Semiconductor device having a package

Abstract: This invention relates to a package construction of a semiconductor device, and provides a semiconductor device in which a vapor-impermeable moistureproof plate is embedded in a bottom surface of a hollow package (1) for an inner surface wallthicknesswise therefrom to provide an excellent moisture-proofness in terms of the package construction.

Description: The present invention relates to a semiconductor device and method for manufacturing the same, and more specifically, to a semiconductor device in which the moisture-proofness of an airtightly sealed semiconductor package is improved and a method for manufacturing the same. In a semiconductor chip such as an IC, LSI or VLSI, a highly integrated circuit surface tends to be affected by changes in ambient temperature or humidity, or fine dust. Furthermore, since a semiconductor chip itself tends to be broken by mechanical vibrations or shocks, it is used as a semiconductor device together with a package for covering the semiconductor chip. The package type is roughly divided into an airtightly sealed type and a resin mold type. In the former, the airtightly sealed type, an airtight space is formed in the central portion of a package member, and a semiconductor chip is arranged within the airtight space. As material for such a hollow package, ceramics having a high moisture-proofness are generally used, but use is also being made of synthetic resin, which is low in cost and easy to work. In a semiconductor device using such a hollow package made of synthetic resin, a semiconductor chip secured to a bonding surface (a chip bonding surface) of a hollow portion by means of adhesives is electrically connected to a lead frame by means of a bonding wire. Opposite ends of the lead frame are opened to an inner side and an outer side of the package. The top surface of the hollow package is airtightly sealed by securing a cover member or lid such as a transparent or opaque synthetic resin plate, a glass plate or the like thereto by adhesives. In the case where such a hollow package made of synthetic resin is used. there arises a problem in that even if the cover member is secured by means of adhesives, a small amount of moisture enters into the package as time passes. As a result, electrodes of the semiconductor chip or internal circuitry becomes corroded to lower the performance thereof, and finally reaches an unusable condition. Even if careful seal means are applied to an adhesive portion of the cover or a seal portion of the lead frame which are contemplated to be an entry route for moisture, it is still not possible to prevent the entry of moisture after the passage of time, a countermeasure for which has been worked out by the present inventors. The present inventors have repeatedly made studies to clear up the cause of the entry of a small amount of moisture into an airtightly sealed semiconductor package. At the outset, it was assumed that a route for entry of moisture into the semiconductor package is from the adhesive portion of the cover member or the seal portion of the lead frame. Therefore, tests were repeatedly carried out to clear up the cause from that field. However, it was found that the entry of moisture from the adhesive portion of the cover member or the seal portion of the lead frame pose not much of a problem since various countermeasures are taken. But really unexpectedly. the entry of moisture into the semiconductor package principally results from the moisture which transmits through the molded body from the lowermost surface of the package, that is. the bottom surface of the hollow package. It is to be noted that the entry of moisture into the semiconductor package was tested in the following manner. A package sealed by a transparent cover member was put into a commercially available pressure cooker tester (PCT) and heated and pressurized under the conditions of temperature (121°C), humidity (100%RH) and gauge pressure (1 kg/cm 2 ) for steps of definite hours, after which it was removed to examine if there was formed condensation caused by moisture which had entered inside the transparent cover member under normal temperature. Those which had no condensation contained less entered moisture, and continued to be heated and pressurized every definite hour till condensation appeared. Accordingly, in the present invention, the quality of the moisture-proofness of the package can be determined by the length of heating and pressurizing time till condensation appears in the inside of the transparent cover member. JP-63-044745 discloses a circuit board for the mounting of electronic components. A circuit board has a recessed part for mounting electronic components, where a metal-coated layer is formed on the inner surface of said recessed part. A metal layer is formed under the recessed part, where this metal layer is in contact with the metal-coated layer inside the recess. The metal layer has the purpose of transferring heat and of protecting from moisture. JP-1-011356 relates to a hollow-package for a semiconductor device. The package consists of a resin substrate containing leads for connecting the interior to the exterior as well as a die pad onto which a chip is mounted. The chip is then wire bonded to the extracting leads and a cap is placed onto the substrate to thereby isolate the