguide gratings, which may utilize a contrast of about Δn=0.8%.

For low contrast applications, lower refractive indices are typically used, and the light is optically spread out across a greater portion of the core. FIG. 2 illustrates this concept. FIG. 2 shows a cross-section of a fiber and a graph illustrating the energy distribution of the light energy λE in the core. As shown in FIG. 2, as the contrast Δn increases, more of the light energy is confined within the core, and less light energy is in the cladding layer. Therefore, as Δn increases thinner cores may be used to carry the optical signals, and as Δn decreases thicker cores are typically used to carry the optical signals.
Embodiments of the present invention are particularly useful for forming PSG optical cores having a low refractive index. Exemplary embodiments of the present invention may be used to form a PSG optical core having a refractive index in the range of 1.45 to 1.46 measured at 1550 nm, which corresponds to a refractive index range of about 1.46 to 1.475 measured at 633 nm. When a PSG optical core is deposited on an underlayer (e.g., an undercladding layer) that has a refractive index of about 1.46 measured at 633 nm on thick thermal oxides, an index contrast of about 0.6% to about 1.0% between the core and the cladding can be achieved. Among other factors, the above referenced range of refractive indices for the core can be achieved by incorporating between about 6.5 to 11.0 wt. % phosphorous into the optical core (e.g., See FIG. 9 below). Optical cores in this refractive index range can be formed with a thickness of between about 4.5 to 7.2 μm or more. However, those skilled in the art will understand that the thickness of the optical core will depend on the application and the particular targeted refractive index. Typically, a larger refractive index corresponds to a thinner optical core because of the higher light confinement illustrated in FIG. 2.
FIG. 3 is a cross-sectional view of a optical waveguide 300 including a PSG optical core according to one embodiment of the present invention. The optical waveguide 300 may be manufactured on a substrate 310, which may be a silicon substrate, for example. However, it is to be understood that a variety of substrates could be used, such as a glass (i.e., silica) substrate, a lithium-niobate LiNbO3 substrate, a silicon-germanium SiGe substrate, a III-V compound substrate (e.g., indium-phosphide InP or gallium-arsenide GaAs), or other suitable material. The optical waveguide 300 includes an optical waveguide core 330, which is completely surrounded by an optical cladding layer 320. The core 330 is the inner part of the fiber through which light is guided. The cladding layer 320 generally has a lower refractive index so that a light ray in the core that strikes the core/cladding boundary at a glancing angle is confined within the core by total internal reflection.
FIG. 4 illustrates a method of forming a PSG optical core using a PECVD process according to one embodiment of the present invention. At step 410, the PECVD deposition chamber is preheated. The deposition chamber may be heated to greater than 300° C., for example. At step 420, an oxygen containing gas such as N2O is introduced into the chamber. At step 430, a silicon containing gas, such as SiH4, and a phosphorous containing gas, such as 5% PH3 diluted in 95% N2, are introduced into the deposition chamber. At step 440, RF power is turned on, a plasma is created in the deposition chamber, and a PSG optical core is deposited on a substrate located in the chamber. After an amount of time corresponding to the desired thickness of the optical core, the silicon containing gas and phosphorous containing gas are evacuated from the chamber at step 450 (i.e., the silicon and phosphorous sources are turned off). However, according to some embodiments of the present invention the oxygen containing gas, which in this particular case may be N2O, is maintained in the chamber to reduce optical imperfections (i.e., hazing) in the PSG optical core. At step 460, the RF power is turned off, and the oxygen containing gas is evacuated from the chamber.
A suitable process recipe for depositing a PSG optical core on a 200 mm substrate using a PECVD process according to one embodiment of the present invention is summarized in Table 1 below:









