Connection structure between components for semiconductor apparatus

Abstract: A connection structure between lead frames (3) and a base plate (1) of aluminum nitride, to be applied as a connection structure between components of a semiconductor apparatus, comprises the base plate formed of a sintered body of aluminum nitride on which a semiconductor device is to be mounted, the lead frames including, as a main material, iron alloy containing nickel in 29 wt.% and cobalt in 17 wt.%, and silver solder (9) for joining the base plate and the lead frames. A surface of the lead frame to be joined to the base plate is formed of oxygen-free copper of a high plastic deformativity to relieve, by plastic deformation of itself, a thermal stress caused by a difference between a thermal expansion coefficient of the base plate and that of the lead frame in a cooling process at the time of soldering. Preferably, only a portion of each lead frame (3) to be joined to the base plate comprises an inner layer portion of iron alloy containing nickel in 29 wt.% and cobalt in 17 wt.%, and an outer layer portion of oxygen-free copper.

Description: The present invention relates to a connection structure between components for a semiconductor apparatus and particularly to a connection structure where a high thermal conductivity is required for mounting of a semiconductor device involving considerable heat generation such as a high power transistor or a laser diode. A connection structure between components of a semiconductor apparatus generally comprises an insulating base member and connection members joined thereto. For example, the connection structure comprises an insulating base plate on which a semi conductor device is mounted, and lead frames connected by soldering using silver solder in predetermined portions where connection circuits or the like are formed on the insulating base plate. In such-a case, the insulating base plate generally needs to have a good electrical insulation, a high mechanical strength and a high thermal conductivity for dissipation of heat from the semiconductor device, so as to maintain insulation from the semiconductor device. The lead frames need to have a low electric resistance and a high mechanical strength. For example, concrete characteristics required for the lead frames are indicated in MIL STD 883B. This standard specifies such characteristics as a tensile strength, a bending strength and fatigue required for the lead frames. Alumina (Al 2 O 3 ) which satisfies the above mentioned requirements is generally selected in the prior art as a material of an insulating base plate used for such connection structure between components of a semiconductor apparatus. For the lead frames, iron-nickel alloys such as Kovar (trade name, i.e, an alloy of Fe-29 wt.% Ni-17 wt.% Co) and 42 alloy (i.e, an alloy of Fe-42 wt, % Ni) are generally selected as the materials satisfying the requirements of the above mentioned standard. The technical paper "Ceramic Substrate and IC Package" pp. 59-67 in the special issue of Electroceramics of "Kagakukogyo" of March 1984 discloses a connection structure in which lead frames of Kovar ( trade name ) are fixed by silver solder to metallized layers formed as connection circuits on an insulating base plate of alumina, the connection structure-being used as a base on which a semiconductor apparatus is mounted. Japanese Patent Laying-Open No. 21032/1984 discloses a base plate formed of a material containing 2 to 30 wt. % of copper in tungsten or molybdenum as a base plate having a high thermal conductivity for mounting a semiconductor device. However, this base plate does not have electric insulation and alumina not having a good heat dissipation is used as a material surrounding the device. Special intererest has been drawn recently to aluminum nitride (AlN) as an insulating base metal for mounting a semiconductor device involving considerable heat generation since aluminum nitride has substantially as good a thermal conductivity as that of beryllia, i.e., 200 Wm -1 K -1 and it has no toxicity and has as good electric insulation and mechanical strength as those of alumina. However, if lead frames are soldered to an aluminum nitride base plate by a metal solder, e.g., a silver solder (Ag-Cu), an average thermal expansion coefficient of aluminum nitride in a range from a room temperature to a silver soldering temperature (780°C) is as small as 4,3 x 10 -6 K -1 , while an average thermal expansion coefficient of iron-nickel alloy of the lead frames is very high, e.g., 10 x 10 -6 K -1 in the case of Kovar and 11 x 10 -6 K -1 in the case of 42 alloy. Consequently, the difference between the thermal expansion coefficient of the aluminum nitride base plate and that of the lead frames causes a strain due to a large thermal stress as a residual stress in the aluminum nitride base plate in a cooling process for silver soldering of the lead frames to the aluminum nitride base plate. Accordingly, if the lead frames are drawn in a direction of detachment from the base plate, the connection structure is easily broken and a sufficient strength of junction of the lead frames can not be assured. Japanese Patent Laying Open No. 167833/1987 discloses that the thermal expansion coefficient of the lead frames is suitably changed by adjustment of composition of an