Mounting a component to a substrate

Abstract: A method of mounting an optical component such as a laser chip (4) on a substrate (1) is described. The method comprises mounting the chip (4) on a bridge (9); and positioning the bridge (9) on the substrate (1). Locator means in the form of depending legs (10) are provided on the bridge (9) so that the bridge is located in the vertical direction relatively to the substrate (1) and hence the laser chip (4) is also located. Finally, the chip (4) is secured to the substrate (1) by, for example, soldering.

Description: The invention relates to a method of mounting a component to a substrate, for example the mounting of an optical component such as a laser chip to a substrate. Recent developments in optical technology have lead to the construction of laser chips and photosensor chips which have relatively small dimensions of the order of 200 µm. It is now proposed that these components should be mounted on substrates and accurately aligned with optical waveguides or other optical components. One of the difficulties with this is that it is difficult to hold the component accurately using a micromanipulator or the like during mounting of the component on a substrate. US-A-4 546,478 published on the 8th October 1985 discloses a heat sink or metal mount, on which a semiconductor chip is mounted, which comprises two parts, namely a main mount or larger portion and a sub-mount or smaller portion. The semiconductor laser chip is soldered by a solder layer on the sub-mount utilizing a microscope so as to assure an accurate position and an accurate direction with respect to the sub-mount. Then, the sub-mount is soldered on the main mount by solder layer with an accurate relation both in position and direction by engaging a linear ridge as a first engaging means provided on the upper face of the main mount with a straight groove and a rear end face as a second engaging means, or by engaging a square recess as a first engaging means and the square bottom of the sub-mount as a second engaging means with each other. As a result of the above-mentioned structure, accurate position and direction of the semiconductor laser chip with respect to the mount is obtainable. The present invention provides a method of mounting an optical component on a substrate, the method comprising the steps of: mounting the optical component on a support; positioning the support on the substrate; at least one of the support and the substrate including locator means for locating the support relatively to the substrate; and securing the support to the substrate; characterised in that the support is bridge-shaped, the optical component is mounted on the undersurface of the support and located between the support and the substrate when the support is secured to the substrate with the optical component not in contact with the substrate, and the locator means is such as to locate the support in two directions relative to the substrate whereby the optical component can be aligned in two directions, prior to the securing step, with a device associated with the substrate. The location of the support in a direction relatively to the substrate automatically locates the component also in that direction. Thus, the locator means can be positioned remotely of the component and will not interfere with the mounting of the component. The invention also provides a combination of a substrate and an optical component mounted on the substrate by means of a support, wherein at least one of the support and the substrate includes locator means for locating the support relatively to the substrate, characterised in that the support is bridge-shaped, the optical component is mounted on the undersurface of the support, the support is secured to the substrate with the optical component located between the support and the substrate with the optical component not in contact with the substrate, and the locator means is such as to locate the support in two directions relative to the substrate whereby the optical component can be aligned, in two directions, with a device associated with the substrate. In one simple arrangement, the locator means comprises two or more legs positioned on either the support or the substrate, the support being positioned on the legs spaced from the substrate by a predetermined amount determined by the length of the legs. Preferably, however, complementary locating portions are formed in the support and the substrate. This has the advantage that the support is located in two directions relative to the substrate. For example, the locating portions may comprise complementary ridges and recesses. Where the substrate comprises a single crystal such as silicon it is particularly convenient if the complementary ridges and recesses have a V-shaped cross-section since these can be formed using known masking and anistropic etching techniques. The component may be directly bonded to the substrate, for example by soldering, or indirectly by bonding the support to the substrate. Preferably, both the support and component are bonded to