TABLE 1







Exemplary


Process Parameter
Range
Value

















F(SiH4) (sccm)
10-1000
100


F(N2O) (sccm)
200-25k
3600


F(PH3) (sccm)
Up to 100
110


F(N2) (sccm)
up to 1900
209


Source RF Density - (Watts/cm2)
1-2
1.5


Pressure (Torr)
less than 10
2.8


Temp (° C.)
greater than 300
550









For the recipe shown in Table 1, the phosphorous containing gas is diluted in N2, However, as mentioned above, other embodiments may use argon. As a noble gas, argon stabilizes the plasma because noble gases ionize more easily, thereby helping the dissociation of other species in the plasma. It is to be understood, however, that other carrier gases such as helium, neon, xenon, or krypton could also be used.
FIGS. 5A-5E illustrate a method of forming an optical waveguide according to one embodiment of the present invention. The technique used in FIGS. 5A-5E includes forming a continuous core layer across the entire substrate, then defining the optical cores by patterning, and then etching away unwanted portions of the core layer. This approach is sometimes referred to as a blanket-etch approach. As shown in FIG. 5A, a silica substrate 510 may be used as the base for the optical waveguide. An undercladding layer 520 is then formed over substrate 510 (FIG. 5B). In one particular embodiment, undercladding layer 520 is formed of undoped SiO2 (“USG”), but more generally, the undercladding layer 520 may be formed from any material having a lower refractive index than the core material. The undercladding layer 520 is typically a thermal oxide or a high-pressure oxide.
Next, a continuous optical core layer 530 is formed over undercladding layer 520 (FIG. 5C). Optical core layer 530 is formed using a PECVD process as discussed herein. In the embodiment shown in FIGS. 5A-5E, a continuous optical core layer 530 is formed across the entire wafer surface. Because the deposition surface is substantially flat and includes no trenches, no gap filling is required. Accordingly, the high-density plamsa process may be used with no RF bias. This has the effect of further reducing the hydrogen content in the optical core layer 530 because RF bias has been found to cause increases in hydrogen content in the deposited core.
After deposition of the continuous optical core layer 530, the layer is etched to form at least one optical core (FIG. 5D). Techniques for etching doped silicate glass layers, such as PSG and others, are well-known to those skilled in the art and are not described here. Two optical cores 530A and 530B are illustrated in FIG. 5D. An anneal step may also be included before etch to further improve the propagation loss. An anneal step may utilize a temperature between 1000-1100° C. for between 2-4 hours.
An uppercladding layer 525 is then formed over the optical cores 530A and 530B (FIG. 5E). In some embodiments of the invention, uppercladding layer 525 is a BPSG layer. It will be appreciated that if more than one optical core is formed, for example optical cores 530A and 530B, then gaps between the optical cores 530A and 530B are typically filled as fully as possible with appropriate gap filling techniques during the deposition of layer 525.
FIG. 6A-E illustrate another method of forming an optical waveguide according to another embodiment of the present invention. The technique used in FIGS. 6A-6E includes laying down the undercladding layer across the entire substrate and then forming trenches in the undercladding layer for receiving and defining optical cores. This approach is sometimes referred to as a gap-fill or “Damascene” approach. As shown in FIG. 6A, a substrate 610 may be used as the base for the optical waveguide. An undercladding layer 620 is formed over substrate 610 (FIG. 6B). The undercladding layer 620 may typically be a thermal oxide or a high-pressure oxide. For the present embodiment, the undercladding layer 620 is deposited to a thickness sufficient to include the thickness of the optical core material. Next, the undercladding layer 620 is etched to form at least one trench 621A and/or 621B (FIG. 6C). Optical cores 630A and 630B are then deposited using a PECVD process, as discussed herein, in the trenches 621A-B in the undercladding layer 620 (FIG. 6D).
In the embodiment shown in FIGS. 6A-6E, the optical cores are deposited in trenches, and therefore, trench filling techniques are required. Accordingly, an RF bias may be used in this case with the PECVD process to improve trench filling during the optical core deposition. For example, in one embodiment, an RF bias power density of between 0 to 16 watts per square centimeter is used to improve trench filling of the deposition. Additionally, a chemical mechanical polish (“CMP”) or etch will typically be used to planarize the surface and expose optical cores 630A and 630B, and undercladding layer 620, before deposition of the uppercladding layer 625. After a CMP, uppercladding layer 625 may then be formed over the exposed optical cores 630A and 630B, and exposed undercladding layer 620 (FIG. 6E).