iron nickel cobalt alloy used as the material the lead frames, thereby to make disappear or reduce residual stress and strain caused by soldering of the lead frames to a sintered body of ceramics having a low thermal expansion coefficient, such as aluminum nitride. However, according to this technique, it is necessary to finely adjust the composition of the iron-nickel-cobalt alloy as conventionally used as the material of the lead frames and consequently the manufacturing process is complicated. Another example of a connection structure between components for semiconductor apparatus is a cap for airtightly enclosing a semiconductor device mounted on an insulating base plate. The material of the cap for enclosing of the semiconductor device, for which a high reliability is required, is for example a low thermal expansion alloy material such as 42 alloy or Kovar (trade name), or a ceramic material such as alumina or mullite. Since semiconductor devices have a tendency to considerably increase heat generated therefrom, it is of urgent necessity to develop caps having excellent heat dissipation. For example, if a cap of a metal material having a high thermal conductivity is used, it is necessary to provide insulator portions as described above, which causes not only increase in manufacturing cost but also deterioration in thermal conductivity. Therefore, caps using materials having high thermal conductivity and excellent insulation have become of much interest. Beryllia (BeO), silicon carbide (SiC) and aluminum nitride (AlN) may be considered as materials satisfying such requirements. However, beryllia and silicon carbide are disadvantageous in view of toxicity and electric characteristics thereof and also because of irregularity of supply thereof. Accordingly, aluminum nitride is most suitable. However, in order to manufacture caps using aluminum nitride as the covering material, it is necessary to apply metallization to portions to be joined to the frame member on the surface of the covering material of aluminum nitride and then to solder the the covering member to the frame member by metal solder. In such a case, if metal soldering, e.g., silver soldering (Ag-Cu) is applied, there is a considerable difference between the average thermal expansion coefficient of aluminum nitride and that of iron-nickel alloy, as described previously. More specifically, the average thermal expansion coefficient of aluminum nitride is low, i.e., 4.3 x 10 -6 K. -1 in the range from the room temperature to the temperature for silver soldering (780°C), while the average thermal expansion coefficient of iron-nickel alloy of low thermal expansion used generally for the frame member is extremely high, i.e., 10 x 10 -6 K -1 (in the case of kovar) to 11 x 10 -6 K -1 (in the case of 42 alloy). As a result, a residual strain occurs due to strong thermal stress caused in the covering member of aluminum nitride. The residual strain causes cracks in the covering member of aluminum nitride and warping or deformation in the frame member. Thus, the above described prior art does not make it possible to provide a caps with high dimensional precision, good air-tightness and high reliability. The citation EP-A-0 153 618 refers to a method for preparing a highly heat conductive substrate which comprises interposing an active layer with a thickness of 0,5 - 10 µm comprising silver and an active metal, optionally copper, between an aluminum nitride substrate and a copper member. The aluminium nitride substrate and the copper member are joined which each other by heating. The citation PATENT ABSTRACT of Japan, vol. 10, no. 192 (E-417) [2248], 5th July 1986; & JP-A-61 39560 intends to prevent the strength of a lead from deteriorating due to diffusion of a silver solder material by a method wherein a coated film comprising nickel as its main constituent element is formed on at least a part of the lead, where it is wetted with the silver solded material. The citation PATENT ABSTRACT of Japan, vol. 10, no. 347 (E-457) [2403], November 21, 1986; & JP-A-61 150351 proposes to restrain the generation of cracks by alleviating the comprehensive thermal stress by enclosing a base and a lid subjected to sintering with AlN with the plural layers of glass which are different slightly in their thermal expansion coefficients. In other words, the base and the lead out of AlN are joined together by plural layer soft glass. It is an object of the present invention to provide a connection structure between components of a semiconductor apparatus, which uses a base member of alumixium nitride having good heat dissipation suitable for mounting of a semiconductor device involving considerable heat generation and makes it possible to join a connection member to the base member with an appropriate junction strength. This object is solved by a connection structure between components of a semiconductor package according to claim 1. The present invention has been accomplished based on the findings by the inventors of the present invention that the residual strain due to thermal stress caused in the cooling process in soldering can be relieved effectively if a specified thermal stress relieving member is interposed between a base member of aluminum nitride and a connection member including as a main material any of iron-nickel alloys and iron-nickel-cobalt