Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-10-26
2002-09-10
Cuneo, K. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S255000, C361S777000, C257S774000
Reexamination Certificate
active
06448505
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a substrate which is mountable with an optical component such as optical semiconductor device and optical device including optical fiber and lens, a method for producing such a substrate, and an optical module using such a substrate.
In recent years, there has been the demand for larger capacity and more functions of optical communication systems and, accordingly, miniaturization, higher integration and lower production costs have been required for optical devices such as optical transmitters and optical receivers. Particularly, in order to reduce the assembling cost of optical devices, attention has been given to the technique for mounting optical components such as optical semiconductor devices, optical fibers and lenses on a single substrate, in particular, to the so-called passive alignment technique such as an optical hybrid mounting technique, and an alignment-free mounting technique using a silicone platform.
According to the above techniques, an optical axis adjustment and an optical coupling can be established without an aligning operation only by mounting optical components on the substrate with respect to a positioning groove formed in the substrate or mounting optical components in mounting grooves formed in the substrate. This enables a significant reduction in the assembling costs. In order to mount optical components without aligning operation, the optical component mounting grooves formed in the substrate are required to have highly precise dimensions of top opening width and depth. Further, the relative positional relationship between the grooves is required to be precise in the order of sub-micrometers or less than one millionth of a meter.
However, there has been the big problem that a plurality of grooves whose respective depths are different from each other cannot be simultaneously made by a single etching operation for the following reasons.
Generally, in the case where a groove is made in a substrate made of a silicon monocrystal by an etching, there will occur an undercut
100
below the non-patterned photoresist layer
200
as shown in FIG.
8
. More specifically, in the case that a V-shaped groove is formed in a substrate made of a silicon monocrystal having a (100) surface as a principle surface by anisotropic etching using an alkaline solution (etching solution: KOH (43 vol %) solution, solution temperature 60° C.), an undercut occurs in proportion to the depth of etching as can be seen from a graph of FIG.
9
.
The undercut amount represented by the vertical axis in
FIG. 9
means a difference between a designed pattern width and an actual top opening width (V-shaped groove width) of a V-shaped groove after etching. It will be seen from
FIG. 8
that the undercut amount (2L) is calculated by subtracting a designed width (DW) of the patterned hole from an actual width (AW) of the V-shaped groove, that is, 2L=AW−DW. To make a groove having a depth of 800 &mgr;m, for example, it is necessary to apply etching for approximately 50 hours, and an undercut of about 30 &mgr;m occurs during this etching. It means that in the case of making a groove having a final depth of 30 &mgr;m, the designed pattern width should be at 0 &mgr;m. This is to say that a groove having a width of 30 &mgr;m or smaller cannot be practically made. The occurrence of undercuts will make it impossible to form a plurality of grooves having different depths in a single substrate by a single etching.
Further, in the case of making a plurality of grooves having trapezoidal cross sections and different depths to their bottom surfaces, it has been difficult to simultaneously form them by a single etching.
In view of this problem, a substrate for mounting an optical component (hereinafter, referred to as “optical component mounting substrate”) which is formed with different depth grooves has been conventionally produced by steps shown in
FIGS. 10A
to
10
E. In order to facilitate the description, in these figures, forming of two V-shaped grooves in a substrate will be described.
First, as shown in
FIG. 10A
, a protection film
52
made of a silicon oxide or a silicon nitride is formed on the entire top surface of a substrate
51
made of a silicon monocrystal. The protection film
52
is resistant to an etching solution for etching the substrate
51
. Further, a photoresist film
53
is formed on the entire top surface of the protection film
52
. Patterning is applied to the photoresist film
53
by photolithography using a photomask for part of a V-shaped groove to be formed in a first etching. Thereafter, the part of the protection film
52
which corresponds to the V-shaped groove is also removed by a silicon oxide etching solution.
Subsequently, as shown in
FIG. 10B
, a V-shaped groove
54
is formed by etching the surface part where the substrate
51
is exposed using an etching solution containing sodium hydroxide (NaOH), potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), etc.
Subsequently, as shown in
FIG. 10C
, a protection film
55
made of a silicon oxide or a silicon nitride is formed on the entire top surface of the substrate
51
including the V-shaped groove
54
after the protection film
52
is entirely removed, and a photoresist film
56
is formed on the entire top surface of the protection film
55
. A photomask for a V-shaped groove to be formed in a second etching is positioned with reference to a marker provided on the substrate
51
to position the photomask, and patterning is applied to the photoresist film
56
by photolithography. Thereafter, the part of the protection film
55
which corresponds to the groove to be formed by the second etching is removed.
Subsequently, as shown in
FIG. 10D
, a groove
57
is formed by the same procedure as the V-shaped groove
54
, and the protection film
55
is finally removed to produce an optical component mounting substrate J
1
formed with the grooves as shown in FIG.
10
E.
In the above-mentioned conventional method, it will be apparent that high-precision patterning cannot be accomplished for the second formed photoresist film
56
and protection film
55
because of the presence of the V-shaped groove
54
formed in the first etching. In view of this problem, Japanese Unexamined Patent Publication No. 3-132031 proposes making of a plurality of V-shaped grooves having different depths by applying patterning only to the top flat surface of the substrate. However, this proposal cannot successfully eliminate the likelihood that the position of a second-placed photomask is different from that of a first-placed photomask, which causes undesired displacement of V-shaped grooves having different depths.
Specifically, a reference marker is provided at a specified position of the substrate, e.g., at an end of the substrate. In the first and second etchings, the photomask is arranged over the substrate by positioning a marker provided on the photomask with respect to the reference marker on the substrate. The photoresist film or protection film is likely to have an irregular thickness around the previously-formed groove, which makes it difficult to distinguish the marker provided on the substrate. Further, an exposure device, e.g., a contact-type exposure device, is usually incapable of positioning in the order of sub-microns.
Even if the above problems could be cleared, the substrate is liable to be warped due to a heat history since the positioning marker provided on the substrate have been subjected to various heat treatment processes. This warping causes shift of the positioning marker.
It will be seen to be extremely difficult to make the marker provided on the photomask agree with the marker provided on the substrate. Accordingly, in the above-mentioned conventional method, the greater the number of grooves having different depths becomes, the greater the number of placing photomask becomes, which consequently accumulates displacements of markers, and finally results in the unacceptable disagreement among grooves.
SUMMARY OF THE INVENTION
Hiraoka Michiaki
Nakashima Keiko
Takemura Koji
Alcalá José H.
Cuneo K.
Hogan & Hartson LLP
Kyocera Corporation
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