FIG. 7 illustrates an optical waveguide structure according to one embodiment of the present invention. The optical waveguide 700 may include PSG optical cores 710A and 710B deposited using embodiments of the PECVD process discussed herein. The optical waveguide 700 also includes a 15 μm USG layer 702 formed on a substrate 701. The USG layer 702 is the undercladding layer of the optical cores 710A and 710B. The uppercladding layer comprises a 15 μm BPSG layer 703. Typical PSG optical cores 710A and 710B may be 4.5 to 7.2 μm or more thick and separated by 1-2 μm for a refractive index of between about 1.467 to 1.473 measured at 633 nm, for example.
2. Exemplary PECVD Processing System
The PECVD process for forming a PSG optical core according to embodiments of the present invention can be carried out in a variety of different plasma CVD chambers. An example of one suitable chamber is set forth below and discussed with respect to FIGS. 8A and 8B, which are vertical, cross-sectional views of a CVD system 110, having a vacuum or processing chamber 115 that includes a chamber wall 115a and chamber lid assembly 115b. 
CVD system 110 contains a gas distribution manifold 111 for dispersing process gases to a substrate (not shown) that rests on a heated pedestal 112 centered within the process chamber. During processing, the substrate (e.g. a semiconductor wafer) is positioned on a flat (or slightly convex) surface 112a of pedestal 112. The pedestal can be moved controllably between a lower loading/off-loading position (depicted in FIG. 8A) and an upper processing position (indicated by dashed line 114 in FIG. 8A and shown in FIG. 8B), which is closely adjacent to manifold 111. A centerboard (not shown) includes sensors for providing information on the position of the wafers.
Deposition and carrier gases are introduced into chamber 115 through perforated holes of a conventional flat, circular gas distribution or faceplate 113a. More specifically, deposition process gases flow into the chamber through the inlet manifold 111 (indicated by arrow 140 in FIG. 8B), through a conventional perforated blocker plate 142 and then through holes 113b in gas distribution faceplate 113a. 
Before reaching the manifold, deposition and carrier gases are input from gas sources 107 through gas supply lines 108 (FIG. 8B) into a mixing system 109 where they are combined and then sent to manifold 111. Generally, the supply line for each process gas includes (i) several safety shut-off valves (not shown) that can be used to automatically or manually shut-off the flow of process gas into the chamber, and (ii) mass flow controllers (also not shown) that measure the flow of gas through the supply line. When toxic gases are used in the process, the several safety shut-off valves are positioned on each gas supply line in conventional configurations.
The deposition process performed in CVD system 110 can be either a thermal process or a plasma-enhanced process. In a plasma-enhanced process, an RF power supply 144 applies electrical power between the gas distribution faceplate 113a and the pedestal so as to excite the process gas mixture to form a plasma within the cylindrical region between the faceplate 113a and the pedestal. (This region will be referred to herein as the “reaction region”). Constituents of the plasma react to deposit a desired film on the surface of the semiconductor wafer supported on pedestal 112. RF power supply 144 is a mixed frequency RF power supply that typically supplies power at a high RF frequency (RF1) of 13.56 MHz and at a low RF frequency (RF2) of 360 KHz to enhance the decomposition of reactive species introduced into the vacuum chamber 115. In a thermal process, RF power supply 144 would not be utilized, and the process gas mixture thermally reacts to deposit the desired films on the surface of the semiconductor wafer supported on pedestal 112, which is resistively heated to provide thermal energy for the reaction.
During a plasma-enhanced deposition process, the plasma heats the entire process chamber 110, including the walls of the chamber body 115a surrounding the exhaust passageway 123 and the shut-off valve 124. When the plasma is not turned on or during a thermal deposition process, a hot liquid is circulated through the walls 115a of the process chamber to maintain the chamber at an elevated temperature. A portion of these heat exchanging passages 118 in the lid of chamber 110 is shown in FIG. 8B. The passages in the remainder of chamber walls 115a are not shown. Fluids used to heat the chamber walls 115a include the typical fluid types, i.e., water-based ethylene glycol or oil-based thermal transfer fluids. This heating (referred to as heating by the “heat exchanger”) beneficially reduces or eliminates condensation of undesirable reactant products and improves the elimination of volatile products of the process gases and other contaminants that might contaminate the process if they were to condense on the walls of cool vacuum passages and migrate back into the processing chamber during periods of no gas flow.
