WO1991002991A1

WO1991002991A1 - Diffraction grating assembly

Info

Publication number: WO1991002991A1
Application number: PCT/GB1990/001295
Authority: WO
Inventors: Anthony David Welbourn
Original assignee: British Telecommunications Public Limited Company
Priority date: 1989-08-24
Filing date: 1990-08-16
Publication date: 1991-03-07
Also published as: AU6169490A; GB8919220D0

Abstract

A diffraction grating assembly is constructed from a silicon substrate (2). The diffraction grating assembly comprises a support member (1a) formed in a cavity (4) etched in the substrate (2). A diffraction grating (1) is formed on the support member (1a), and means (3) are provided for supporting the support member on the substrate (2) for pivotal movement. Means (5a, 5b) are also provided for controlling the pivotal movement of the support member (1a).

Description

DIFFRACTION GRATING ASSEMBLY

This invention relates to an adjustable diffraction grating assembly, and to a method of making such an assembly.

Adjustable (tunable) diffraction gratings have many applications, for example where tunable optical filters or beam deflection are required. Particular uses of adjustable diffraction gratings are in long external cavity (LEC) lasers, wave division multiplexers (WDM), wavelength selectors for lasers and optical switching. In order to adjust (tune) such diffraction gratings, it is usual to use piezoelectric transducers to rotate the angle between the optical beam and the plane of the grating. Unfortunately, the use of piezoelectric transducers results in a very limited angular range through which a diffraction grating can be adjusted. This is because of the inherent limitations of piezoelectric material, in which the strain in the material is restricted to a relatively small value - otherwise the material would physically break.

It is known to form adjustable gratings monolithically by ruling grating lines into a torsion member formed by micromachining (EP A 0219357). The ruling of the grating is a mechanical process which is not generally compatible with micromachining processes. Damage is introduced to the crystalline material, which will interfere with the process of cavity formation. Moreover, debris formed during the ruling of the grating lines will interfere with the microlithographic processes. A primary aim of the invention is to provide an adjustable diffraction grating assembly that has greater adjustability than known assemblies

The present invention provides a diffraction grating assembly constructed from a monocrystalline substrate, the diffraction grating assembly comprising a support member formed in a cavity etched in the substrate, a diffraction grating formed on the support member by an anisotropic etching process, means for supporting the support member on the substrate for pivotal movement, and means for controlling the pivotal movement of the support member.

The anisotropic etching process reveals angled crystal planes, and ensures extremely accurate alignment of the grating lines.

Advantageously, the substrate is a silicon substrate, preferably a {hhl} silicon substrate, where h and 1 are the Miller indices of the plane of the substrate. Conveniently, the substrate is a (hhl) silicon substrate, whereby the faces of the grating are the (111) and (111) planes, the grating lines lying in the direction [1Ϊ0].

In a preferred embodiment, torsion bars constitute the means for supporting the support member on the substrate, the torsion bars being integral with the substrate and the support member. Advantageously, the torsion bars lie in the direction [1Ϊ0]. The anisotropic etching process forms the grating so that the whole device is formed by standard chemical micromachining processes. This gives the added benefit of improved grating efficiency, because the grating surfaces are smoother, and improved orientation alignment between the grating and the torsion members.

Conveniently, electrodes are formed on the support member and within the cavity, the electrodes constituting the means for controlling the pivotal movement of the support member. The invention also provides a diffraction grating assembly as defined above in combination with at least one micromachined device, said at least one micromachined device being formed on the substrate.

The invention also provides a method of constructing a diffraction grating assembly, the method comprising the steps of forming a support member in a cavity etched in a monocrystalline substrate, the support member being attached to the substrate by a pair of aligned torsion bars, forming a diffraction grating on the support member by an anisotropic etching process, and providing electrodes on the support member and within the cavity, whereby the support member can be controllably pivoted about the torsion bars.

The cavity may be formed in the substrate by an anisotropic etching process using, for example, ethylene diamine pyrocatechol and water (EDP) or KOH. Conveniently, the etching is carried out from that side of the substrate adjacent to the support member. Alternatively, the etching is carried out from that side of the substrate remote from the support member. In the latter case, the method further comprises the step of forming a base to the cavity by bonding a further silicon or glass substrate to that side of the first-mentioned substrate remote from the support member.

The further substrate may incorporate at least one built-in electrode, and at least one further electrode may be formed on that surface of the support member adjacent to the further substrate. Advantageously, the electrodes may be formed by evaporation of, for example, aluminium. Alternatively, the electrodes are formed by evaporation of a ferromagnetic material.

