Method of fabricating a semiconductor device

Abstract: A method for fabricating a semiconductor device comprises coating the surface of a semiconductor element (2) with a curable silicone composition (7) in which there is dispersed a filler (8) having an average particle diameter of 0.01 to 500 micrometers and a specific gravity of 0.01 to 0.95 and thereafter curing said composition (7) after the elapse of sufficient time for the filler (8) in the layer of the composition adjoining the element to migrate into the layer of said composition remote from the element (2).

Description: This invention relates to a method of fabricating a semiconductor device. More particularly, it relates to semiconductor devices having cured silicone covering the surface of the semiconductor elements. These coatings are designed to resist swelling when cleaned by solvents such that the semiconductor elements and bonding wires of these devices are neither ruptured nor deformed by cleaning. The surface of semiconductor elements are typically coated with cured silicone to protect them from moisture and external stresses. This is true for both face-up semiconductor devices, in which the semiconductor element and lead frame are electrically connected through bonding wires, or for face-down semiconductor devices, in which the semiconductor element is electrically connected to the lead frame through solder bumps. The use of a solvent to clean the solder flux off these devices can cause problems. For instance, solvent cleaning can cause swelling of the silicone coating which results in possible warpage or destruction of the semiconductor element, potential deformation or breakage of the bonding wires, and delamination of solder bumps. Also, when such devices are used in a static-prone location, static electricity will gradually accumulate on the cured silicone, covering the surface of the semiconductor element, and will either fail or operate erratically. This occurs, for instance, in applications such as image sensors, used in facsimile equipment for reading the document to be transmitted, or thermal print heads, used in printers for printing on the output paper. To solve the solvent cleaning problem, the art suggests using (a) semiconductor devices in which the surface of the semiconductor element is coated with cured silicone, and the element is then additionally sealed in a ceramic or plastic package; or (b) semiconductor devices in which the surface of the element is covered with cured silicone containing a high loading of dispersed filler. The former solution has increased costs and a reduced efficiency in mass production. The latter solution causes problems during heat cycling, such as damage to the surface of the semiconductor element by the filler, and deformation or separation of the semiconductor element and bonding wires. JP-A-62092344 describes a method for preventing the cracking of a semiconductor element which is coated by a resin which comprises both solid body fillers and hollow body fillers. Due to differences in the specific gravity of these fillers, the solid body fillers accumulate around the semiconductor element. In related case JP-A-61191056, a semiconductor element is described which has been sealed by an epoxy resin which has added to it hollow body filler and solid body filler. The resin is heated and hardened and at this time the solid body filler sinks to the peripheral part of the semiconductor element due to the difference in specific gravity and the hollow body filler becomes positioned at the upper part of the semiconductor element. JP-A-5206325 describes a method for protecting a semiconductor element with a resin selected from an epoxy resin, urethane resin, silicone resin, polyamide resin and unsaturated polyester resin. The resin comprises a filler which contains air, the volume percentage of air contained in the filler should be 5 to 85%, and the filler is selected from silica balloon, shirasu balloon, polyethylene balloon or polyurethane balloon. JP-A-5009270 describes a semiconductor resin sealing composition which comprises epoxy resin, phenol novolak resin, a curing accelerator and an inorganic filler. The inorganic filler comprises a hollow filling material which is preferably a glass ceramic material. JP-A-61051834 describes a method for improving the moisture resistance and long term reliability of a semiconductor element sealed with a resin composition made of an epoxy resin, by including in the resin a filler having a specific weight smaller than the epoxy resin. When the resin is heated, the filler is present in the upper half of the resin. The present invention takes as its object a method of fabricating a semiconductor device whose semiconductor element is coated with a cured silicone which resists swelling when cleaned with a solvent. The semiconductor element and bonding wires are, therefore, neither separated nor deformed by solvent cleaning. The present invention is a method of fabricating a semiconductor device which comprises a substrate having a semiconductor element mounted thereon. A coating comprising a cured silicone in which there is dispersed a filler having an average particle diameter of 0.01 to 500 micrometers and a specific gravity of 0.01 to 0.95 covers the semiconductor