The remainder of the gas mixture that is not deposited in a layer, including reaction byproducts, is evacuated from the chamber by a vacuum pump (not shown). Specifically, the gases are exhausted through an annular, slot-shaped orifice 116 surrounding the reaction region and into an annular exhaust plenum 117. The annular slot 116 and the plenum 117 are defined by the gap between the top of the chamber's cylindrical side wall 115a (including the upper dielectric lining 119 on the wall) and the bottom of the circular chamber lid 120. The 360° circular symmetry and uniformity of the slot orifice 116 and the plenum 117 help achieve a uniform flow of process gases over the wafer so as to deposit a uniform film on the wafer.
From the exhaust plenum 117, the gases flow underneath a lateral extension portion 121 of the exhaust plenum 117, past a viewing port (not shown), through a downward-extending gas passage 123, past a vacuum shut-off valve 124 (whose body is integrated with the lower chamber wall 115a), and into the exhaust outlet 125 that connects to the external vacuum pump (not shown) through a foreline (also not shown).
The wafer support platter of the pedestal 112 (preferably aluminum, ceramic, or a combination thereof) is resistively-heated using an embedded single-loop embedded heater element configured to make two full turns in the form of parallel concentric circles. An outer portion of the heater element runs adjacent to a perimeter of the support platter, while an inner portion runs on the path of a concentric circle having a smaller radius. The wiring to the heater element passes through the stem of the pedestal 112.
Typically, any or all of the chamber lining, gas inlet manifold faceplate, and various other reactor hardware are made out of material such as aluminum, anodized aluminum, or ceramic. An example of such a CVD apparatus is described in U.S. Pat. No. 5,558,717 entitled “CVD Processing Chamber,” issued to Zhao et al. The U.S. Pat. No. 5,558,717 patent is assigned to Applied Materials, Inc., the assignee of the present invention, and is hereby incorporated by reference in its entirety.
A lift mechanism and motor 132 (FIG. 8A) raises and lowers the heater pedestal assembly 112 and its wafer lift pins 112b as wafers are transferred into and out of the body of the chamber by a robot blade (not shown) through an insertion/removal opening 126 in the side of the chamber 110. The motor 132 raises and lowers pedestal 112 between a processing position 114 and a lower, wafer-loading position. The motor, valves or flow controllers connected to the supply lines 108, gas delivery system, throttle valve, RF power supply 144, and chamber and substrate heating systems are all controlled by a system controller 134 (FIG. 8B) over control lines 136, of which only some are shown. Controller 134 relies on feedback from optical sensors to determine the position of movable mechanical assemblies such as the throttle valve and susceptor which are moved by appropriate motors under the control of controller 134.
System controller 134 controls all of the activities of the CVD machine. The system controller executes system control software, which is a computer program stored in a computer-readable medium such as a memory 138. Preferably, memory 138 is a hard disk drive, but memory 138 may also be other kinds of memory. The computer program includes sets of instructions that dictate the timing, mixture of gases, chamber pressure, chamber temperature, RF power levels, susceptor position, and other parameters of a particular process. Other computer programs stored on other memory devices including, for example, a floppy disk or other another appropriate drive, may also be used to operate controller 134.
The above reactor description is mainly for illustrative purposes, and other types of plasma CVD equipment may be employed to form PSG optical cores. Additionally, variations of the above-described system, such as variations in pedestal design, heater design, RF power frequencies, location of RF power connections and others are possible. For example, the wafer could be supported by a susceptor and heated by quartz lamps. The layer and method for forming such a layer of the present invention is not limited to any specific apparatus or to any specific plasma excitation method.
3. Further Exemplary Embodiments
Another suitable process recipe for depositing an PSG optical core on a 200 mm substrate using a PECVD process according to one embodiment of the present invention is summarized in Table 1 below:










