Method of fabrication of a micro-electromechanically tunable vertical cavity photonic device

ABSTRACT

A tunable Fabry-Perot vertical cavity photonic device and a method of its fabrication are presented. The device comprises top and bottom semiconductor DBR stacks and a tunable air-gap cavity therebetween. The air-gap cavity is formed within a recess in a spacer above the bottom DBR stack. The top DBR stack is carried by a supporting structure in a region thereof located above a central region of the recess, while a region of the supporting structure above the recess and outside the DBR stack presents a membrane deflectable by the application of a tuning voltage to the device contacts.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is schematically illustrated a tunable vertical cavity device, generally designated , constructed according to one embodiment of the present invention. The device is designed like a Fabry-Perot vertical cavity based device, having two semiconductor DBRs and , and an air-gap cavity therebetween, and presents a tunable optical filter. The air-gap cavity is located within an etched-through recess formed in a spacer , which is located on top of the bottom DBR and is completely covered by a supporting structure , which carries the top DBR stack . The top DBR stack is located on a region of the supporting structure so as to be centered around a vertical axis passing through the center of the recess . The top DBR stack has a lateral dimension smaller than that of the recess . A region of the supporting structure outside the region (carrying the top DBR stack ) presents a membrane deformable by the application of a tuning voltage to the device contacts .

In the present example, the bottom DBR comprises 30 pairs of AlGaAs/GaAs n-type layers grown on a n-type GaAs substrate and having the reflectivity of 99.5% at 1.55 μm. The spacer is a stack of six pairs of AlGaAs/GaAs layers with the same thickness and composition values as in the bottom DBR stack . In distinction to the layer structure of the bottom DBR stack, the layers in the spacer have alternating n-type and p-type doping. The recess with a lateral dimension of 300×300 μm2 is made by etching all six layers of the spacer , such that the depth of the recess is equal to about 1.5 μm, which defines the thickness of the air-gap cavity , and the bottom surface of the recess coincides with the top of the bottom DBR stack

The top DBR stack is a mesa containing 25 pairs of AlGaAs/GaAs layers, and having the reflectivity of 99.7% and the lateral dimension of 80×80 μm2. The top DBR stack is located on the supporting structure (within the region thereof), which consists of 4 pairs of AlGaAs/GaAs layers with the same thickness and composition as the layers in the top DBR stack , and terminates with a InGaP etch-stop layer . The layer has the thickness of 30 nm and is located at the interface between the top DBR and the supporting structure . The lateral continuation of the supporting structure within the region thereof (outside the region ) forms the membrane which completely covers the recess .

The fabrication of the filter device will now be described with reference to FIG. .

In the first step, the etched-through recess with the lateral size of 300×300 μm2 is formed in the spacer (consisting of a stack of six pairs of AlGaAs/GaAs layers with alternating n-type and p-type doping) by reactive plasma dry etching in Cl2—CH4—Ar and selective chemical etching in a HF—H2O solution. This procedure allows to precisely stop the etching, when reaching the top GaAs layer of the bottom AlGaAs/GaAs DBR stack (grown on a substrate ), which results in the recess depth of about 1.5 μm.

In the second step, a wafer fusion is applied between the surface of the supporting structure of a top DBR wafer and the structured surface of the spacer . The top DBR wafer contains a DBR (in which the top DBR is then formed) grown on a GaAs substrate , and the supporting structure grown on top of the DBR . Hence, the surface of the supporting structure is fused face to face with the structured surface of the spacer forming a fused interface within a surface region of the spacer outside the recess. The fusion is performed at 650° C. by applying a pressure of 2 bar to the fused interface. Thereafter, although not specifically shown here, the GaAs-substrate is selectively etched in a H2O2—NH3OH solution till reaching the first AlGaAs layer of the DBR structure (i.e., bottom layer of the structure bonded to the spacer), which acts as an etch-stop layer and which is also selectively etched in a HF—H2O solution.

In the third step, a mesa is etched in the DBR by dry etching in Cl2—CH4—Ar and selective chemical etching in a HF—H2O solution till reaching the etch stop-layer to form the top DBR stack (FIG. ), which is centered around a vertical axis passing through the center of the recess and has the lateral dimension of 80×80 μm2. As a result of this etching, the membrane is formed as the lateral continuation of the supporting structure (its region ) completely covering the recess . By this, the air-gap cavity is formed being confined at its bottom side by the top surface of the bottom DBR stack and at its top side by the supporting structure . The device fabrication is completed by forming the electrical contacts .

In the present example, the spacer structure and the supporting structure are made of pairs of GaAs/AlGaAs layers. It should, however, be noted that these structures, as well as those of the DBR stacks, can also be made of GaAs, or other types of dielectric layers. In order to stabilize the transmitted optical mode, a mesa can be formed on the bottom of the recess being centered around the vertical axis passing through the center of the recess and having the lateral size of less than 10 and height of less than {fraction (1/30)} of the device operation wavelength.

