# Patent

## Title
Conformal metallization process for the fabrication of semiconductor laser devices

## Abstract

A method of fabricating a semiconductor laser device by forming a semiconductor structure at least part of which is in the form of a mesa structure having a flat top. The steps include depositing a passivation layer over the mesa structure, forming a contact opening in the passivation layer on the flat top of the mesa structure; and depositing a metal contact portion, with the deposited metal contact portion contacting the semiconductor structure via the contact opening. The contact opening formed through the passivation layer has a smaller area than the flat top of the mesa structure to allow for wider tolerances in alignment accuracy. The metal contact portion comprises a platinum layer between one or more gold layers to provide an effective barrier against Au diffusion into the semiconductor material.

## Background
This application is a divisional of application Ser. No. 13/333,493, filed Dec. 21, 2011, which is incorporated herein by reference in its entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the fabrication of a semiconductor laser device, and in particular to the formation of a metal contact forming part of the semiconductor laser device.
2. The Background Art
Semiconductor laser devices are widely available. One type of semiconductor laser device is tuneable around the 1.5 μm wavelength for use in DWDM optical communications. Such semiconductor laser devices may be based on an InP semiconductor system.
One way of forming a p-metal contact on an InP semiconductor system is to deposit a lattice-matched InGaAs layer, followed by the deposition of layers of Titanium (Ti), Platinum (Pt) and Gold (Au) to form a so-called non-alloyed ohmic contact. The Au layer forms a contact pad, while the Pt layer acts as a barrier to inhibit diffusion of gold into the semiconductor structure. The article “Au/Pt/Ti contacts to p-In0.53Ga0.47As and n-InP layers formed by a single metallization common step and rapid thermal processing” by A. Katz et al, J. Appl. Phys. 68 (3), 1 Aug. 1990, the whole contents of which are hereby incorporated by reference, discusses the formation of such a contact by evaporating the Au bonding layer during a single deposition process with the Ti and Pt layers, followed by rapid thermal processing.
Sudden failure is a problem which can afflict semiconductor laser devices. One cause of such sudden failure is Au migration into the InP material, particularly as Au migrates rapidly in InP material.
SUMMARY OF THE INVENTION
The present invention aims to prevent sudden failure of semiconductor laser devices by providing a more effective barrier between the metal contact pad and the semiconductor material.
One aspect of the present invention provides a method of fabricating a semiconductor laser device having a semiconductor structure in the form of a mesa structure having a substantially flat top. A passivation layer is deposited over the mesa structure, and then a contact opening is formed in the portion of the passivation layer on the flat top of the mesa structure. A metal contact portion is then formed in the contact opening. The area of the contact opening is made less than the area of the flat top of the mesa structure. In this way, small misalignments during the fabrication process do not result in the metallic layers of the contact being deposited on the side surfaces of the mesa structure, which the inventors have identified as an issue in the sudden failure of semiconductor laser devices.
Another aspect of the invention provides a semiconductor laser device having a mesa structure with a flat top. A metal contact portion is formed in an opening in the passivation layer on the flat top of the mesa structure. The area of the metal contact portion is less than that of the flat top of the mesa structure.
In an embodiment, a layer of platinum of at least 80 nanometers is provided between the semiconductor structure and one or more gold layers forming part of the metal contact portion. In this way, Au diffusion into the semiconductor structure is impeded.
In an embodiment, the metal contact portion is formed by depositing, in a single deposition process: a layer of titanium so as to contact the semiconductor substrate through the contact opening; then a layer of platinum; then a layer of gold; then a layer of chromium; and then a second layer of gold. In this way, the uniformity of the deposited layers is improved, thereby reducing the possibility of Au migration into the semiconductor material by virtue of an inhomogeneity in the deposited metal contact portion.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic side view of a semiconductor laser device following the deposition of a passivation layer on a mesa-structure;
FIG. 2 is a schematic side view of a semiconductor laser device following the deposition and exposure of a photo-resist material on the semiconductor laser device of FIG. 1;
FIG. 3 is a schematic side view of a semiconductor laser device following an etching operation performed on the semiconductor laser device of FIG. 2;
FIG. 4 is a schematic side view of a semiconductor laser device following the removal of the photo-resist material remaining on the semiconductor laser device of FIG. 3;
FIG. 5 is a schematic side view of a semiconductor laser device following the deposition, exposure and etching of a second photo-resist material on the semiconductor laser device of FIG. 4;
FIG. 6 is a schematic side view of a semiconductor laser device following the deposition of a Ti layer on the semiconductor laser device of FIG. 5;
FIG. 7 is a schematic side view of a semiconductor laser device following the deposition of a Pt layer on the semiconductor laser device of FIG. 6;
FIG. 8 is a schematic side view of a semiconductor laser device following the deposition of a first Au layer on the semiconductor laser device of FIG. 7;
FIG. 9 is a schematic side view of a semiconductor laser device following the deposition of a Cr layer on the semiconductor laser device of FIG. 8;
FIG. 10 is a schematic side view of a semiconductor laser device following the deposition of a second Au layer on the semiconductor laser device of FIG. 9; and
FIG. 11 is a schematic side view of a semiconductor laser device following a lift-off process performed on the semiconductor laser device of FIG. 10.



