Manufacture of silica waveguides with minimal absorption

Details Manufacture of silica waveguides with minimal absorption Manufacture of silica waveguides with minimal absorption

2001-03-07

2003-03-25

Meeks, Timothy (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Plasma

C427S163200, C427S167000, C427S255370, C427S379000, C427S397700, C385S129000, C385S130000

Reexamination Certificate

active

06537623

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved method for manufacturing silica waveguides with minimal absorption.
BACKGROUND OF THE INVENTION
The manufacture of optical devices which employ silica waveguides, such as optical Multiplexers (Mux) and Demultiplexers (Dmux), entails depositing silica films onto a silicon wafer. The silica films are ideally of optical quality, characterised in that they are transparent in the 1.30 &mgr;m bi-directional narrow optical band and/or in the 1.55 &mgr;m video signal optical band. Such optical quality silica films are extremely difficult to produce in reality because hydrogen and nitrogen atoms are typically present in the films. These impurities in the silica films result in excessive optical absorption in the 1.30 and 1.55 &mgr;m optical bands.
Fourier Transform Infrared (FTIR) spectroscopy can be used to monitor the quality of optical silica films. The FTIR spectra of optical quality silica films, containing no undesirable optical absorption peaks, are characterised by the presence of only four fundamental optical absorption peaks: (1) an intense and small Full Width at Half Maximum (FWHM) Si—O—Si “rocking mode” absorption peak ranging between 410 and 510 cm
−1
, centred at 460 cm
−1
(21.739 &mgr;m); (2) a small FWHM Si—O—Si “bending mode” absorption peak ranging between 740 and 880 cm
−1
, centred at 810 cm
−1
(12.346 &mgr;m); (3) an intense and small Full Width at Half Maximum (FWHM) Si—O—Si “in-phase-stretching mode” absorption peak ranging between 1000 and 1160 cm
−1
, centred at 1080 cm
−1
(9.256 &mgr;m) indicating stoichiometric silica films with the optimum Si—O—Si bond angle of 144° and the optimum density; and (4) an almost eliminated Si—O—Si “out-of-phase-stretching mode” absorption peak ranging between 1080 and 1280 cm
−1
, centred at 1180 cm
−1
(8.475 &mgr;m), as compared to the Si—O—Si in-phase-stretching mode absorption peak.
The position in the infrared spectrum of these four fundamental (first mode) infrared absorption peaks, respectively centered at 21.739 &mgr;m, 12.346 &mgr;m, 9.256 &mgr;m, and 8.475 &mgr;m, is far away from the infrared bands of interest at 1.30 and 1.55 &mgr;m. However, residual absorption of optical quality silica is never completely eliminated because the higher harmonics of these four residual optical absorption peaks do cause small residual optical absorption peaks in the 1.30 and 1.55 &mgr;m optical band. The very high harmonics (i.e. very little absorption effect) falling within this range are: the sixth (1.302 to 1.543 &mgr;m) and seventh (1.116 to 1.323 &mgr;m) harmonics of the Si—O—Si “out-of-phase-stretching mode” infrared absorption peak; the sixth (1.437 to 1.667 &mgr;m) and seventh (1.232 to 1.429 &mgr;m) harmonics of the Si—O—Si “in-phase-stretching mode” infrared absorption peak; the eighth (1.420 to 1.689 &mgr;m) and ninth (1.263 to 1.502 &mgr;m) harmonics of the Si—O—Si “bending mode” infrared absorption peak; and the thirteenth (1.508 to 1.876 &mgr;m) and fourteenth (1.401 to 1.742 &mgr;m) and fifteenth (1.307 to 1.626 &mgr;m) harmonics of the Si—O—Si “rocking mode” infrared absorption peak.
The FTIR spectra of optical quality silica films are also characterised by a net separation between the Si—O—Si “in-phase-stretching mode” absorption peak (1080 cm
−1
) and the Si—O—Si “bending mode” absorption peak (810 cm
−1
) with a deep valley between 850 and 1000 cm
−1
.
Silica films may be deposited onto a silicon wafer using a silane (SiH
4
) and nitrous oxide (N
2
O) gas mixture at a low temperature according to the following reaction:
SiH
4
(
g
)+2N
2
O(
g
)→SiO
2
+2N
2
(
g
)+2H
2
(
g
)
Theoretically, it is possible to achieve optical quality silica films from this reaction. However, in reality, numerous side reactions occur, forming a mixture of undesirable Si—O