TABLE 2







Process Parameter
Value



















F(SiH4) (sccm)
100



F(N2O) (sccm)
3600



5 PH3 in 95% N2 (sccm)
220



Source RF Density - (Watts)
400-470



Pressure (millitorr)
2.8



Temp (° C.)
400-550










The process in Table 2 may be carried out in a PECVD system similar to system 110 described above. For example, if system 110 is used, a ceramic heater and nitride faceplate may be used. The spacing between the pedestal surface and manifold may be set to 565 mils.
In order to better appreciate and understand the present invention, exemplary data derived from the process of Table 2 is set forth below. FIG. 9 shows experimental data illustrating the relationship between phosphorous concentration and PH3 flow for temperatures of 400° C. and 550° C. The data illustrates that the phosphorous concentration changes with PH3 flow according to an approximately linear relationship. Furthermore, it can be seen that more phosphorous is incorporated into the optical core at lower deposition temperatures.
FIG. 10 shows experimental data illustrating the relationship between PH3 flow and refractive index for temperatures of 400° C. and 550° C. For a flow greater than 600 sccm, the RF power was increased from 400 W to 470 W. The refractive index can be seen to increase with the PH3 flow rate as the phosphorous concentration increases in the optical core (See FIG. 9).
As mentioned above, a post deposition anneal step may be added to further improve the optical qualities of the core. FIG. 11 illustrates an anneal step according to one embodiment of the invention. The material is soaked in a steam ambient at a temperature of about 1050° C. for about 1-2 hours. The loading temperature is about 450° C., with a ramp up time of about 66 minutes. After the soaking time, the temperature is ramped back down to an unloading temperature of about 600° C. over the course of about 77 minutes. Thus, the total load-unload time may be between about 3 hours and 23 minutes to 4 hours and 23 minutes.
FIG. 12 illustrates the relationship of phosphorous content and steam anneal on the refractive index of a 7 μm thick PSG films at 400° C. and 550° C. according to one embodiment of the present invention. The refractive index of the PSG optical core can be seen to increase with phosphorous content and deposition temperature. Post anneal refractive index also increases with phosphorous content according to an approximately linear relation. However, the refractive index is substantially independent of deposition temperature.
FIG. 13 illustrates the relationship of phosphorous content and steam anneal on the refractive index range (i.e., uniformity) of a 7 μm thick PSG film deposited at 400° C. and 550° C. according to one embodiment of the present invention. At 400° C., the refractive index varied between 0.0006 to 0.0011 before anneal and 0.0004 to 0.0008 post anneal, which corresponds to a post anneal uniformity of about ±0.0002. At 550° C., the refractive index varied between 0.0005 to 0.001 before anneal and 0.0004 to 0.0011 post anneal, which corresponds to a post anneal uniformity of about ±0.0004. Thus, the process of Table 2 has been used to obtain a uniformity of at least ±0.0004, which is suitable for optical applications.
FIG. 14 illustrates the effect of multiple steam anneal cycles on the refractive index range of a 7 μm thick PSG film embodiments of the present invention. While multiple anneal cycles do not substantially change the refractive index, the uniformity can be seen to improve after two cycles.
FIG. 15 illustrates the effect of phosphorous content and steam anneal on stress. FIG. 16 illustrates the effect of multiple anneal cycles on stress. As the phosphorous content is increased the PSG film becomes less compressive. While the PSG film becomes more compressive after one anneal cycle for lower phosphorous concentrations, FIG. 16 illustrates that stress is generally reduced for multiple anneal cycles. In fact, PSG optical cores having phosphorous concentrations levels above about 6.5 wt. % achieved stresses of less than −100 MPa. As evident from FIGS. 15 and 16, the stress of the PSG film will increase after anneal as shown in FIG. 15, but more annealing cycles will cause the stress to decrease as shown in FIG. 16.
FIG. 17 illustrates the effect of phosphorous content and steam anneal on the change in thickness of PSG films. Steam anneal will not result in a substantive change in the thickness of a 7 μm PSG optical core. However, steam anneal will cause a substantive change in the thickness of a 1 μm PSG layer. As the phosphorous content is increased, the change in thickness on a 1 μm PSG layer is exacerbated. For phosphorous content between 1 wt. % to 10 wt. %, the PSG layer experienced a change in thickness of about 8-17%. It is believed that the change in thickness is due to the growth of thermal oxide between the PSG layer and underlying silicon materials. Higher phosphorous concentrations in the PSG layer can lead to greater changes in thickness because H2O can diffuse faster in more heavily doped PSG material than in less heavily doped material. Accordingly, thinner PSG structures will be more susceptible to changes in thickness, and must have such changes taken into account during the routine design process of the particular PSG optical core as would be known to those skilled in the art.
FIG. 18 illustrates a PECVD process recipe 1800 for depositing a PSG optical core according to another embodiment of the present invention. Some of the primary control variable listed along the column 1801 include step time, pressure, RF power, heater temperature, heater spacing, and flow rates. Process recipe includes a post deposition termination step wherein all deposition source gases are turned off except the oxygen containing gas (e.g., the silicon containing gas and phosphorous containing gas are turned off) so that optical imperfections in the PSG optical core are reduced.
FIG. 18 illustrates a method of forming a PSG optical core using a PECVD process according to one embodiment of the present invention. At step 1810, the PECVD deposition chamber is preheated to 550° C. At step 1820, an oxygen containing gas such as N2O is introduced into the chamber at a flow rate of 3600 sccm. At step 1830, a silicon containing gas, such as SiH4, and a phosphorous containing gas, such as 5% PH3 diluted in N2, are introduced into the deposition chamber at flow rates of 100 sccm and 220 sccm, respectively. At step 1840, an RF power of 400 Watts is applied to create a plasma in the deposition chamber, and a PSG optical core is deposited on a substrate located in the chamber. After an amount of time corresponding to the desired thickness of the optical core (e.g., 135 seconds), the silicon containing gas and phosphorous containing gas are evacuated from the chamber at step 1850 (i.e., the silicon and phosphorous sources are turned off). However, the oxygen containing gas, N2O, is maintained in the chamber to reduce optical imperfections (i.e., hazing) in the PSG optical core. At step 1860, the RF power is turned off, and the oxygen containing gas is evacuated from the chamber.
Having fully described several embodiments of the present invention, many other equivalent or alternative methods of forming a PSG optical waveguide core using a PECVD process according to the present invention will be apparent to those skilled in the art. These alternatives and equivalents are intended to be included within the scope of the present invention.