element. Moreover, concentration of filler in the cured silicone is higher in the part of the coating remote from the element than in the part of the coating adjoining the element. This invention provides a method for fabricating a semiconductor device comprising: coating the surface of a semiconductor element with a curable silicone composition comprising a curable silicone polymer and a filler having an average particle diameter of 0.01 to 500 micrometers and a specific gravity of 0.01 to 0.95 wherein said filler comprises a heat-distortable organic resin powder; allowing sufficient time to elapse for the filler in the part of the curable silicone composition adjoining the element to migrate into the part of the curable silicone composition remote from the element; and curing the curable silicone composition by heating said composition to at least the heat-distortion temperature of the organic resin powder. The present invention is based on our unexpected discovery that the silicone coatings of the present invention can be designed to inhibit swelling when cleaned with a solvent. This inhibits damage to the device itself and also prevents deformation and separation of its bonding wires. The reason for this improvement is the higher concentration of filler in that part of the coating, remote from the element, as compared to that part of the coating adjoining the element. Figure 1 depicts the cross section of a semiconductor device according to the present invention as fabricated in Examples 1 and 3. Figure 2 depicts the cross section of a semiconductor device according to the present invention as fabricated in Example 2 and 5. Figure 3 depicts the cross section of a face-down semiconductor device according to the present invention. The structure of the semiconductor device of the present invention is not critical. Generally, the device contains a semiconductor element such as, a transistor, IC, LSI, and so forth, mounted in the semiconductor device. These semiconductor devices are exemplified by face-up semiconductor devices, such as those of Figures 1 and 2 or face-down semiconductor devices such as those of Figure 3. For simplicity, the present invention is explained with reference to the semiconductor device in Figure 1. This device has a structure in which a semiconductor element 2 mounted on substrate 1 is electrically connected to the lead frame 5 through bonding wires 4 . The surface of said element 2 is coated with a cured silicone 7 in which filler 8 is dispersed. The specific cured silicone 7 is not crucial, and it may be, for example, a gel or rubber. This cured silicone is formed by curing a curable silicone composition comprising a curable silicone polymer and a filler. No specific limitations attach to the composition, curing mechanism, etc., of the curable silicone in our curable silicone composition. The curing mechanism of the curable silicone is exemplified by addition reaction-curing, condensation reaction-curing, radical reaction-curing, and ultraviolet-curing mechanisms. The addition reaction-curing mechanism is preferred. The filler 8 is to have an average particle diameter of 0.01 to 500 micrometers, and preferably 0.1 to 100 micrometers. The specific gravity of the filler is 0.01 to 0.95. An important feature of the coated semiconductor device manufactured according to the present invention is that the concentration of filler 8 , present in the layer of cured silicone 7 remote from the semiconductor element 2 , is higher than the concentration of said filler 8 present in the layer of cured silicone 7 adjoining the semiconductor element 2 . No limitations otherwise apply. The variation in filler concentration in the silicone may be continuous or discontinuous as one moves from the layer of cured silicone 7 adjoining the element 2 to the layer of cured silicone 7 remote from the element 2 . When the filler in the present invention is used to inhibit damage due to cleaning with solvents, the higher concentration of the filler 8 , dispersed in the layer of cured silicone 7 remote from the semiconductor device 2 , functions to inhibit swelling of the cured silicone 7 when the semiconductor device is subjected to solvent cleaning after mounting on a circuit substrate. Moreover, the lower concentration of the filler 8 , dispersed in the layer of cured material 7 adjoining the element 2 , inhibits damage to the surface of the element 2 by the filler 8 dispersed in this layer of the cured material 7 when the device is thermally cycled. Further, this lower concentration inhibits deformation or destruction of the element 2, and deformation or snapping of the bonding wires 5. In preferred embodiments, there is a high concentration of filler 8 in the layer of cured silicone 7 , remote from the semiconductor element 2; and the filler 8 is almost completely absent from the layer of the cured material 7 adjoining the element 2 . For fillers used according to the present invention to inhibit cleaning damage cleaning with solvents, the material constituting the filler 8 is a heat-distortable powder of an organic resin such as nylon resins, polyethylene resins, polypropylene resins, polystyrene resins, acrylic resins, fluororesins, epoxy resins, phenolic resins, and polyethylene terephthalate resins; or hollow forms of these