alloys. If a material of any of soft metals and soft alloys is used as the stress relieving member between the base member of aluminum nitride and the connection member, the stress relieving member becomes further softened near a temperature for soldering and it becomes readily plastically deformative. As a result, most of the thermal stress caused by the difference in the thermal expansion coefficients of the aluminum nitride base member and the connection member is absorbed by the plastic deformation of the stress relieving member, whereby residual stress in the base member of aluminum nitride can be made disappear. Consequently, a desired strength of soldering can be attained. A preferred embodiment of the above described connection structure is claimed in claim 2. If a lead frame as the connection member has a size of 0.1 mm in thickness and 8 mm in width, the thickness of the stress relieving member is preferably in the range from 0.01 to 1 mm. If it is less than 0.01 mm the stress relieving member is too thin to allow a sufficient amount of plastic deformation and cannot absorb the thermal stress in a Satisfactory manner. If the thickness of the stress relieving member is more than 1 mm, a thermal stress caused by thermal expansion of the stress relieving member itself at the time of soldering cannot be disregarded. More specifically, even if, the thermal stress caused by the difference between the thermal expansion coefficients of the lead frames and the aluminum nitride base member can be absorbed by plastic deformation of the stress relieving member, the considerably large thermal stress of the stress relieving member itself adversely affects the lead frames and the base member of aluminum nitride, resulting in a thermal strain in those members. A preferred embodiment of the connection structure mentioned above is claimed in claim 3 by using this metallized layer, the connection structure has the desired junction strength and thermal conductivity. A further preferred embodiment of the above mentioned connection structure is claimed in claim 4. If a plated layer is formed on a surface of the metallized layer to be joined with solder material, uniform and stable soldering can be applied thereto. This is because wettability of the solder material and the metallized layer can be improved by the plated layer provided therebetween. The plated layer formed on the surface of the connection member to be joined with the solder member has also the same effect. Those plated layers are formed preferably by nickel plating. Particularly, if treatment such as gold plating is to be applied in an aftertreatment it is preferred to apply nickel plating for the purposes of improving adhesion and deposition of the gold plating and forming a uniform gold plated layer on the nickel plating. Another connection structure between components of a semiconductor package according to the invention is claimed in claim 5. In this connection structures, any of soft metals and soft alloys forming the surface of the connection member joined to the base member of aluminum nitride is plastically deformed to relieve the thermal stress caused by the difference between the thermal expansion coefficient of the base member of aluminum nitride and that of the connection member. Accordingly, if the base member of aluminum nitride and the connection member are soldered, the connection structure can provide a sufficiently high junction strength since the thermal stress caused therebetween has been relieved. Thus, the connection structure has a high reliability and a high resistance to thermal stress. A preferred embodiment of the connection structure as claimed in claim 5, is claimed in claims 6 and 7. If at least the portion of the connection member joined to the base member includes an inner layer portion formed of iron-nickel alloy or iron nickel cobalt alloy; and an outer layer portion formed of a soft metal or a soft alloy, the outer layer portion of the soft metal or the soft alloy to be joined to the member of aluminum nitride is Plastically deformed to relieve the thermal stress caused between the base member of aluminum nitride and the connection member. If the entire connection member has a three-layer structure including the inner layer portion and the outer layer portion, the bending strength of the connection member itself is lowered, which may cause inconvenience in practical use. For this reason, the connection member may be formed so that only the junction portion subjected to thermal stress has a three-layer structure and the other portions have a single-layer structure of iron-nickel alloy or iron nickel cobalt alloy to maintain a high bending strength. Further, if a two-layer: composite structure is adopted in which a single stress relieving layer of a soft metal or a soft alloy is formed on a portion of the connection member such as a lead frame to be soldered, this two-layer composite structure has also an effect of relieving thermal stress. However, in this two-layer composite structure, warping or deformation may occur in the lead frames due to a bimetal effect at the time of soldering at a high temperature and voids will be- easily generated in the junction portions between the lead frames and the base member of aluminum nitride. Thus, the junction strength or the connection member such as the lead frames cannot be uniform and the reliability of the connection structure itself is lowered. For example, let us assume a case in which copper is selected as the soft metal material of the surface of the connection member joined to the base member, Kovar (trade name, i.e., Fe-29 wt. % Ni-17 wt.