Referring to FIG. 3, there is illustrated a tunable vertical cavity device according to another embodiment of the present invention presenting a VCSEL device structure. This device is designed to emit light in the vicinity of 1.55 μm. To facilitate understanding, the same reference numbers are used for identifying those components, which are identical in the devices and . Similar to the device of the previous example, the device is designed like a tunable Fabry-Perot cavity having top and bottom DBRs and , respectively, with maximum reflectivity at 1.55 μm. In distinction to the previously described device , in the device , the spacer is placed on the top of an active cavity material , which is fused to the surface of the AlGaAs/GaAs bottom DBR stack

The active cavity material comprises a multiquantum well InGaAsP/InGaAs layer stack , which has a maximum of photoluminescence emission at 1.55 μm and is sandwiched between two InP cladding layers and . The optical thickness of the active cavity material is equal to 3/2×1.55 μm. The spacer has a total thickness of 1.51 μm and comprises a InP layer with alternating p-n-p-n doping sandwiched between 2 InGaAsP etch-stop layers and . The spacer is grown in the same process with the active cavity material . A mesa made of InGaAsP and having the maximum of photoluminescence (PLmax) at 1.41 μm is located on the bottom of the recess and centered about a central vertical axis passing through the center of the recess .

The device may be pumped optically with 980 nm pump light, for example, through the top DBR , resulting in an emission at 1.55 μm through the bottom DBR and the GaAs substrate . Applying a voltage between contacts results in a deflection of the membrane towards the bottom of the recess , which shortens the air-gap cavity and correspondingly, the emission wavelength of the VCSEL device as well. The mesa introduces a lateral refractive index variation in the optical cavity allowing to stabilize the optical mode. The height and the lateral size of the mesa should be set less than {fraction (1/30)} and less than 10, respectively, of the device operation wavelength.

The fabrication of the tunable VCSEL device will now be described with reference to FIGS. 4 and 5.

First, a multilayer stack structure is grown on a InP substrate . The structure comprises the spacer and the active cavity material . The spacer has the total thickness of 1.5 μm and includes an InP layer with alternating p-n-p-n doping sandwiched between two etch stop InGaAsAP layers, both with PLmax=1.4 μm and thickness of 50 nm. The active cavity material has the total thickness of 725 nm and comprises 6 quantum wells sandwiched between two InP cladding layers.

Then, the fusion of the multilayer stack with the bottom DBR stack is performed by putting them face to face in a forming gas ambient, increasing the temperature to 650° C., and applying a pressure of about 2 bar to the fused interface. This process is followed by selective etching of the InP substrate in a HCl—H2O solution till reaching the InGaAsP etch-stop layer to form the recess . More specifically, the selective etching consists of the following: The InGaAsP etch-stop layer is first etched in an H2SO4—H2O2—H2O solution, and then the InP layer is etched in a HCl—H2O solution. Thereafter, the mesa is formed by etching in a H2SO4—H2O2—H2O solution.

In the next step, the structured surface of the spacer is fused to the substantially planar surface of the supporting structure . The fusion is performed at 650° C. applying a pressure of 2 bar to the fused interface. This is followed by selective etching of the GaAs substrate of the top DBR wafer , and by etching the DBR as described above with respect to the fabrication of the device to form the mesa . The device fabrication is completed by forming the electrical contacts .

Those skilled in the art will readily appreciate that various modifications and changes can be applied, to the preferred embodiment of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims. Note that the publications designated with an astrick* are incorporated herein by reference thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, several embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:

FIG. 1 illustrates an example of a tunable optical filter device according to the present invention;

FIG. 2 illustrates the fabrication of the filter device of FIG. 1;

FIG. 3 illustrates an example of a tunable VCSEL device according to the present invention; and

FIGS. 4 and 5 illustrate the fabrication of the tunable VCSEL device of FIG. .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of PCT/IB02/00682, filed on Mar. 8, 2002 and U.S. Ser. No 09/809,236, filed on Mar. 15, 2001, now U.S. Pat. No. 6,546,029, all of the same title, the applications having common inventors, and the contents of which being incorporated herein by reference thereto. Priority under 35 U.S.C. §119 is claimed to these prior applications.

CLAIMS

1. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, wherein said second DBR structure comprises a supporting structure carrying a layer structure of the second DBR stack, wherein the lateral continuation of the supporting structure within a region thereof outside the region below the mesa forms the membrane completely covering the recess, wherein the thickness of the membrane is about 0.5-1.5 micron.

2. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, wherein said second DBR structure comprises a supporting structure carrying a layer structure of the second DBR stack, wherein the second DBR stack is coupled to the structure of the spacer by applying a wafer fusion between the surface of the supporting structure of the second DBR structure and the structured surface of the spacer.

3. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, comprising formation of a mesa on the bottom of said recess.

4. The method according to claim 3, wherein said mesa on the bottom of the recess is centered about the central vertical axis passing through the center of the recess.