## Claims

1. A method of forming an optical waveguide, the method comprising:
forming an undercladding layer over a substrate;
depositing a phosphorus-doped silicate glass core layer over the undercladding layer with a plasma-enhanced chemical-vapor-deposition process, wherein depositing the core layer comprises:
disposing the substrate in a substrate processing chamber;
flowing a process gas into the substrate processing chamber, the process gas comprising a silicon source, a first oxygen source, and a phosphorus source;
forming a first plasma from the process gas to deposit the phosphorus-doped silicate glass core over the undercladding layer, wherein the phosphorus-doped silicate glass core has a thickness with a thickness greater than 4.5 μm; and
exposing the phosphorus-doped silicate glass core layer to a second plasma, wherein the second plasma is formed from gas that includes a second oxygen source and does not include a silicon source;

etching the phosphorus-doped silicate glass core layer to produce a plurality of phosphorus-doped silicate glass cores separated by at least 1 μm; and
depositing an uppercladding layer over the phosphorus-doped silicate glass cores and in gaps between the phosphorus-doped silicate glass cores.

2. The method recited in claim 1 wherein depositing the phosphorus-doped silicate glass core layer further comprises thermally annealing the phosphorus-doped silicate glass core layer at a temperature greater than 1000° C.

3. The method recited in claim 2 wherein thermally annealing the phosphorus-doped silicate glass core layer is performed in a steam atmosphere.

4. The method recited in claim 1 wherein the phosphorus source comprises PH3.

5. The method recited in claim 4 wherein the phosphorus source is diluted with N2.

6. The method recited in claim 5 wherein the phosphorus source comprises approximately 5% PH3 diluted with 95% N2.

7. The method recited in claim 1 wherein the process gas has a ratio of oxygen atoms to silicon atoms greater than 20:1.

8. The method recited in claim 1 wherein the undercladding layer comprises substantially undoped silicate glass.

9. The method recited in claim 1 wherein the uppercladding layer comprises boron- and phosphorus-doped silicate glass.

10. A method of forming an optical waveguide, the method comprising:
forming an undercladding layer over a substrate;
etching a plurality of trenches within the undercladding layer, each of the plurality of trenches having a depth greater than 4.5 μm and less than a thickness of the undercladding layer;
depositing phosphorus-doped silicate glass cores within the trenches with a plasma-enhanced chemical-vapor-deposition process, wherein depositing the phosphorus-doped silicate glass cores comprises:
disposing the substrate in a substrate processing chamber;
flowing a process gas into the substrate processing chamber, the process gas comprising a silicon source, a first oxygen source, and a phosphorus source;
forming a first plasma from the process gas to deposit the phosphorus-doped silicate glass cores within the trenches; and
exposing the phosphorus-doped silicate glass cores to a second plasma, wherein the second plasma is formed from gas that includes a second oxygen source and does not include a silicon source; and

depositing an uppercladding layer over the phosphorus-doped silicate glass cores.

11. The method recited in claim 10 wherein depositing the phosphorus-doped silicate glass cores further comprises thermally annealing the phosphorus-doped silicate glass cores at a temperature greater than 1000° C.

12. The method recited in claim 11 wherein thermally annealing the phosphorus-doped silicate glass cores is performed in a steam atmosphere.

13. The method recited in claim 10 wherein the phosphorus source comprises PH3.

14. The method recited in claim 13 wherein the phosphorus source is diluted with N2.

15. The method recited in claim 14 wherein the phosphorus source comprises approximately 5% PH3 diluted with 95% N2.

16. The method recited in claim 10 wherein the process gas has a ratio of oxygen atoms to silicon atoms greater than 20:1.

17. The method recited in claim 10 wherein the undercladding layer comprises substantially undoped silicate glass.

18. The method recited in claim 10 wherein the uppercladding layer comprises boron- and phosphorus-doped silicate glass.