In a preferred embodiment, the or each electrode within the cavity is formed by:- (a) etching a via in the substrate from that side of the substrate remote from the support member;

(b) forming a thin insulating layer, for example by oxidation or deposition; and

(c) depositing metal in the via.

In this case, the monocrystalline substrate includes a buried p layer, and the etching of the or each via is carried out in two stages, a first of which uses an

+ anisotropic etchant which terminates at the p buried layer, and the second of which uses an isotropic or selective p etchant.

Preferably, the anisotropic etching process used to form the diffraction grating utilises KOH or EDP as -the etchant.

A laser usually has an internal cavity with reflecting walls at opposite ends thereof. In use, light beams bounce backwards and forwards between the reflecting walls, the output of the laser being a balance between the percentage of light which leaks out through one of the reflecting walls and the number of times a light beam is reflected. The output of the laser is constituted by a plurality of discrete frequency components (modes) covering a relatively broad frequency range.

In order to use a laser at a high operating speed, it is essential for it to have widely-spaced modes, each having a narrow linewidth. A laser having a large internal cavity results in modes having narrow linewidths. Unfortunately, however, the large internal cavity also results in the modes being closely spaced. One way of achieving a large cavity which results in modes having particularly narrow linewidths is to form an external cavity. Such a long external cavity ( EC) laser is formed by coating one end wall of the internal cavity of a laser with an anti-reflection coating (so that almost all the light hitting that end will escape therethrough), and by placing a mirror a predetermined distance away. The mirror and the other end wall of the laser thus define the cavity (known as an external cavity) of the laser. The disadvantage of an LEC laser, apart from the closely-spaced modes, is that the accuracy with which the mirror must be placed cannot, in practice, be achieved.

It is known to replace the mirror of an LEC laser with a diffraction grating, the diffraction grating being suitably angled to diffract light back to the laser. This superimposes grating characteristics on top of the laser characteristics, which leads to widely-spaced modes. Unfortunately, because the grating is mounted on piezoelectric transducers, the arrangement is expensive because it is time-consuming to assemble and align correctly.

The invention further provides a long external cavity laser comprising a laser diode and a diffraction grating assembly, the diffraction grating assembly being as defined above, and the laser diode being mounted in a cavity etched in the substrate. The laser may further comprise beam shaping optics positioned on the substrate between the laser diode and the diffraction grating assembly.

The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic perspective view of of a diffraction grating assembly constructed in accordance with the invention;

Figure 2 is a schematic side elevation of the assembly of Figure 1;

Figure 3 is a diagram illustrating the process for forming the assembly of Figures 1 and 2;

Figure 4 is a diagram illustrating the orientation of the grating assembly; Figure 5 is a diagram illustrating the relationship between grating orientation and the angles of the grating faces of the grating of the assembly;

Figures 6 to 8 are diagrams illustrating a method of forming electrodes for controlling the adjustability of the grating assembly;

Figure 9 is a schematic perspective view of an LEC laser incorporating the grating assembly; and

Figure 10 is a schematic side elevation of the LEC laser.

Referring to the drawings, Figures 1 and 2 show a monolithic adjustable diffraction grating assembly comprising a diffraction grating 1 formed in a (hhl) silicon substrate. Figure 4 shows schematically the orientation of the grating 1 on the substrate 2, the grating lines lying in the direction [1Ϊ0] of the (hhl) substrate, h and 1 being the Miller indices of the plane of the substrate. In this case, for convenience of analysis, the usual notation of a (100) substrate is replaced by the symmetrically - related plane (001), so that the grating 1 is orientated along the [110] direction.

The diffraction grating 1 is formed by etching (in a manner described below) the substrate 2. This etching process is such as to form a pair of aligned bars 3 which define a torsion paddle. The bars 3 are integral with the grating 1 and the substrate 2, and are positioned centrally so as to define a central axis about which the grating can pivot. This etching process also defines a cavity 4 beneath the grating 1, this cavity permitting the grating to pivot so as to be suitably angled. Pairs of electrodes 5a and 5b are fixed to the grating 1, and to the base of the cavity 4 respectively, so that the angular position of the grating can be controlled electrostaticall . The grating 1 of the assembly shown in Figures 1 and 2 is made in a manner described below with reference to Figure 3. A low doped (either n or p) epilayer 6 is formed on the substrate 2 which is doped p (using boron). The grating 1 is then formed by an anisotropic etch, using KOH or EDP, through a suitably-defined masking layer of silicon oxide or silicon nitride. With this type of etchant, a blazed grating 1 results in which the blaze angle is related to the crystal structure. Where the substrate 2 is a (112) substrate, the grating has a vertical alignment edge. Alternatively, if (100) silicon were used, a symmetrical V-grooved grating would result. In either case, the etch stops along the crystal planes (111) and (111) to define the grating lines (as shown in Fig 5).