5. The method according to claim 3, wherein said mesa on the bottom of the recess has a lateral size and a height of less than 10 and less than {fraction (1/30)}, respectively, of a certain wavelength selected as an operational wavelength of the device.

6. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, comprising formation of an active cavity material between the first DBR stack and the spacer.

7. The method according to claim 6, wherein the formation of the active cavity material comprises the steps of growing a multiquantum well layer stack sandwiched between two cladding layers.

8. The method according to claim 6, wherein said active cavity material is fused to the surface of the first DBR stack.

9. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, wherein the spacer is a stack formed by layers having the same thickness and composition values as in the first DBR stack.

10. The method according to claim 9, wherein the layers in the spacer have alternating n-type and p-type doping.

11. The method according to claim 1, wherein the second DBR stack comprises AlxGa1-xAs layers with different values of x.

12. The method according to claim 1, wherein the supporting structure comprises pairs of AlxGa1-xAs layers with different values of x.

13. The method according to claim 12, wherein the supporting structure comprises the same pairs of AlxGa1-xAs layers as the second DBR stack.

14. The method according to claim 1, wherein each of the first and second DBR stacks comprises pairs of AlxGa1-xAs layers with different values of x.

15. The method according to claim 12, wherein the second DBR structure comprises an etch stop layer presenting an interface between the second DBR stack and the supporting structure.

16. The method according to claim 12, wherein said etch stop layer is an InGaP layer.

17. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, wherein the first DBR stack comprises 30 pairs of AlGaAs/GaAs n-type layers grown on an n-type GaAs substance.

18. The method according to claim 17, wherein said spacer is a stack having six pairs of AlGaAs/GaAs layers with alternating n-type and p-type doping.

19. The method according to claim 18, wherein said recess is formed in the spacer by reactive plasma dry etching in Cl2—CH4—Ar and selective chemical etching in a HF—H2O solution.

20. The method according to claim 19, wherein the etching is stopped, when reaching the top GaAs layer of the first AlGaAs/GaAs DBR stack.

21. A method of fabrication of a Fabry-Perot tunable vertical cavity device comprising first and second distributed Bragg reflector (DBR) stacks with a tunable air-gap cavity therebetween, the method comprising the steps of: (a) forming a spacer above the first DBR stack, wherein said spacer has a structured surface formed by a recess presenting a location for the tunable air-gap cavity; (b) providing a second DBR structure and coupling it to the structured surface of the spacer so as to completely cover said recess by said second DBR structure, thus forming the air-gap cavity between the first DBR stack and the second DBR structure, and; (c) selectively applying material removal to the second DBR structure to form a mesa, presenting the second DBR stack, above a region of said recess, and to form a membrane above said recess outside said second DBR stack, wherein the second DBR structure contains a GaAs substance carrying a layer structure of the second DBR stack, and a supporting on top of said layer structure.

22. The method according to claim 21, wherein the second DBR stack is coupled to the structured surface of the spacer by applying a wafer fusion between the surface of the supporting structure of the second DBR structure and the structured surface of the spacer.

23. The method according to claim 22, wherein the fusion is performed at 650° C. by applying a pressure of 2 bar to the fused interface.

24. The method according to claim 23, wherein said selective material removal comprises selectively etching the GaAs-substrate in a H2O2—NH3OH solution till reaching the first AlGaAs layer of the second DBR layer structure, said first layer acting as an etch-stop layer.

25. The method according to claim 24, comprising selectively etching said etch-stop layer in a HF—H2O solution.

26. The method according to claim 21, wherein said selective material removal comprising etching the mesa in the second DBR structure by dry etching in Cl2—CH4—Ar and selective chemical etching in a HF—H2O solution.

27. The method according to claim 26, wherein said etching of mesa continues until an etch stop-layer in the second DBR structure is reached, thus presenting an interface between a support structure for supporting the DBR stack in the second DBR structure.

28. The method according to claim 3, comprising formation of an active cavity material between the first DBR stack and the spacer.

29. The method according to claim 28, wherein the formation of the active cavity material comprises the steps of growing a multiquantum well layer stack sandwiched between two cladding layers.

30. The method according to claim 28, wherein said active cavity material is fused to the surface of the first DBR stack.

31. The method according to claim 28, wherein said first DBR stack is AlGaAs/GaAs, and said active cavity material comprises a multiquantum well InGaAsP/InGaAs layer stack sandwiched between two InP cladding layers.

32. The method according to claim 31, wherein said spacer comprises a InP layer with alternating p-n-p-n doping sandwiched between 2 InGaAsP etch-stop layers.

33. The method according to claim 28, wherein said mesa on the bottom of the recess is centered about the central vertical axis passing through the center of the recess.

34. The method according to claim 33, wherein said mesa on the bottom of the recess has a lateral size and a height of less than 10 and less than {fraction (1/30)}, respectively, of a certain wavelength selected as an operational wavelength of the device.

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