bonded chip. It is an object of the present invention to provide a semiconductor device of an airtight seal type having excellent moisture-proofness which can effectively prevent the entry of moisture. Accordingly, the present invention provides a device having the features of claim 1. Preferred embodiments can be found in the subclaims. According to a first series of embodiments having a moisture-proof plate, an island portion connected to a lead frame through a support lead is formed on the bottom surface of the hollow package of Claim 1 or on an inner portion wall-thicknesswise therefrom. This support lead is important for fixing the island portion in an accurate position, but even if the support lead is present in a state cut interiorly after molding, the effect of preventing the entry of moisture of the semiconductor device is not lowered. Accordingly, in molding, the island portion as well as the lead frame itself are set beforehand within a mold to effect insert moulding whereby the integrally molded hollow package can be manufactured. This island portion is preferable in that it is connected to the lead frame through the support lead so that a semiconductor package with the island portion fixed in an accurate position at the time of molding can be molded. Furthermore, in said first series of embodiments, there may be provided a semiconductor device in which said island portion is provided internally (lower) of the bonding surface and internally (upper) of the bottom surface of the hollow package, particularly closer to the bottom surface of the hollow package by at least 100 µm or more from the bonding surface, whereby the prevention or impediment of the entry of moisture into the hollow package is further effectively achieved. Moreover, in the first series of embodiments, after a lead frame having an island portion has been set beforehand to a predetermined position within a mold, synthetic resin is subjected to injection molding or transfer molding whereby the island portion, the lead portion and the hollow package can be integrated. According to a second series of embodiments having a vapor-impermeable moistureproof plate, a moistureproof plate formed from a vapor-impermeable plate-like member is embedded on the bottom surface of an airtight sealed hollow package or on an inner portion wall-thicknesswise therefrom. In this second series of embodiments, since the moisture-proof plate is an element separate from the lead frame, there is the merit that the material, size, etc. of the moisture-proof plate can be suitably selected irrespective of the material or size of the lead frame. The present invention will now be described by reference to the following drawings, in which: Figs. 1 to 3 show the first series of embodiments of the present invention; Fig. 1 is a side sectional view showing one example of a hollow package integrated by insert molding; Fig. 2 is a side sectional view showing the example of Fig. 1 in which an airtightly sealed semiconductor device is completed; Fig. 3 is a side sectional view showing a further example in which an airtightly sealed semiconductor device is completed; Fig. 4 is a top view showing one example of the shape of a lead frame and island before insert molding in the first series of embodiments; Fig. 5 is a graph showing the results of pressure cooker tests according to the differences in position in which an island is formed in the first series of embodiments; Figs. 6 and 7 show the second series of embodiments of the present invention; Fig. 6 is a side sectional view showing one example of a hollow package integrated by insert molding; and Fig. 7 is a side sectional view showing the example of Fig. 6 in which an airtightly sealed semiconductor device is completed. A semiconductor device according to an embodiment of this invention comprises a semiconductor chip within a hollow package, a lid (cover member) and a moisture-proof plate, therefrom to provide an airtightly sealed semiconductor package construction having excellent moisture-proofness. The hollow package 1 aforementioned is made of synthetic resin, which is preferably molded of thermosetting resins such as epoxy resin, polyimide resin (for example polyaminomaleimide, polypyromeritimide), phenol resin, unsaturated polyester resin, silicon resin, etc., or heat resistant thermoplastic resins such as a liquid-crystal polymer, polyphenylsulfide resin, polysulfone resin, etc. Among the synthetic resins aforementioned, epoxy resin such as bisphenol-A type, orthocresol novolak type glycidyl amine type, etc., is preferred in view of its mouldability and moisture-proofness, and a hollow package can be provided through injection or transfer moulding. As the moistureproof plate 11, there can be used a vapor impermeable plate-like member, particularly, those in which a material such as a metal such as iron, copper, nickel, aluminum, alloys and their oxides, ceramics, glass, etc. are worked into a plate-like member. In the case where out of these materials, metals such as aluminum, copper and iron, alloys or their oxides which have a heat transfer rate in excess of 0.01 cal/cm. sec. °C are used, not only the moisture-proofness is improved but also an effect in which heat generated by the semiconductor chip 7 is radiated outside the package is obtained, and the operating stability of the semiconductor element