organic resin powders. This filler 8 is a heat-distortable or thermally deformable organic resin powder. The use of a heat-distortable organic resin powder is preferably accompanied by the use of an addition reaction-curing curable silicone composition. Curing of the composition at temperatures at or above the heat-distortion temperature of the organic resin powder will cause heat distortion and melt-adhesion of at least a portion of the organic resin powder 8 in the layer of the cured silicone 7 remote from the semiconductor element 2 . This will make this layer relatively harder as a result. When fillers are used to additionally inhibit static electricity damage, the material constituting the filler is not crucial as long as the filler is either intrinsically electrical conductive, or has a surface that is electrically conductive. The electrically conductive filler is exemplified by powders coated with an electrically conductive material or hollow powders of copper, aluminum, silver, zinc, carbon, and so forth. Examples of coated powders include organic resin powders, hollow organic resin powders, and hollow inorganic powders, in each case having a surface coating of an electrically conductive material such as copper, aluminum, silver, zinc, or carbon. The organic resins constituting said organic resin powders and hollow organic resin powders are exemplified by nylon, polyethylene, polypropylene, polystyrene, acrylic resins, fluororesins, epoxy resins, phenolic resins, and polyethylene terephthalate. The inorganic hollow powders are exemplified by hollow glass powders, hollow silica powders, hollow alumina powders, and hollow ceramic powders. The electrically conductive filler is also exemplified by the hollow powders of copper, aluminum, silver, zinc, and carbon. If a coating is used to inhibit static electricity damage, the surface of the cured silicone is preferably grounded in the case of relatively large semiconductor devices. Methods are exemplified by grounding the surface of the cured silicone using a conductive trace or lead and by the formation of a dam with a volume resistivity from 1 x 10 6 to 1 x 10 11 ohm-cm around the circumference of the substrate-mounted semiconductor element. The latter method is preferred. This grounding inhibits the accumulation of static electricity on the cured silicone 7 that coats the surface of the semiconductor element 2 and also serves to rapidly dissipate any charge that may accumulate. The quantity of filler addition is not crucial, and the appropriate addition is selected, for example, based on the average particle diameter and specific gravity of the filler, the application of the obtained semiconductor device, and so forth. The range of 0.1 to 80 wt% of filler in the composition is generally preferred. The cured silicone coating the surface of the semiconductor element of the present invention resists swelling when the device is cleaned with solvent. This avoids deformation or destruction of the element and deformation or separation of the bonding wires. As a result, the element need not be sealed in a ceramic or plastic package as with prior-art semiconductor devices. This permits a substantial simplification in the structure of said semiconductor device. The fabrication method of the present invention commences with coating the above-described curable silicone composition on the surface of a semiconductor element. This is typically done using a dispenser. The composition is then allowed to stand for a period of time long enough for the filler, in the layer of curable silicone composition adjoining the semiconductor element, to migrate into the layer of the composition that is remote from the element. This standing period is followed by curing. The length of this period cannot be rigorously specified because it will vary as a function of the difference in specific gravities between the filler and non-filler fraction of the curable silicone composition, the viscosity of the curable silicone composition, the average particle diameter of the filler, the quantity of filler addition, and so forth. However, when the curable silicone composition has a low viscosity and there is a large difference in specific gravities between the filler and non-filler fraction of the curable silicone composition, the standing period requirement will usually be satisfied immediately after the composition has been coated on the surface of the semiconductor element. The use of very long times will permit the formation of a discontinuous dispersion of the filler in the cured silicone layers, in which there will be almost no filler dispersed in the cured silicone layer adjoining the semiconductor element and the filler will be dispersed only in the cured material remote from the element. The use of brief, but still adequate standing times, permits the formation of silicone layers in which the concentration of the filler continuously increases moving from the layer of cured material adjoining the element to the layer of cured material remote from the element. The cured silicone coating on the surface of the semiconductor element used to resist solvent swelling during cleaning comprises filler which is a heat-distortable or thermally deformable organic resin powder. Additionally, the use of such a filler