the substrate. Some examples of methods and combinations in accordance with the invention will now be described with reference to the accompanying drawings, in which: - Figure 1 is a side elevation of a first example with some parts omitted for clarity; Figure 2 is a plan of the first example with some parts omitted for clarity; and Figures 3, 4, and 5 are a side elevation, partial end elevation, and plan respectively of a second example. Figures 1 and 2 illustrate a lithium niobate substrate 1. Titanium is diffused into a narrow rectilinear section 2 of the top surface of the substrate 1 to define an optical waveguide by modifying the refractive index of the substrate. A generally U-shaped recess or slot 3 is then cut in the surface of the substrate 1 orthogonal to the waveguide 2 by using a suitable technique such as ion beam milling or reactive ion etch. As can be seen in Figure 1, the slot 3 has a rectangular cross-section. It will be seen that the formation of the slot 3 divides the optical waveguide 2 into two subsidiary optical waveguides 5, 6 which are automatically in alignment with one another. A conventional laser chip 4 is then mounted in the slot 3 (by a method to be described below) with opposite facets 7, 8 in alignment with the subsidiary waveguides 6, 5 respectively. The depth (D) of the slot 3 is chosen so that the light emitting stripe in the laser chip 4 is matched to the optical waveguides 5, 6. The length (L) of the slot 3 is chosen to allow the maximum possible light transmission between the laser chip 4 and the substrate 1 and it can be tailored to suit a given chip length. The third dimension (W) is not critical and is chosen to allow adjustment of the laser chip 4 along the slot 3 to enable the optimum position of the laser chip relatively to the optical waveguides 5, 6 to be found and to permit a number of chips to be mounted side by side. Typically, L is about 200 µm and D < 15 µm. To mount the laser chip 4, the laser chip is initially soldered to the undersurface of a metal bridge 9. The bridge 9 has a pair of integral depending legs 10 which rest on an upper surface of the substrate 1 with the laser chip 4 suspended in the slot 3. This locates the laser chip 4 in the vertical direction by controlling the extent to which the laser chip 4 is received in the slot 3. Furthermore, it is easier for a micromanipulator to hold the bridge 9 than the laser chip itself. Optimum alignment of the laser chip 4 with the optical waveguides 5, 6 is achieved using an optical method by monitoring the power transmitted along the waveguides while the activated laser chip is moved along the slot 3. Once the optimum position has been found, corresponding to maximum power coupling with the waveguides 5, 6, the laser chip 4 and the legs 10 of the bridge 9 are soldered to the substrate 1. There are a number of advantages in providing two optical waveguides 5,6. In general, the spectral performance of conventional laser chips needs to be improved and this can be achieved by monitoring the laser output from the facet 8 while the main laser output is generated from the facet 7. In addition, this access to both facets could be used in a combined transmitter/receiver or simply to monitor the output power. A transverse lower connection 11 to the laser chip 4 extends along the base of the slot 3 (Figure 2). One of the advantages of providing an elongate slot 3 is that a number of laser chips could be mounted side by side. This is shown in Figure 2 where additional laser chips 12, 13 are provided in alignment with optical waveguides 14, 15; 16, 17 respectively, each pair having a similar form to the optical waveguides 5, 6. In Figure 3, bridges corresponding to the bridge 9 and supporting the laser chips 12, 13 have been omitted. The advantage of lithium niobate is that it can be used to form electro-optic components which would be incorporated into areas of the substrate adjacent the slot 3 with suitable connections being made with the optical waveguides. Figures 3 to 5 illustrate a second example in which a silicon substrate 18 is used. One of the advantages of silicon is that it can be very accurately etched using its anisotropic etching properties to produce grooves with depths accurate to 1 micron and with accurately determined included angles. In the example shown in Figures 3 to 5, initially a flat bottomed channel 19 is formed having a generally U-shaped cross-section with sloping sides by etching the 111 faces of the crystal. Subsequently a pair of parallel V-shaped grooves 20 are etched parallel with the channel 19 and on either side of the channel 19. A V-shaped groove 21 is etched at right angles to the channel 19 and grooves 20, having a depth approximately equal to half the diameter of a monomode optical fibre 22 which is subsequently to be mounted in the groove. A photodiode 23 is bonded (e.g. soldered) to the sloping surface of the channel 19 facing the optical fibre 22. A second silicon substrate or chip 24 is provided which corresponds to the bridge 9 in the previous example. The chip 24 has two pairs of