x
—H
y
—N
z
compounds. For example,
FIG. 1
presents the various potential Si—O
x
—H
y
—N
z
compounds that may result from the combination of silane (SiH
4
) and nitrous oxide (N
2
O) gases. It shows 35 products that could be found in silica films deposited from a silane (SiH
4
) and nitrous oxide (N
2
O) gas mixture. N
2
, O
2
, HNO, NH
3
, H
2
O, and H
2
gaseous by-products are eliminated from the surface or from the micro-pores of the silica films during these chemical reactions. As a result of the production of these side-products, the incorporation of oxygen atoms, a key factor to achieve optical quality silica, competes with the incorporation of nitrogen and hydrogen atoms in the silica films. Thus, the silica films as deposited on the silicon wafer are not optical quality silica films, due to the absorption by the undesirable Si—O
x
—H
y
—N
z
compounds formed.
To resolve this problem caused by Si—O
x
—H
y
—N
z
impurities in the films, techniques have been used wherein the silica films are subject to a high temperature (typically, between 600 and 1350° C.) thermal treatment under vacuum, argon (Ar), or a nitrogen atmosphere as a means for reducing the optical absorption of silica films in the 1.30 and 1.55 &mgr;m optical regions. In general, the higher the temperature of this high temperature thermal treatment, the lower the optical absorption of the silica films. However, unlike fused silica optical fibres, that are heated at a temperature exceeding about 2000° C. during the drawing process, the high temperature thermal treatment of the silica films on silicon wafers is performed at a temperature ranging from 600° C. to a maximum temperature of about 1350° C., close to the fusion point of the silicon wafer. The temperature is typically limited by the high compressive mechanical stress induced in the silica films from the difference of thermal expansion between the silica films and the underlying silicon wafer. This temperature limitation results in silica films with undesirable residual infrared oscillators and in their associated undesirable residual optical absorption peaks in the 1.30 and 1.55 &mgr;m wavelength optical bands.
Thus, using a high temperature thermal treatment in the presence of nitrogen on the thirty-five products of silane and nitrous oxide given in
FIG. 1
, results in a maximum of only twelve of the thirty-five potential Si—O
x
—H
y
—N
z
products being converted to SiO
2
. The same twelve compounds could also lead to the formation of SiO
2
in an inert (Ar) atmosphere or under vacuum, since in none of these twelve chemical reactions is nitrogen incorporated.
Following a high temperature thermal treatment in a nitrogen atmosphere, the other twenty-three Si—O
x
—H
y
—N
z
potential compounds can lead to the formation of: SiNH, SiN
2
, SiOH
2
, SiONH, and SiON
2
. Therefore, high temperature thermal treatments under nitrogen, argon, or in a vacuum are incapable of transforming twenty-three potential initial Si—O
x
—H
y
—N
z
products formed from the reaction of silane and nitrous oxide into SiO
2
. Thus, the silica films that result from these high temperature thermal treatments under nitrogen, argon, or in a vacuum are composed not only of SiO
2
, but are solid mixtures of six possible compounds: SiO
2
, SiNH, SiN
2
, SiOH
2
, SiONH and SiON
2
. Gaseous by-products that result from the thermal decomposition of silica films are: nitrogen (N
2
), hydrogen (H
2
), and ammonia (NH
3
).
FIG. 3
lists some FTIR fundamental infrared absorption peaks and their corresponding higher harmonic peaks associated with SiO
2
, SiNH, SiN
2
, SiOH
2
, SiONH, and SiON
2
. The higher harmonics of the absorption peaks corresponding to these six residual potential compounds contribute to the optical absorption in the 1.30 and 1.55 &mgr;m optical bands, as follows: the second vibration harmonics of the HO—H oscillators in trapped water vapour in the micro-pores of the silica films (3550 to 3750 cm
−1
) increases the optical absorption near 1.333 to 1.408 &mgr;m; the second vibration harmonics of the SiO—H oscillators in the silica films (3470 to 3550 cm
−1
) increases the optical absorption near 1.408

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