Semiconducting device mounting

Abstract: A technique for assembling semiconductor devices in which semiconductor elements, especially lasers or LEDS, are mounted on a first packaging element such as a heat spreader that is mounted on a second packaging element such as a heat sink. Handling of solder preforms is avoided by coating at least one of each of the pairs of surfaces to be brought into contact with a layer of solder and heating the assembly to melt both solder layers in a single operation. In a preferred embodiment a heat spreader is coated in solder on both sides and interposed between a semiconductor element and a heat sink and the assembly is heated. Molybdenum may be used as a layer to aid wetting and adhesion and gold/tin solder may be used. Slightly different solidification temperatures and compositions for the layers may result from different take up of metallisation layers from the components into the solder.

Description: This invention relates to the mounting of devices and is especially applicable to bonding devices such as lasers to heat sinks. Many devices are attached to heat sinks for thermal stability and heat dissipation, but for devices such as semiconductor lasers that dissipate large amounts of power over a very small surface area (typically a few hundred microns by tens of microns) it is usually necessary to bond the device to a high conductivity heat spreader such as diamond which is then in turn bonded to a heat sink. The bonding process is a crucial stage and for satisfactory device life and it is necessary for the bond to exhibit good thermal conductivity. It is also important for the bond to exhibit good mechanical stability and for the bonding process to cause minimal or no change to the actual device. For reasons of mechanical stability, which in an optical communications system may need to be of the order of less than one micron over a period of 25 years, it is not possible to use many of the soft solders because these are subject to creep. Also, as there are two soldering stages of heat sink to heat spreader and device to heat spreader it is necessary to ensure that the soldering stages do not interfere with one another. The current state of the art procedure is for the heat sink and heat spreader to be bonded together in a first step and for the device to be soldered to the heat spreader subsequently using a lower melting temperature solder so that the first solder joint is not remelted. The heat sink and heat spreader are usually bonded together with a 88 weight percent gold 12 weight percent germanium alloy which is a eutectic composition with a melting temperature of 356°C. In order to facilitate adhesion the components are usually plated with gold, or nickel then gold, the nickel preventing diffusion of copper (which is a common material for the heat sink) into the solder layer. The solder is usually a preform that is laid on to the heat sink, the heat spreader is laid on top, and the assembly heated until the solder melts, usually about 10 to 20 seconds. During the melted stage the solder interacts with the gold layers on the sink and spreader and so the resulting solidified solder layer is therefore slightly off the eutectic composition. Bonding of the device chip to the spreader then takes place in a similar way using a preform of 80wt% gold 20wt% tin solder that melts at 282 degrees centigrade. The spreader and chip are usually plated with titanium and then gold, often with an intervening barrier layer of platinum, in order to aid adhesion. An alternative method in which the semiconductor device is first mounted on the heat spreader and the heat spreader is subsequently mounted to the heat sink is described in US-A-4 546 478. The problem with either bonding procedure is that it is laborious and involves difficult handling stages: in particular the solder preforms are only of the order of 10 to 12 µm in thickness with their other dimensions slightly smaller than the smallest adjacent component so that after melting and spreading it is of the same size as the smallest adjacent component. A process is described in US-A-3 922 775 where a preformed planar structure and a heat spreader are simultaneously bonded to a heat sink by thermocompression bonding, followed by mounting the semiconductor device on the heat spreader. The present invention is directed towards enabling a simpler bonding procedure and eliminating the need for solder preform handling. Accordingly the invention provides a method of manufacturing a device package having a semiconductor device that dissipates large amounts of power over a very small surface area, such as a semiconductor laser for example, bonded to a first surface of a heat spreader and a second surface of the heat spreader bonded to a heat sink, the method comprising applying a first solder layer to the first surface of the heat spreader, applying a second solder layer to at least one of the second surface of the heat spreader and the heat sink, positioning the heat spreader with respect to the semiconductor device and with respect to the heat sink and applying heat to melt the solder layers and bond both the semiconductor device and the heat sink simultaneously to the heat spreader in a single heating operation. In order to eliminate the need for handling preforms the present invention proposes coating the heat spreader with solder. The problems encountered in satisfactorily achieving this and a good solder bond are various. Firstly the gold-tin alloy was sputtered on to a titanium/platinum/ gold plated diamond heat spreader. However during the bonding process the solder interacted with the plating metallisation so that the solder became unsatisfactory. A similar procedure was attempted using diamond heat spreaders coated only with platinum but again an unsatisfactory mixture of ternary gold/tin/platinum occurred. A search for suitable materials to aid adhesion to the heat spreader and not significantly migrate into the solder during the melting stage resulted in selection of molybdenum as a preferred coating for the heat spreader. However once the upper surface of the heat sink is coated with gold/tin solder to avoid having to use the small preform it is then not possible to use a solder of higher melting temperature for the lower solder layer, and thus it is not possible to use the usual germanium/gold solder. This could be overcome by soldering the heat spreader to the heat sink first, but this is not desirable from a mass production point of view since it would mean more sputtering stages because only a comparatively small number of mounted heat spreaders could be coated in a single operation. Another option available would be to thermocompression bond the coated spreader to the heat sink using a temperature of about 250°C, but this process is not reliably reproducible there being a tendency to affect the gold/tin solder surface. The present invention departs from the established two-stage bonding process by forming both bonds at the same time, thereby eliminating the necessity for different melting temperature solders. In a preferred method and embodiment a heat spreader is coated on both sides with a barrier layer that aids adhesion and wetting by solder, molybdenum is particularly suitable for this, and then both sides of the coated heat spreader are coated with solder, an 80 weight percent gold 20 weight percent tin solder being preferred. Thus, according to a second aspect of the present invention, preferred embodiments may comprise a device made according to the first aspect of the present invention, comprising a semiconductor chip bonded to a heat spreader, which heat spreader is bonded to a heat sink, wherein the bonding is provided by a gold/tin alloy. Preferably, the bonding is further provided by a molybdenum barrier layer. Typically the molybdenum layer is about 0.25µm thick. This solder coated heat spreader is then interposed between a heat sink and device chip, held in position, and heated via the heat sink for about 15 seconds to melt both solder layers. The heat sink, which may be a packaging header or a separate block, is gold plated in the usual manner to a thickness of about 1 µm, and so when the solder between the heat spreader and heat sink melts the solder takes up some of that gold and solidifies as a non-eutectic paste. This can be tolerated in this location as residual stresses here will not affect device performance. There is usually metallisation on the device chip surface, typically a 0.3 µm layer of gold and so the top solder layer is not able to take up as much gold as the lower layer (since there is less available) and thus it solidifies at or closer to the eutectic composition and remains substantially stress free. In practice it is possible to remove the collet that is used to hold the chip in position and cease heating as soon as the upper layer has melted. Of course it is not essential for the solder used to be of eutectic composition. Acceptable results may be achieved with alloy compositions a few weight percent to either side of the eutectic composition, provided that after heating, the solder between the heat sink and the heat spreader has a higher melting point than the solder provided on the surface of the heat spreader remote from the heat sink. The limits to solder composition are of course dependent on the heat sensitivity of the components of the assembly : normally the semiconductor device will be the most temperature sensitive component. The ease and extent to which the significant solder portions pick up alloying elements e.g. gold, during the soldering process are also significant. Attention should therefore be paid to the composition and thickness of the surface metallisation of the heat sink and the semiconductor device. It is likely that for the gold tin solders a gold content of at least 70 weight percent is required, the upper limit on gold content being determined by the maximum permissible soldering temperature, and almost certainly being less than 83 weight percent. If a gold tin solder is used the gold content preferably is between 75 weight percent and 82 weight percent. Although this procedure requires the heat spreaders to be coated, it is possible to coat very large numbers of spreaders in a single operation, turn them over and coat the other side. Once this is done all subsequent assembly stages are simplified: instead of handling three components and two delicate solder preforms and performing two separate heating stages no preforms need to be handled and only one heating stage is required. A particular advantage of the procedure is that it maybe readily automated for mass production with the coated heat spreaders stacked in and dispensed from a hopper. If the solder melting stage is performed very quickly it may be possible in some applications to omit a barrier/wetting layer on the heat spreader.