The table below shows the relationship between the blaze angles a and b of gratings for a variety of low-index substrates:-

(hhl) An le a An le b An le C

Boron is then diffused into the region la surrounding the grating 1 to form a p region which forms a support member for the grating . At the same time, boron is diffused into the regions (not shown in Figure 3) which are to form the torsion bars 3. These regions thus form p regions. The grating 1 is then coated with a metal such as aluminium, chromium, gold or silver, by evaporation. A passivating dielectric coating may also be required to protect the metal during subsequent processing. The grating pitch is between 0.5 and 1.5 w for optical transmission at 1.3 or 1.55 m, these being the standard transmission windows for optical fibre communications at which silicon is transparent.

Using an anisotropic etchant such as EDP, the epilayer 6 surrounding the region la is etched away to form the cavity 4, the torsion bars 3 and the "floating" grating 1, the cavity being defined by patterning a window in a surface masking layer (not shown). The p regions la and 3 and the heavily-doped substrate 2 are not affected by this etching step.

The electrodes 5a can be plated on top of the grating 1, after a dielectric insulation layer (not shown) has been deposited and before the cavity 4 is etched out. The heavily-doped substrate 2 forming the base of the cavity 4 could itself form the electrodes 5b. Alternatively, a conformal metallic coating (for example CVD tungsten) or suicide could be used for the electrodes 5b, thereby reducing the contact resistance of the electrodes.

Figures 6 to 8 illustrate an alternative method of forming the electrodes 5b. In this method, a heavily-boron-doped p region 2b is formed at the surface of a low-doped substrate 2a by diffusion or ion implantation. A low-doped (n or p) epilayer 6 is formed on the (p) substrate 2, so the region 2b becomes a buried layer. A grating 1 is then formed within a cavity 4 in the manner described above with reference to Fig. 3, the cavity being defined by patterning a window in a surface masking layer (not shown). The mask used to form the grating 1 (a nitride mask) is retained in place on top of the grating, and an oxide layer 7 is thermally grown (see Figure 7). The oxide layer 7 has a thickness in the range of from 0.1 to 1.0 w, the upper limit being set by the requirement that the oxide layer should not be thick enough to bend the grating 1. The mask protects the grating 1 during this thermal oxidation step. Two vias 8 are then formed in the substrate 2a by an anisotropic etching process from the back of the substrate. This anisotropic etching process terminates at the P⁺ buried layer 2b. This stage of the process is shown in Figure 7.

The etching process is then continued using an isotropic etchant (such as a mixture of HF, HNO and acetic acid) or a selective P etchant (such as Dash etch). This etching process terminates at the oxide layer 7. A thin insulating layer 9 is then formed on the exposed silicon surfaces of the vias 8, for example by oxidation. Electrodes 5b are then formed by a standard metal deposition and patterning method.

In an alternative method of forming the grating 1, the cavity 4 could be etched out from beneath, in which case a further silicon or glass wafer would be bonded to the substrate 2 to form the base of the cavity 4. This additional wafer would have the electrodes 5b built-in. The advantage of this alternative method is that the electrodes 5a could be formed on the underneath surface of the grating 1, for example by evaporation of aluminium. Alternatively, the electrodes 5a and 5b could be formed by depositing a ferromagnetic material, in which case electromagnetic forces would be used to angle the grating 1 instead of electrostatic forces.

Instead of using ferromagnetic material, solenoids could be incorporated into the substrate 2, so that ferromagnetic forces could again be used to control the angle of the grating 1. Figures 9 and 10 show a LEC laser incorporating a monolithic adjustable diffraction grating assembly constructed in accordance with the invention. The LEC laser includes a laser diode 10 which is mounted in a laser cavity 11 etched in the substrate 2 in which diffraction grating assembly is formed. One end wall 10a of the laser diode 10 is coated with anti-reflection material (not shown) so that light readily passes therethrough. The axis of the torsion bars 3 is arranged to lie at right-angles to the beam of light from the laser diode 10, so that the grating 1 can be suitably angled to diffract light from the laser diode back to the laser diode, thereby defining an LEC laser.