can also be maintained. Where the moisture proof plate consists of a part of the lead frame, the lead frame is desirably formed from a material selected out of copper, iron, aluminum or a group comprising an alloy thereof, or preferably 42 alloy or a copper alloy. The lead frame 2 is wholly or partly as necessary applied with a surface treatment, for example, applied with plating such as gold, silver, nickel, solder, etc. For example, nickel plating containing no lustering agent can be used. More specifically, saccharin or the like can be added to smooth the plated surface. (Embodiment 1). In Fig. 1, showing in a side sectional view one example of a package for a semiconductor device in which an island portion 3 is formed internally from a bottom surface, reference numeral 1 designates a hollow package made from synthetic resin; and 2, a lead frame. The island portion 3 is connected to the lead frame 2 through a support lead 3' and secured to a position lower than the other lead portion. Reference numeral 4 designates a bonding surface on which a semiconductor chip 7 is secured by means of adhesives, and numeral 5 designates a cover adhesive portion for sealing a space in the package. The hollow package 1 is molded from the aforesaid resins such as epoxy resin, polyimide resin. The lead frame 2 (including the island portion 3) is formed of a material aforementioned and preferably selected are 42 alloy or a copper alloy. Other plate-like materials such as a copper plate can be joined to the island portion 3 for improved heat radiation properties. In this manner, the plate-like member such as a copper plate is joined to the island portion 3 whereby not only the moisture-proofness of the semiconductor device is improved but also the effect of efficiently radiating heat generated in the semiconductor chip 7 outside the package is obtained to maintain the stability of operation of the semiconductor chip 7. The plate-like member may have the same size as that of the island portion 3, or an area larger than the former. In the present embodiment, the island portion 3 is connected to the lead frame 2 through the support lead 3' as shown in Fig. 4. The island portion 3 is integrally formed on the same plane as the lead frame 2 at the time of fabrication as a part of the lead frame 2, but thereafter only said portion is depressed by a suitable amount by post-working. Accordingly, the island portion 3 and the lead frame 2 are formed of the same material. However, in order to improve the anticorrosion and radiation properties, only the island portion 3 may be partly plated or other material such as a copper plate may be joined as mentioned above. As described above, preferably, the island portion 3 is formed inwardly (in the figure, below the bonding surface 4) from the bonding surface 4 and inwardly (in the figure, above the bottom surface 14 of the package) from the bottom surface 14 of the hollow package 1, particularly, closer to the bottom surface 14 by at least 100 µm or more from the bonding surface. The registration of the upper surface of the island portion 3 with the bonding surface is not preferable since moisture-proofness is then lowered. The forming position of the island portion 3 is suitably adjusted by operation when the lead frame 2 is pressed, for example. The thickness of the island portion 3 is normally 50 to 1000µm, preferably, 100 to 500µm, and has a size which is substantially the same as that of the bottom surface of the semiconductor chip 7. The larger size thereof brings forth preferable results according thereto. The lid adhesive portion 5 is preferably provided with a shoulder as shown in order to obtain a better sealing effect. The semiconductor chip 7 is secured to the bonding surface 4 of the hollow package 1, the chip and the lead frame 2 are connected by a bonding wire 6, and thereafter the cover member 8 or lid is adhered to the cover adhesive portion 5 of the package 1 by adhesives such as epoxy group, imide group or acrylic group adhesives to provide an airtight seal. As the bonding wire 6, gold or aluminum wire is normally used. As the cover member 8 or lid, there can be used transparent cover member 8 such as a glass plate a sapphire plate, a transparent alumina plate, a transparent plastic plate, etc. or translucent or opaque cover members 8 such as a colored glass plate, a ceramics plate, a colored plastic plate, etc. While Fig. 2 shows an example in which the lower surface of the island portion 3 is at a position of the bottom surface of the hollow package, it is to be noted that the island portion 3 may be arranged further inwardly of the bottom surface of the hollow package. A semiconductor device according to this embodiment is manufactured in the following procedure. The lead frame 2 having the island portion 3 depressed by a predetermined distance from other portions is set beforehand to a predetermined position within the mold. and subsequently, epoxy resin such as bisphenol A type resin, orthocresol novolak type resin, glycidyl amine type resin, etc., are integrated by injection or transfer molding. According to this method, a position of the island portion 3 of the lead frame 2 sealed into the hollow package 1 is depressed by suitable pressing means, for example, a press machine, so that it can be located in advance to an accurate position. Therefore, a single