is preferably accompanied by the use of an addition reaction-curing curable silicone composition. When an addition reaction-curing curable silicone composition filled with a heat-distortable organic resin powder is coated on the surface of the semiconductor element, and then heated to at least the heat-distortion temperature of the powder, at least a portion of the powder in the cured silicone layer remote from the semiconductor element will be thermally distorted and melt-adhered. This produces a relatively hard layer. The method of the present invention also makes it unnecessary to additionally seal the semiconductor element with a metal, ceramic or plastic package, or similar coating. This provides for a substantial abbreviation of the manufacturing process for highly reliable semiconductor devices. Semiconductor devices of the present invention and their method of fabrication will be explained in greater detail through working examples. The reported viscosity values were measured at 25°C. Example 1 An opaque white curable silicone composition was prepared by homogeneously mixing 100 weight parts of a curable organopolysiloxane, comprising a mixture of an organopolysiloxane polymer having alkenyl groups, an organopolysiloxane polymer having silicon-bonded hydrogen atoms and a platinum catalyst for hydrosilylation curing (viscosity = 400 mPa.s (centipoise), specific gravity = 1.0) and 20 weight parts of polypropylene resin powder (average particle diameter = 50 micrometers, specific gravity = 0.9, heat-distortion temperature = 100°C). The organopolysiloxane then cured through an addition reaction to give a transparent silicone rubber with a durometer reading (JIS A) of 32. A semiconductor element 2 carrying an aluminum circuit pattern was mounted on the surface of a glass substrate 1 , and the bonding pads 3 on the top edges of the element 2 were then electrically connected to the lead frame 5 through gold bonding wires 4 . The aforementioned curable silicone composition was thereafter carefully coated on the surface of the semiconductor element 2 without overflowing the metal frame 6 (height = 2 mm). This was followed by standing at room temperature for 30 minutes and then heating in a forced circulation oven at 150°C for 30 minutes to yield the product semiconductor device. Inspection of this semiconductor device with a stereoscopic microscope showed that the polypropylene resin powder 8 , in the cured silicone 7 coated on the surface of semiconductor element 2 , was present and dispersed only in the layer of the cured material 7 remote from the element 2 . It was also observed that a portion of the powder 8 had undergone heat distortion and melt-adhesion. Finally, almost no polypropylene resin powder 8 was observed in the layer of cured silicone 7 adjoining the semiconductor element 2 , which as a result was transparent. The fabricated semiconductor device was subsequently immersed in 1,1,1-trichloroethane at 25°C for 30 minutes, then removed, and again inspected with the stereoscopic microscope. Almost no swelling of the cured silicone 7 was observed. In addition, neither destruction or deformation of the semiconductor element 2 in the semiconductor device, nor deformation or separation of the bonding wires 4 were observed. Comparative Example 1 A semiconductor device was fabricated as in Example 1, with the exception that in this case the curable organopolysiloxane described in Example 1 was used by itself as the curable silicone composition (no filler). Substantial swelling of the cured silicone 7 was observed when this semiconductor device was immersed in 1,1,1-trichloroethane as in Example 1. While the semiconductor element 2 in the semiconductor device was not destroyed or deformed, deformation of some of the bonding wires 4 was observed.

Claims: A method for fabricating a semiconductor device comprising: coating the surface of a semiconductor element with a curable silicone composition comprising a curable silicone polymer and a filler having an average particle diameter of 0.01 to 500 µm and a specific gravity of 0.01 to 0.95 wherein said filler comprises a heat-distortable organic resin powder; allowing sufficient time to elapse for the filler in the part of the curable silicone composition adjoining the element to migrate into the part of the curable silicone composition remote from the element; and curing the curable silicone composition by heating said composition to at least the heat-distortion temperature of the organic resin powder. The method of claim 1 wherein the curable silicone is cured by a curing mechanism selected from the group consisting of addition reaction-curing, condensation reaction-curing, radical reaction-curing, and ultraviolet-curing. The method of claim 1 wherein the curable silicone polymer is an addition reaction-curing polymer.

Inventors: ISHIKAWA TAKAE, MINE KATSUTOSHI, NAITO HIROYOSI, YAMAKAWA KIMIO, ISHIKAWA, TAKAE, MINE, KATSUTOSHI, NAITO, HIROYOSI, YAMAKAWA, KIMIO, Ishikawa, Takae, Dow Corning Toray Silicone Co.Ltd, MINE, KATSUTOSHI, DOW CORNING TORAY, Naito, Hiroyosi, Dow Corning Toray, YAMAKAWA, KIMIO, DOW CORNING TORAY
Applicants: DOW CORNING TORAY SILICONE, DOW CORNING TORAY SILICONE COMPANY LIMITED
Application year: 1995
Main classification: H01L23
IPC codes: H01L23