% Co.) is selected as a main material of the connection member and a composite metal plate formed by combination of those materials is soldered to the base member of aluminum nitride by silver solder. In this case, copper is plastically deformed in a temperature range from the silver soldering temperature of 780°C or a higher temperature to about 200°C, whereby the thermal stress caused between the Kovar and the base plate of aluminum nitride can be relieved. Although the thermal expansion coefficient of copper is as high as 18 x 10 -6 K -1 , which value is considerably larger than that of aluminum nitride, it hardly causes residual strain due to thermal stress generated in the cooling process of the soldering, because copper has a high plastic deformativity to be plastically deformed in the cooling process. In addition, if oxygen free copper is used as the copper for the junction surface, it has a particularly remarkable effect of relieving the thermal stress. The Kovar as one of iron-nickel-cobalt alloys is selected as the main material of the connection member not only because it is industrially suited as the material of the lead frames and the like but also because it has a thermal expansion coefficient close to that of aluminum nitride among the metal materials. reason for forming the connection member such as the lead frame by a composite metal plate is that a lead frame formed of only a soft metal material itself has low tensile and bending strength and it would involve a disadvantage in practical application. Another connection structure in form of a cap for airtigtly enclosing a semiconductor device mounted on an insulating base plate according to the invention is claimed in claim 12. In the cap in accordance with the present invention the stress relieving member is plastically deformed, so as to relieve a thermal stress caused by the difference between the thermal expansion coefficient of the covering member and that of the frame member in the cooling process at the time of soldering. Further, it becomes possible to suppress generation of cracks in the covering member of aluminum nitride and to suppress the bimetal effect, whereby warping and deformation of the frame member can be reduced. Thus, it possible to provide a cap with a high dimensional precision and a high reliability, ensuring good airtightness. Since the surface layer portion of the junction surface to be soldered is at least formed of a soft metal material, the soft metal material serves to relieve the thermal stress applied to the junction surface and accordingly the junction strength between the respective members of the cap is sufficiently high, which also enhances reliability. For example, let us assume a case in which copper is selected as the soft metal material, Kovar (trade name) is selected as the main material of the frame member and a three-layer composite metal plate formed by combination thereof is soldered by silver solder to the covering member of aluminum nitride. In this case, the copper is plastically deformed in a temperature range from the silver, soldering temperature, i.e ., 780°C or a higher temperature to about 200 °C, so as to relieve the therminal stress applied between the Kovar and the aluminum nitride. In this example, the materials of the respective members have the same function as described above in connection with the example of the connection structure between the lead frames and the base plate of aluminum nitride. Further preferred embodiments of the invention are claimed in the remaining subclaims. These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A, 1B and 1C are plan and sectional views showing a connection structure between components for semiconductor apparatus in accordance with an embodiment of the present invention, e.g. , a connection structure among lead frames, a stress relieving member and a base plate of aluminum nitride. Figs. 2A, 2B and 2C are plan and sectional views showing a connection structure between components for semiconductor apparatus in accordance with another embodiment of the present invention, e.g., a connection structure between lead frames of a composite metal plate and a base plate of aluminum nitride. Figs. 3A, 3B, 3CI and 3CII are plan and sectional views showing a connection structure between components for semiconductor apparatus in accordance with a further embodiment of the present invention, e.g., a connection structure between lead frames of a three-layer composite structure and a base plate of aluminum nitride. Fig. 4 is a sectional view showing a still further embodiment of the invention in which a connection structure between components for semiconductor apparatus is applied to a cap. Fig. 5 is a side view showing a dimensional relation in an example of a connection structure between a lead frame and a base plate of aluminum nitride. Fig. 6 is a side view for explaining a test method for measuring a junction strength of a lead frame joined to a base plate of aluminum nitride. Fig. 7 is an illustration showing a portion where a warping in a frame member of an example of a cap is measured. Fig. 8 is an illustration for explaining a test method for measuring airtightness of the cap of Fig 11. The base member formed of a sintered body of aluminum nitride includes preferably aluminum nitride as a major component and contains 0.01 to 1. 