insert step at the time of molding will suffice, and the trouble of cumbersome positioning can be omitted. The conditions of insert molding differ with the kinds of resins used. For example, when using epoxy resin, preferably, pressing and heating are carried out under the general conditions of 10 to 800 kg/cm 2 of pressure, 150 to 200°C of temperature, 1 to 5 minutes of time, and post-curing is added as needed. The lead frame 2 having the depressed island portion 3 is subjected to insert molding whereby the island portion 3 with excellent moisture-proofness is secured with high precision to the bottom surface of the hollow package 1 or to an inner portion therefrom. The operating steps can be simplified and in addition, positive fixing can be obtained. The present inventors have conducted the following experiments in order to measure the moisture-proofness of a semiconductor package according to the present series of embodiments. It is to be noted that the entry of moisture into the semiconductor package was tested by the following method (Method A for evaluation). A package sealed by a transparent cover member 8 was put into a commercially available pressure cooker tester and heated and pressurized for two hours under the conditions of temperature (121°C), humidity (100%RH) and gauge pressure (1 kg/cm 2 ), after which it was removed. The package on the side of the transparent cover member was pressed against a hot plate and heated for 15 seconds at 80°C, and thereafter the transparent cover member was pressed against a brass block for 10 seconds at 25°C to examine if condensation caused by entered moisture was formed inside the transparent cover member. Those which had no condensation had less entered moisture, which was judged to produce no practical problem. Accordingly, in the present invention, the quality of the moisture-proofness of the package was determined by the length of time heating in the pressure cooker tester till condensation appeared inside the transparent cover material. Experiment 1 A lead frame 2 (thickness: 250µm) made of 42 alloy subjected to nickel plating with a lustering agent (saccharin) combined and having an island portion 3 as shown in Fig. 4 was press molded. Only the island portion 3 was depressed and set to a predetermined position within the mold of a transfer molding machine. Next, molding material of an orthocresol novolak type epoxy resin and phenol novolak curing agent was transfered into the mold under conditions such that the temperature was 180°C, pressure 120 kg/cm 2 and time 3 minutes, after which it was subjected to post-curing for three hours at 180°C to obtain a molded article (thickness of bottom 1 mm) as shown in Fig. 1. Subsequently, the cover adhesive portion 5 was coated with the epoxy resin to form an adhesive layer 9, to which a cover member 8 made from a transparent glass plate was adhered to airtightly seal a hollow package 1. This hollow package 1 was put in a pressure cooker tester under the above-mentioned conditions. The package was removed every two hours to examine if condensation had appeared on the inside of the glass cover member 8. As a result, no condensation was recognized till 12 hours but condensation was first recognized after 14 hours. The results obtained by measuring the difference in moisture-proofness from varying the forming position from the package die pad surface (bonding surface 4) to the upper surface of the island portion 3 was as indicated by the dotted line (marked by Δ) in Fig. 5. The thickness of the package used (from the bottom surface of the semiconductor chip 7 (bonding surface) to the bottom surface of the package) was 1 mm. The time till a blur occurred inside was measured by the same method. On the other hand, in a package fabricated in exactly the same manner, except that an incorporation of the island portion was not embodied, condensation was recognized in four hours by a similar pressure cooker test. Experiment 2 An experiment was conducted in a manner similar to that of the aforesaid Experiment 1 except that nickel plated 42 alloy with a lustering agent not combined was used as the lead frame 2. The results obtained by measuring the difference in moisture-proofness from suitably varying a forming position (t) from the bonding surface 4 to the upper surface of the island, portion is shown by the solid line (marked by ) in Fig. 5. It has been found from the aforesaid results that the position in which the island portion 3 is formed is preferably provided at an inner portion from the bonding surface 4, and internally of the bottom surface of the hollow package 1, and particularly, closer to the bottom surface by at least 100 µm or more from the bonding surface. Experiment 3 An experiment was conducted in a manner similar to that of the aforesaid experiment 1 except that 42 alloy not applied with plating was used as the lead frame. As a result, the same results as shown by the mark  in Fig. 5 were obtained. (Embodiment 2) In this embodiment, a moisture-proof plate 11 separate from a lead frame 2 is embedded from the bottom surface of a hollow package 1 inwardly wall-thicknesswise, as shown in Fig. 6. In Fig. 6, the reference numeral 1 designates a hollow package formed of thermosetting resins such as epoxy resin, polyimide resin; 2, a lead frame formed of 42 alloy, copper alloy, etc. ; 4, a bonding surface