DESCRIPTION OF THE PREFERRED EMBODIMENT
Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiments nor the relative dimensions of the depicted elements, and are not drawn to scale.
As shown in FIG. 1, in an embodiment of the present invention a mesa structure 1 forms part of a semiconductor laser device. In this embodiment, the semiconductor laser device is a tuneable laser in the frequency region around 1.5 μm fabricated using an indium Phosphide (InP) semiconductor system. In particular, the semiconductor laser device is formed by the deposition of lattice-matched layers on an InP substrate. FIGS. 1 to 11 schematically illustrate respective stages during a process to form a metal contact portion on the flat top of the mesa structure 1.
Initially, as shown in FIG. 1, a lattice-matched p-InGaAs layer 3 is deposited on top of the mesa-structure, and then a passivation layer 5 is formed over the mesa-structure 1. In this embodiment, the passivation layer 5 is formed by depositing a sequence of dielectric sub-layers. More specifically, the passivation layer 5 is formed by depositing a 100 nm SINx dielectric layer followed by depositing a 150 nm SiO2 dielectric layer.
Turning to FIG. 2, next photo resist material 7a,7b, in this embodiment PR-1827 photo-resist, is deposited, followed by a spinning process at 4000 rpm and a pre-bake at 115° C. for 1 minute. A contact mask is then applied and the photo-resist material is exposed for 14 seconds to form an exposed region 9. As shown in FIG. 2, the area of the exposed region 9 is less than the area of the flat top of the mesa structure 1. Further, the exposed region 9 is generally centred at the centre of the flat top of the mesa structure 1, so that the sidewalls of the mesa structure 1 are not covered by the exposed region 9. It will be appreciated that by making the area of the exposed region 9 less than the area of the flat top of the mesa structure 1, misalignment during the exposure process is less likely to result in the exposed portion 9 extending over any sidewall of the mesa structure 1.
With reference to FIG. 3, next an etching operation is performed to remove the exposed portion 9 of the photo-resist material and the portion of the passivation layer 5 covered by the exposed portion 9 of the photo-resist material. In this way, an opening is formed to a portion 11 of the p-InGaAs layer 3. In this embodiment, the exposed portion 9 is etched using MF319 developer, the SiO2 dielectric material is etched using Sirius Reactive Ion Etchant (RIE) for one hundred seconds, and the SiNx dielectric material is etched using Branson RIE for thirty-five seconds.
The remaining photo-resist material 7a,7b is then removed to form a semiconductor structure as shown in FIG. 4. In this embodiment, the remaining semiconductor material is removed using PRR1165 at 120° C. for five minutes, followed by rinsing with deionised water for two minutes.
Next a bondpad opening covering the mesa-structure 1 is formed by depositing lift-off photo-resist material, in this embodiment LOR-10A, and further photo-resist material, in this embodiment PR1811, spinning at 4000 rpm and baking at 115° C. for one minute. A mask is then applied to define the bondpad pattern, followed by an eight second exposure. The photo-resist is then etched, in this embodiment for live minutes using MF319 developer. As shown in FIG. 5, the bondpad pattern results in the mesa-structure being free from photo-resist material following etching, but the areas of the semiconductor laser device surrounding the mesa structure 1 being covered by photo-resist material 13.
There then follows a single deposition process in which:

    
    
        a layer 15 of Titanium is deposited (see FIG. 6);
        a layer 17 of Platinum is deposited on the Ti layer 17 (see FIG. 7);
        a first layer 19 of Gold is deposited on the Pt layer 17 (see FIG. 8);
        a layer of Chromium 21 is deposited on the first Au layer 19 (see FIG. 9); and
        a second layer 23 of Gold is deposited on the Cr layer 21 (see FIG. 10).
    
    


In this embodiment, the Ti layer 15 has a thickness of 50 nm and the Pt layer 17 has a thickness of 80 nm. The thickness of the Pt layer 17 is more than normal to provide an improved barrier against Au migration into the semiconductor material.
In this embodiment, the thickness of the first Au layer 19 is 300 nm, the thickness of the Cr layer 21 is 15 nm and the thickness of the second Au layer 23 is 1200 nm. As discussed in the article “The effect of Cr barrier on interfacial reaction of Au/Zn/Au/Cr/Au contacts to p-type InGaAs/InP” by J. S. Huang and C. B. Vartuli, Thins Solid Films 446 (2004) 132-137, the whole contents of which are hereby incorporated by reference, the Cr layer 21 serves as an adhesion layer.
By depositing the Ti layer 15, Pt layer 17, first Au layer 19, Cr layer 21 and second Au layer 23 in a single deposition process, a more uniform deposition is obtained as processing operations performed between deposition processes can result in non-uniformity of the underlying layers for later deposition processes.
A lift-off process is then performed to remove the photo-resist material 13 and the metal layers formed thereon. In this embodiment the lift-off process uses PRR1165 for ten minutes at 120° C. Finally, an O2 plasma clean is performed for five minutes.
As shown in FIG. 11, the resulting semiconductor laser device has a mesa-structure 1 covered by a metal contact portion. The metal contact portion contacts the portion 11 of the p-InGaAs layer 3. By making the portion 11 smaller than the area of the top of the mesa structure 1, sensitivity to alignment is reduced. Further, by making the Pt layer 17 thicker than normal, a better barrier against Au diffusion is formed. In addition, by depositing the metal contact portion in a single deposition process, a more uniform laminar structure is formed.
It will be appreciated that the depth of the Pt layer 17 could be increased further to improve the barrier against Au diffusion into the semiconductor material still further.