Claims: A method of manufacturing a device package having a semiconductor device that dissipates large amounts of power over a very small surface area, such as a semiconductor laser for example, bonded to a first surface of a heat spreader and a second surface of the heat spreader bonded to a heat sink, the method comprising applying a first solder layer to the first surface of the heat spreader, applying a second solder layer to at least one of the second surface of the heat spreader and the heat sink, positioning the heat spreader with respect to the semiconductor device and with respect to the heat sink and applying heat to melt the solder layers and bond both the semiconductor device and the heat sink simultaneously to the heat spreader in a single heating operation. A method according to Claim 1 wherein said positioning comprises interposing the heat spreader between the semiconductor device and the heat sink. A method according to either preceding claim in which the solder is a gold and tin solder. A method according to any one of the preceding Claims wherein the first and second solder layers have the same or substantially the same melting point. A method according to any one of the preceding Claims in which the semiconductor device and at least one of the adjacent surfaces of the heat spreader and the heat sink comprises a gold layer. A method as claimed in any preceding Claim wherein the first and second solder layers are applied respectively to the first and second surfaces of the heat spreader, and wherein the surface of said semiconductor device to which said first surface is to be bonded and the surface of the heat sink to which said second surface is to be bonded each have at least a surface of a material which is readily absorbable by the respective solder layer when the solder is in a molten state whereby during the single heating operation the composition or compositions of the solder layers changes with a consequent change in liquidus temperature. A method as claimed in Claim 6 wherein the first and second solder layers have substantially the same composition prior to bonding, the thickness of said readily absorbable material on said surface of said semiconductor device being substantially less than on said surface of the heat sink, whereby in the bonding process less of said readily absorbable material is absorbed by said first solder layer than by said second solder layer, such that after bonding the second solder layer has a higher liquidus temperature than said first solder layer. A method as claimed in any one of the preceding Claims wherein the first and second solder layers each have compositions at or close to a eutectic composition. A method as claimed in any one of Claims 6 or 7 or Claim 8 as dependent on Claim 6, wherein the first and second solder layers each comprise a gold tin solder, and wherein said readily absorbable material is gold. A method according to any preceding claim in which a layer is deposited prior to the solder application that aids adhesion and wetting by the solder. A method according to Claim 10 in which said layer comprises molybdenum. A method according of any one of Claims 1 to 11 in which both the bonding surfaces of the heat spreader are each provided with a solder layer comprising a gold/tin alloy, and a molybdenum barrier layer. A method as claimed in claim 12 wherein the solder on each of said surfaces has substantially the same composition. A method as claimed in claim 12 or 13 to wherein the gold/tin alloy of each of said solder layers comprises 80 weight percent gold and 20 weight percent tin.

Inventors: REGNAULT JOHN CHRISTOPHER, REGNAULT, JOHN CHRISTOPHER
Applicants: BT & D TECHNOLOGIES LTD, BT&D TECHNOLOGIES LIMITED
Application year: 1989
Main classification: H01L21
IPC codes: H01L21, H01S5, H01L23, H01S3