for securing a semiconductor chip; and 5, a cover adhesive portion for sealing a semiconductor device, the cover adhesive portion being provided with a shoulder for providing a better sealing effect, similarly to the aforementioned Embodiment 1. Reference numeral 11 designates a moisture-proof plate which comprises the technical feature of the present embodiment, and which is selected from aforementioned materials. If alloys or their oxides which have a heat transfer rate in excess of 0.01 cal/cm. sec. °C are used, not only is the moisture-proofness improved but also the effect in which heat generated by the semiconductor chip 7 is radiated outside the package is obtained, and the operating stability of the semiconductor element can also be maintained. The thickness of the moistureproof plate 11 is normally 50 to 1000 µm, preferably, 100 to 500 µm. The size of the moistureproof plate 11 between the bottom surface of the semiconductor chip 7 and the bottom surface of the hollow package is selected preferably, as shown in Fig. 6, to be substantially the same as the size of the bottom surface (lowermost surface) of the hollow package 1. Preferably, the moistureproof plate 11 is embedded on the bottom surface 14 of the hollow package 1 or inwardly thereof as shown in Fig. 6. In the case where a function as a radiation plate is added to the moistureproof plate 11, the moistureproof plate 11 may be provided to be closer to the semiconductor chip 7 ( for example by adjusting the length of a lead member 10 ). Furthermore, as shown in Fig. 7, a lead member 10 which is hung directly below the bonding surface 4 and connected to the moistureproof plate 11 may be provided. Naturally, in this case, the lead member 10 is preferably formed of a material having a heat transfer rate in excess of 0.01 cal/cm sec.°C. With the provision of the lead member 10, heat generated from the semiconductor chip 7 is transmitted to the moistureproof plate 11 to enhance the radiation effect. A lead frame 2 made of 42 alloy and a moisture-proof plate 11 made of copper alloy having a thickness of 250 µm and a heat transfer rate of 0.53 cal/cm. sec.°C were inserted to a predetermined position within a mold of a transfer molding machine. Next, a novolak type epoxy resin as a molding material together with a curing agent were insert-molded under the conditions of temperature 180°C, pressure 80 kg/cm 2 and time 2 seconds, after which they were subjected to post-curing for 3 hours at a a temperature of 180°C to obtain a hollow package 1 as shown in Fig. 6. Then, a cover member 8 made from a transparent glass plate was adhered to a cover adhesive portion 5 of the hollow package 1 by an epoxy resin (adhesive layer 9). The aforementioned insert molding conditions differ with the kind of resin used. However, in the case of epoxy resin, pressurization and heating under conditions wherein in general pressure is 10 to 500 kg/cm 2 , temperature is 150 to 200°C, and time is 1 to 5 minutes are preferable. The moistureproof plate 11 is subjected to insert molding simultaneously with the lead frame 2 whereby the moistureproof plate 11 is fixed with high precision on the bottom surface of the hollow package 1 or inwardly therefrom. Not only are the operating steps simplified but also accurate fixing is obtained as compared with the method for adhering the moistureproof plate 11. The airtightly sealed package was evaluated under the following conditions (Method B for evaluation). Specifically, this package was subjected to a pressure cooker test under conditions of temperature 121°C, humidity 100%RH and gauge pressure 1kg/cm 2 . It was removed every 5 hours and was left at room temperature for an hour to examine if condensation could be recognized inside the glass cover at room temperature. Incidentally, this package was not forcibly cooled under this Method B for evaluation. However, under the aforesaid Method A for evaluation, that package was forcibly cooled at that stage. As a result, condensation was not recognized till after 45 hours but was recognized first after 50 hours. On the other hand in a package fabricated in a manner similar to the former, except that an incorporation of a moisture-proof plate was not embodied, condensation was recognized in 20 hours by a similar pressure cooker test, and the effect of the moistureproof plate was conspicuous. As described above, according the present embodiment, by the formation of a layer of a vapor-impermeable moisture-proof plate 11 on the bottom surface of the hollow package 11 or inwardly therefrom, the entry of moisture from the bottom surface of a package in which moisture permeability into the semiconductor package is highest is efficiently prevented. Furthermore, by insert molding of the moistureproof plate 11 along with the lead frame 2, the moisture proof plate 11 can be fixed to the inner surface or outer surface of the hollow package 1 with high precision and by simple steps. The semiconductor chip 7 may be a solid state image pickup element such as a Charge Coupled Device, Metal Oxide Semiconductor or Charge Priming Device. It is sealed in the aforesaid hollow package with a lid (cover member 8) which is made from a transparent glass. When a solid state image pickup device is produced with these members, the device has the features of airtightness, vaportightness and high reliability.