## Claims

1. A semiconductor laser device comprising:
a semiconductor structure at least part of which is in the form of a mesa structure having a flat top;
a passivation layer over the mesa structure, the passivation layer defining a contact opening on the flat top of the mesa structure; and
a metal contact portion contacting the semiconductor via the contact opening, wherein the contact opening defined by the passivation layer has a smaller area than the flat top of the mesa structure,
wherein the metal contact portion comprises at least one layer comprising gold and a layer comprising platinum disposed between said at least one layer comprising gold and said semiconductor structure, and
wherein said at least one layer comprising gold comprises a first gold layer and a second gold layer separated by a chromium layer.

2. The semiconductor laser device of claim 1, wherein the mesa structure has sloping sidewalls and the passivation layer covers the sloping sidewalls and the periphery of the flat top of the mesa structure.

3. The semiconductor laser device of claim 1, wherein said platinum layer has a thickness of at least 80 nanometers.

4. The semiconductor laser device of claim 1, wherein the metal contact portion comprises a layer of titanium disposed between said platinum layer and said semiconductor structure.

5. The semiconductor laser device according to claim 4, wherein said semiconductor structure comprises a layer of p-doped semiconductor material at the top of the mesa structure.

6. The semiconductor laser device of claim 5, wherein said Titanium layer is disposed adjacent said p-doped layer.

7. The semiconductor laser device of claim 6, wherein the p-doped layer comprise indium gallium arsenide.

8. The semiconductor laser device of claim 7, wherein the semiconductor structure comprise an indium phosphide substrate.

9. A semiconductor laser device comprising:
an indium phosphide semiconductor structure, at least part of which is in the form of a mesa structure having a flat top with a p-InGaAs layer adjacent the flat top;
a passivation layer over the mesa structure, the passivation layer defining a contact opening on the flat top of the mesa structure; and
a metal contact portion contacting the semiconductor via the contact opening,
wherein the contact opening defined by the passivation layer has a smaller area than the flat top of the mesa structure,
wherein the metal contact portion comprises:
a layer of titanium in contact with the semiconductor structure via the contact opening;
a layer of platinum on the titanium layer, wherein the platinum layer has a thickness of at least 80 nanometers;
a first layer of gold on the platinum layer;
a layer of chromium on the first gold layer; and
a second layer of gold on the chromium layer.

10. The semiconductor laser device of claim 9, wherein the mesa structure has sloping sidewalls and the passivation layer covers the sloping sidewalls and the periphery of the flat top of the mesa structure.

11. The semiconductor laser device of claim 9 wherein the semiconductor structure comprises lattice-matched layers.

12. The semiconductor laser device of claim 11 wherein the passivation layer comprises a SiNx dielectric layer followed by a SiO2 dielectric layer.

13. The semiconductor laser device of claim 9 wherein the passivation layer comprises a sequence of dielectric sublayers.

14. The semiconductor laser device of claim 9 wherein the semiconductor laser device is a tunable laser in the frequency range around 1.5 μm.

15. A semiconductor laser device comprising:
a semiconductor structure comprising lattice-matched layers, at least part of which is in the form of a mesa structure having a flat top;
a passivation layer over the mesa structure, the passivation layer defining a contact opening on the flat top of the mesa structure; and
a metal contact portion contacting the semiconductor via the contact opening,
wherein the contact opening defined by the passivation layer has a smaller area than the flat top of the mesa structure,
wherein the metal contact portion comprises at least one layer comprising gold and a layer comprising platinum disposed between said at least one layer comprising gold and said semiconductor structure,
wherein said at least one layer comprising gold comprises a first gold layer and a second gold layer separated by a chromium layer, and
wherein the semiconductor laser device is a tunable laser in the frequency range around 1.5 μm.

16. The semiconductor laser device of claim 15, wherein the mesa structure has sloping sidewalls and the passivation layer covers the sloping sidewalls and the periphery of the flat top of the mesa structure.

17. The semiconductor laser device of claim 15 wherein the passivation layer comprises a sequence of dielectric sublayers.

18. The semiconductor laser device of claim 15 wherein the passivation layer comprises a SiNx dielectric layer followed by a SiO2 dielectric layer.