0 wt.% of an element of the group IIIa in the periodic table and 0.001 to 0.5 wt.% of oxygen, with a thermal conductivity thereof being higher than 180 Wm -1 K -1 . In forming the base member, first at least a compound containing a rare earth element is mixed with powder of aluminum nitride so that the compound containing the rare earth element is contained in 0.01 to 1.0 wt.% evaluated by conversion of the rare earth element. Paraffin, PVB, PEG or the like is used as a shaping additive agent. Phenol resin or the like causing a residual carbon by decomposition, or carbon powder, graphite powder or the like may be added thereto so as to control the residual carbon in the sintered body. The rare earth compound is for example stearic acid, palmitic acid, alcoxide nitrate, carbonate or hydroxide. Preferably, a high polymer compound such as stearic acid is used. It is believed that such materials make it possible to decrease the content of the rare earth and to prepare mixture with aluminum nitride powder in good condition. Particularly, stearic acid is the most preferred in view of not only the function as the shaping additive but also the property of being mixed aluminum nitride powder, the quantity of residual carbon, and the like The aluminum nitride powder needs to be in fine and uniform grains. The average grain diameter of the aluminum nitride powder is preferably 1 µm or less and the content of oxygen in the powder is preferably 2.0 wt.% or less. Such aluminum nitride powder cannot be easily obtained by a direct nitriding method (i.e, a method using nitrization of metal aluminum). It can be obtained by a reducing nitriding method (i.e., a method by utilizing reducing nitrization of aluminum oxide.) If the direct nitriding method is adopted, it is necessary to pay particular attention to control of the reaction, classification of the grain diameter and the like. After the mixture powder is shaped in a desired form, it is sintered in a non-oxidizing atmosphere containing nitrogen. In order to make the sintered body have a high thermal conductivity, sintering is performed preferably at a temperature from 1000 to 2100°C for more than five hours to cause an average grain diameter to be more than 5 µm. Cooling after the sintering is preferably performed rapidly. If the cooling is performed gradually, sintering additives are deposited and the sintered surfaces are considerably deteriorated. For this reason, the cooling is effected preferably down to 150°C at a rate of 200°C/hour. A process for forming a metallized layer on a surface of the base member of the sintered body of aluminum nitride thus obtained is applied in the following manner. First, the base plate of the sintered body of aluminum nitride is prepared by the above described method. As the material of the metallized layer, metal paste is prepared by mixing calcium compound powder, aluminum compound powder and metal powder of tungsten or molybdenum, and adding an organic binder such as vehicle thereto. Compositions of the metal paste are metal powder of 40. to 98 wt.%, aluminum compound of 1 to 25 wt.% and calcium oxide of 1 to 35 wt.%. In order to apply a sintering process as aftertreatment at a low temperature, copper or nickel may be added as catalyzer for lowering the sintering temperature. The metal paste thus prepared is coated on a major surface of the base plate formed of the sintered body of aluminum nitride. The base plate of the sintered body of aluminum nitride is fired in an inactive atmosphere of nitrogen or the like at a temperature from 1500 to 1800°C, whereby a metallized layer is formed on the surface of the base plate. If the metallized layer is formed to contain aluminum oxide of 1 to 10 wt.% as the aluminum compound and calcium oxide of 1 to 20 wt.% using tungsten powder as the metal powder, or if the metallized layer is formed to contain aluminum oxide of 1 to 10 wt.% as the aluminum compound and calcium oxide of 1 to 35 wt.% by using molybdenum powder as the metal powders it is made possible to provide a connection structure with a desired adhesion strength between the base plate of the aluminum nitride sintered body and the metallized layer and having a good thermal conductivity. The steps of forming a metallized layer on the surface of a base plate formed by the aluminum nitride sintered body may be performed as follows, by sintering an aluminum nitride formed body coated with the metal paste at one time. First, a base plate of the aluminum nitride formed body is prepared by shaping the aforementioned mixed powder into a prescribed configuration such a a green sheet. A material for the metallized layer is prepared by kneading powder of tungstem and at leash one additives selected from a group of aluminum oxide, aluminum nitride, calcium oxide, yttrium oxide and stearic acid yttrium and the like, to provide metal paste, similarly to the above. The metal paste thus provided is applied to the surface of the base plate formed by the aluminum nitride formed body, by printing or