Claims: A semiconductor device having a semiconductor chip (7) secured to a bonding surface (4) of a hollow package (1) formed of synthetic resin so as to be airtightly sealed therein and provided with a lead frame (2) integrally molded in said hollow resin package (1); characterized in that: a vapour-impermeable moisture-proof plate (3, 3'; 11) is integrally molded within said hollow resin package and is embedded within a base portion of said hollow package (1) at a predetermined distance (t) from said bonding surface (4) between said bonding surface (4) and a bottom surface (14) of said hollow package (1), whereby a portion of said resin is interposed between said moisture-proof plate (3, 3'; 11) and said bonding surface (4). A semiconductor device according to claim 1, wherein said moisture-proof plate (3, 3') comprises an island portion (3) connected to said lead frame (2) by support leads (3'), said island portion (3) having an area which is the same as or larger than a bottom surface of the semiconductor chip (7) secured to said bonding surface (4), and said island portion (3) being positioned lower than a surface of said lead frame (2). A semiconductor device according to claim 1 or claim 2, wherein said predetermined distance (t) from said bonding surface (4) is at least 100 µm or more. A semiconductor device according to claim 2 or claim 3, further comprising a plate-like member for efficiently radiating heat generated in the semiconductor chip (7), said plate-like member having an area the same as or larger than the island portion (3) and being joined to said island portion (3). A semiconductor device according to claim 1, wherein said moisture-proof plate (11) is formed of a material selected from a group comprising aluminum, copper, iron, oxide thereof or alloys whose heat transfer rate is in excess of 0.01 cal/cm.sec. A semiconductor device according to claim 1, wherein a lead member (10) which is directed from said bonding surface (4) to said moisture-proof plate (11) is connected to said moisture-proof plate (11) and said bonding surface (4) of said hollow package (1). A method for manufacturing a semiconductor device having a moisture-proof plate (3, 3') embedded into a hollow package (1), comprising the steps of: depressing an island portion (3) of said moisture-proof plate (3, 3') formed substantially in a central portion of a lead frame (2) by a predetermined distance (t) from the surface of the lead frame (2); thereafter locating and setting said lead frame (2) and said island portion (3) connected thereto at a predetermined position within a mold; and pouring synthetic resin into said mold to thereby embed the island portion (3) into the synthetic resin and cover the island portion (3) with said resin; and hardening said resin to integrate the lead frame (2) with the hollow package (1) formed of synthetic resin. A solid state image pickup device using a semiconductor device as claimed in any of claims 1 to 6.

Inventors: KATAYAMA SHIGERU, TOMINAGA KAORU, YOSHITAKE JUNICHI, KATAYAMA, SHIGERU, TOMINAGA, KAORU, YOSHITAKE, JUNICHI, Katayama, Shigeru, c/o Mitsui Petrochem. Ind. Ltd., Tominaga, Kaoru, c/o Mitsui Petrochem. Ind. Ltd., Yoshitake, Junichi, c/o Mitsui Petrochem. Ind. Lt
Applicants: MITSUI CHEMICALS INC, MITSUI CHEMICALS, INC.
Application year: 1991
Main classification: H01L23
IPC codes: H01L23