depending, V-shaped ridges 25 and a central depending ridge 26. The included angle of each ridge 25 is substantially the same as the included angle of the V-shaped grooves 20 in the substrate 18. A laser diode 27 is bonded to the ridge 26. The substrate 24 is then mounted on the substrate 18 with each pair of ridges 25 being received in the corresponding groove 20 and straddling the groove 21. The depth of the grooves 20 and the height of the ridge 26 are chosen such that when the substrate 24 is mounted on the substrate 18, the laser diode 27 is accurately located and aligned with an optical fibre 22 in the groove 21 (Figure 4). The provision of the grooves 20 and complementary ridges 25 assists in accurately positioning the laser diode 27 in two directions and this should be contrasted with the previous example in which the bridge 9 permits a certain degree of movement transverse to the slot 3. A feature of this example is that the position of the laser diode 27 with respect to the optical fibre 22 can be adjusted in the direction of the grooves 20 to obtain maximum power coupling into the optical fibre. In addition, the separation of the end of the optical fibre 22 from the laser diode 27 can also be altered by sliding the fibre within the groove 21. Once the correct relative positions have been found, the upper and lower silicon substrates 18, 24 are bonded together in such a way that the laser diode attachment to the upper silicon chip 24 is unaffected. In addition, the optical fibre 22 is bonded into the groove 21. Bonding may be achieved using soldering or any other known technique. The photodiode 23 may be used for a variety of purposes similar to those outlined in the previous example for monitoring laser emission from the facet of the laser diode 27 opposite to the optical fibre 22. In a modification of this example (not shown) the upper substrate 24 may include a further depending ridge which clamps the optical fibre 22 into the groove 21.

Claims: A method of mounting an optical component (27) on a substrate (18), the method comprising the steps of: mounting the optical component on a support (24); positioning the support on the substrate; at least one of the support and the substrate including locator means (20, 25) for locating the support relatively to the substrate; and securing the support to the substrate; characterised in that the support is bridge-shaped, the optical component is mounted on the undersurface of the support and located between the support and the substrate when the support is secured to the substrate with the optical component not in contact with the substrate, and the locator means is such as to locate the support in two directions relative to the substrate whereby the optical component can be aligned in two directions, prior to the securing step, with a device associated with the substrate. A method according to claim 1, further comprising, prior to the positioning step, providing the locator means by forming complementary locating portions (20, 25) in the support (24) and the substrate (18). A method according to claim 2, wherein the substrate (18) and the support (24) each comprises a single crystal, the method further comprising the step of anisotropically etching the substrate and the support to produce the complementary locating portions (20, 25). A combination of a substrate (18) and an optical component (27) mounted on the substrate by means of a support (24), wherein at least one of the support and the substrate includes locator means (20, 25) for locating the support relatively to the substrate, characterised in that the support is bridge-shaped, the optical component is mounted on the undersurface of the support, the support is secured to the substrate with the optical component located between the support and the substrate with the optical component not in contact with the substrate, and the locator means is such as to locate the support in two directions relative to the substrate whereby the optical component can be aligned, in two directions, with a device associated with the substrate. A combination according to claim 4, wherein the support (24) and the substrate (18) have complementary locating portions (20, 25) constituting the locator means. A combination according to claim 5, wherein the locating portions (20, 25) comprise complementary ridges (25) and recesses (20). A combination according to claim 6, wherein the locating portions (20, 25) comprise complementary V-shaped ridges (25) and V-shaped recesses (20). A component according to any one of claims 4 to 7, wherein the optical component (27) comprises a laser chip or an optical sensor chip. A combination according to any one of claims 4 to 8, wherein one or both of the substrate (18) and the support (24) comprise a single crystal. A combination according to claim 9, wherein one or both of the substrate (18) and the support (24) comprise silicon.

Inventors: STANLEY IAN WILLIAM DR, STANLEY, IAN WILLIAM, DR.
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: H01S3
IPC codes: G02B7, G02B6, H01S3, G02B26, G01J3, G01D5, H01S5