Method of reducing stress-induced mechanical problems in optical components

Abstract

A method of making optical quality films is described. A silica film is deposited on a wafer by PECVD (Plasma Enhanced Chemical Vapor Deposition). The deposited film is then subjected to a first heat treatment to reduce optical absorption, wafer warp, and compressive stress. A second film is deposited. This step is then followed by a second heat treatment to reduce optical absorption, wafer warp and tensile stress. The two heat treatments have similar temperature profiles.

We claim:  
     
         1 . A method of making optical quality films, comprising the steps of: 
 depositing a first silica film on a wafer by PECVD (Plasma Enhanced Chemical Vapor Deposition);    subjecting the wafer to a first heat treatment to reduce optical absorption, wafer warp, and compressive stress;    depositing a second silica film on the wafer by PECVD; and    subsequently subjecting the wafer to a second heat treatment to reduce optical absorption, wafer warp and tensile stress.    
     
     
         2 . A method as claimed in  claim 1 , wherein the first heat treatment follows a predetermined temperature profile.  
     
     
         3 . A method as claimed in  claim 2 , wherein said first heat treatment comprises a first phase in which said wafer is stabilized at a first predetermined temperature, a second phase in which the temperature is ramped up to a second predetermined temperature, a third phase in which the temperature is maintained at said second predetermined temperature, a fourth phase in which the temperature is ramped down to a final temperature, and a fifth phase in which the wafer is stabilized at said final temperature.  
     
     
         4 . A method as claimed in  claim 3 , wherein the duration of said first phase lies in the range 1.3 to 130 minutes.  
     
     
         5 . A method as claimed in  claim 3 , wherein the duration of said first phase is about 13 minutes.  
     
     
         6 . A method as claimed in  claim 3 , wherein the temperature in said second phase is ramped up at a rate lying in the range 1° C./min to 25° C./min . . . .  
     
     
         6 . A method as claimed in  claim 3 , wherein the temperature in said second phase is ramped up at 5° C./min.  
     
     
         7 . A method as claimed in  claim 3 , wherein said first predetermined temperature lies in the range 300° C. to 700° C.  
     
     
         8 . A method as claimed in  claim 7 , wherein said first predetermined temperature is about 400° C.  
     
     
         9 . A method as claimed in  claim 8 , wherein the temperature in said fourth phase is ramped down at a rate in the range 1° C./min. to 25° C./min.  
     
     
         10 . A method as claimed in  claim 9 , wherein the temperature in said fourth phase is ramped down at 2.5° C./min.  
     
     
         11 . A method as claimed in  claim 3 , wherein said second predetermined temperature lies in the range 800° C. to 1,300° C.  
     
     
         12 . A method as claimed in  claim 11 , wherein said second predetermined temperature is about 900° C.  
     
     
         13 . A method as claimed in  claim 1 , wherein said first and second heat treatments are carried out in the presence of an inert gas.  
     
     
         14 . A method as claimed in  claim 1 , wherein said inert gas is selected from the group consisting of: nitrogen, N 2 , oxygen, O 2 , hydrogen, H 2 , water vapour, H 2 O, argon, Ar, fluorine, F 2 , carbon tetrafluoride, CF 4 , nitrogen trifluoride, NF 3 , and hydrogen peroxide, H 2 O 2    
     
     
         15 . A method as claimed in  claim 13 , wherein the flow rate of said inert gas is constant.  
     
     
         16 . A method as claimed in  claim 15 , wherein the flow rate of said inert gas lies in the range 1 liter/min. to 100 liters/min.  
     
     
         17 . A method as claimed in  claim 3 , wherein the second heat treatment follows a predetermined temperature profile.  
     
     
         18 . A method as claimed in  claim 17 , wherein said second profile follows the same form as said first profile.  
     
     
         19 . A method as claimed in  claim 10 , wherein deposition is carried out in a seven-dimensional space wherein the flow rates of raw material gas, oxidation gas, carrier gas and dopant gas are set at fixed values, the total deposition pressure is set at a fixed value, a post-deposition thermal treatment is carried out at a temperature selected from a group of predetermined temperatures, and the observed FTIR characteristics of the resulting product are used to determine the post deposition thermal treatment temperature.  
     
     
         20 . A method as claimed in  claim 19 , wherein a first independent variable, the SiH 4  flow, is fixed at about 0.20 std litre/min; a second independent variable, the N 2 O flow, is fixed at about 6.00 std litre/min; a third independent variable, the N 2  flow, is fixed at about 3.15 std litre/min; a fourth independent variable, the PH 3  flow, is fixed at about 0.50 std litre/min; a fifth independent variable, the total deposition pressure, is fixed at about 2.60 Torr; a sixth independent variable, the post-deposition thermal treatment is varied among the following choices: 30 minutes duration thermal treatment in a nitrogen ambient at 600° C.; 30 minutes duration thermal treatment in a nitrogen ambient at 700° C.; 30 minutes duration thermal treatment in a nitrogen ambient at 750° C.; 30 minutes duration thermal treatment in a nitrogen ambient at 800° C.; 30 minutes duration thermal treatment in a nitrogen ambient at 850° C.; 30 minutes duration thermal treatment in a nitrogen ambient at 900° C.  
     
     
         21 . A method as claimed in  claim 1 , wherein the first layer is a silica buffer layer and the second layer is a silica core layer.  
     
     
         22 . A method as claimed in  claim 21 , wherein a second buffer layer, symmetrical with said first-mentioned buffer layer, is deposited on the back side of the wafer.  
     
     
         23 . A method as claimed in  claim 22 , wherein a protective layer is deposited on the back face of the buffer layer on the back side of the wafer and a compensating layer is deposited on the front face of the wafer.  
     
     
         24 . A method as claimed in  claim 24 , wherein the protective layer and compensating layer are silicon nitride.  
     
     
         25 . A method of making a photonic device by PECDV (Plasma Enhanced Chemical Vapor Deposition) comprising: 
 a) depositing a thick first silica buffer layer on the back side of a wafer;    b) depositing a thick silica buffer layer on the front side of said wafer;    c) subjecting the wafer to a first heat treatment to reduce optical absorption, wafer warp, and compressive stress;    d) depositing a silica core layer;    e) subsequently to step d subjecting the wafer to a second heat treatment to reduce optical absorption, wafer warp and tensile stress; and    f) depositing a silica cladding layer on said silica core layer.    
     
     
         26 . A method as claimed in  claim 25 , wherein said first and second heat treatments follow a predetermined profile and comprises a first phase in which said wafer is stabilized at a first predetermined temperature, a second phase in which the temperature is ramped up to a second predetermined temperature, a third phase in which the temperature is maintained at said second predetermined temperature, a fourth phase in which the temperature is ramped down to a final temperature, and a fifth phase in which the wafer is stabilized at said final temperature.  
     
     
         27 . A method as claimed in  claim 26 , wherein a sacrificial layer is deposited on the front side of the wafer prior to step a, an etch protective layer is deposited on said buffer layer on the back side of said wafer, said sacrificial layer is removed after depositing said etch protective layer, and a compensating layer is deposited on the front face of the wafer prior to step b.  
     
     
         28 . A method as claimed in  claim 27 , wherein said sacrificial layer is silica.  
     
     
         29 . A method as claimed in  claim 28 , wherein said protective layer and said compensating layer are silicon nitride.  
     
     
         30 . A method as claimed in  claim 25 , wherein said photonic device is a deep-etched optical component.