Beam shaping optics 12 (see Fig.10) are provided for accurately directing the output of the laser diode 10 to the grating 1. The beam shaping optics 12 could be a Fresnel lens etched into a pillar on the substrate 2, thereby providing lateral beam shaping but no vertical control. Alternatively, the output of the laser diode 10 could be coupled directly into a waveguiding slab to give vertical confinement. In this case, a Fresnel lens would be used to provide lateral beam shaping, and both the waveguide and the lens would be produced monolithically. It would also be possible to etch a slot in the substrate, the slot providing an accurate mount for an external lens component.

As an alternative to defining unetched regions by p layers prior to selective etching in EDP or KOH, it would be possible to define these regions by p/n junctions and to use an electrochemical etching technique, in which the p regions are etched and the n regions are not. It would also be possible to form these structures in silicon-on-insulator (SOI), where the cavity 4 would be formed by removing the insulator using any suitable etchant, such as hydrofluoric acid where the insulator is oxide.

Claims

1. A diffraction grating assembly constructed from a monocrystalline substrate, the diffraction grating assembly comprising a support member formed in a cavity etched in the substrate, a diffraction grating formed on the support member by an anisotropic etching process, means for supporting the support member on the substrate for pivotal movement, and means for controlling the pivotal movement of the support member.

2. An assembly as claimed in claim 1, wherein the substrate is a silicon substrate.

3. An assembly as claimed in claim 2, wherein the substrate is a {hhl} silicon substrate, where h and 1 are the Miller indices of the plane of the substrate.

4. An assembly as claimed in Claim 3 wherein the substrate is a (hhl) silicon substrate, whereby the faces of the grating are the (111) and (111) planes, the grating lines lying in the direction [1Ϊ0]

5. An assembly as claimed in any one of claims 1 to 4, wherein torsion bars constitute the means for supporting the support member on the substrate, the torsion bars being integral with the substrate and the support member.

6. An assembly as claimed in claim 5 when appendent to claim 4, wherein the torsion bars lie in the direction [1Ϊ0].

7. An assembly as claimed in any one of claims 1 to 6, wherein electrodes are formed on the support member and within the cavity, the electrodes constituting the means for controlling the pivotal movement of the support member.

8. A diffraction grating assembly substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

9. A diffraction grating assembly as claimed in any one of claims 1 to 8, in combination with at least one micromachined element, said at least one micromachined element being formed monolithically on the substrate.

10. A long external cavity laser comprising a laser diode and a diffraction grating assembly, the diffraction grating assembly being as claimed in any one of claims 1 to 8, and the laser diode being mounted in a cavity etched in the substrate.

11. A laser as claimed in claim 10, further comprising beam shaping optics positioned on the substrate between the laser diode and the diffraction grating assembly.

12. A long external cavity laser substantially as hereinbefore described with reference to, and as illustrated by. Figures 9 and 10 of the accompanying drawings.

13. A method of constructing a diffraction grating assembly, the method comprising the steps of forming a support member in a cavity etched in a monocrystalline substrate, the support member being attached to the substrate by a pair of aligned torsion bars, forming a diffraction grating on the support member by an anisotropic etching process, and providing electrodes on the support member and within the cavity, whereby the support member can be controllably pivoted about the torsion bars.

14. A method as claimed in claim 13, wherein the cavity is formed in the substrate by an anisotropic etching process.

15. A method as claimed in claim 14, wherein the etching is carried out from that side of the substrate adjacent to the support member.

16. A method as claimed in claim 14, wherein the etching is carried out from that side of the substrate remote from the support member.

17. A method as claimed in claim 16, further comprising the step of forming a base to the cavity by bonding a further silicon or glass substrate to that side of the first-mentioned substrate remote from the support member.

18. A method as claimed in claim 17, wherein the further substrate incorporates at least one built-in electrode.

19. A method as claimed in claim 18, wherein at least one further electrode is formed on that surface of the support member adjacent to the further substrate.

20. A method as claimed in claim 19, wherein said electrodes are formed by evaporation of, for example, aluminium.

21. A method as claimed in claim 18, wherein said electrodes are formed by evaporation of a ferromagnetic material.

22. A method as claimed in 15, wherein the or each electrode within the cavity is formed by:-

(a) etching a via in the substrate from that side of the substrate remote from the support member;

(c) depositing metal in the via.

23. A method as claimed in claim 22, wherein the monocrystalline substrate includes a buried p layer, and the etching of the or each via is carried out in two stages, a first of which uses an anisotropic etchant which terminates at the p buried layer, and the second of which uses an isotropic or selective p etchant.

24. A method as claimed in any one of claims 13 to 23, wherein the or each anisotropic etching process utilises KOH or EDP as the etchant.

25. A method of constructing a diffraction grating assembly substantially as hereinbefore described with reference to Figure 3 or Figures 6 to 8 of the accompanying drawings.