coating. The base plate formed by the aluminum nitride formed body is sintered with the metal paste similarly to the above conditions, to be provided with a metallized layer on the surface of a base plate formed by the aluminum nitride sintered body. Thus, the aluminum nitride sintered body with the metallized having high thermal conductivity can be obtained. An embodiment of a connection structure between components of a semiconductor apparatus, for example, a junction structure of lead frames, stress relieving members and a base plate of aluminun nitride will be described with reference to the drawings. Fig. 1A is a plan view showing the embodiment of the invention the invention used for the base plate for mounting the semiconductor apparatus; Fig. 1B is a sectional view thereof; and Fig. 1C is a detailed sectional view of a junction portion of the lead frame 3 and the base plate 1 of aluminum nitride. Referring to the figures, this connection structure includes a metallized layer 2 formed by screen printing and sintering of a metal paste on specified regions of a major surface of the aluminum nitride sintered base plate 1, and lead frames 3 soldered to the metallized layer 2 by metal solder or the like. A stress relieving member 13 of a soft metal such as copper coated with a nickel plated layer is interposed between the metallized layer 2 and each lead frame 3. A semiconductor device 4 such as an FET involving considerable heat generation is mounted on a predetermined position of the aluminum nitride base plate 1. The semiconductor device 4 is connected with the metallized layer 2 or the lead frames 3 by bonding wires 5. Further, a heat sink 6 of a tungsten alloy, e.g. , a copper-tungsten alloy is attached to another surface of the aluminum nitride base plate 1, opposite to the aforementioned major surface. In addition, as shown in Fig. 1C, the junction portion between the aluminum nitride base plate 1 and each lead frame 3 further comprises a thin plated layer 7 formed on the metallized. layer 2, and a plated layer 8 formed, as required, on outer surfaces of a metal layer 23 of Kovar or the like, to ensure wettability of metal solder 9. Another embodiment of a connection structure between components of a semiconductor apparatus, e.g., a connection structure between lead frames having a stress relieving layer of a soft metal material and a metal layer of iron-nickel alloy and an aluminum nitride base plate will be described with reference to the drawings. As shown in Figs. 2A and 2B, a metallized layer 2 is formed on specified regions of surface of the aluminum nitride base plate 1 and lead frames 3 of a composite metal layer are joined to the metallized layer 2 by soldering of metal solder or the like. Each of the lead frames has a stress relieving layer 13 of copper or the like and a metal layer 23 of Kovar or the like. A semiconductor device 4 such as an FET involving considerable heat generation is mounted on a predetermined position of the aluminum nitride base plate 1 and it is connected with the metallized layer 2 or the lead frames 3 of a composite metal plate by means of bonding wires 5. Further, a heat sink 6 is attached to another surface of the aluminum nitride base plate 1. The metallized layer 2 may be the one conventionally used for soldering lead frames to an insulating base plate. For example, it may be formed by the above described steps of coating metal paste including tungsten or molybdenum as a major component on the aluminum nitride base plate and firing it together with the base plate, or it may be formed by a postmetallizing process, or may be a thin film of titanium, chromium or nickel formed by vacuum evaporation or sputtering. The metal soldering material is preferably silver solder; however; it may be other solder materiaL insofar as it can ensure a good junction between a metal material of a composite metal plate and a metallized layer by forming a thin metal coated layers having good wettability with the solder material thereon. Even in the case of using silver solder, it is preferred, for example as shown in Fig. 2, to form; as required, a thin nickel plated layer 7 on the metallized layer 2 if the metallized layer 2 includes tungsten as a major component and to form, as required, a nickel plated layer 8 on the surfaces of each lead frame 3 of a composite metal plate in advance thereby to ensure a stable wettability of the metal solder 9. Another embodiment, of a connection structure between components of a semiconductor apparatus, e.g., a connection structure of lead frames having three-layer composite structure and an aluminum nitride base plate. will be described with reference to the drawings. As shown in Figs. 3A to 3CII, in this connection structure, a metallized layer 2 is formed on specified regions of a major surface of the aluminum nitride base plate 1 and lead frames 3 each including a three-layer composite metal portion 3a and a single-layer metal portion 3b, or only a three-layer composite portion 3a are joined to the metallized layer 2. Only the three-layer composite metal portions 3a of the lead frames 3 are joined to the metallized layer 2 by soldering of metal solder. 9. A semiconductor device 4 such as an FET involving considerable heat generation is mounted on a predetermined position of the aluminum nitride base plate 1 and it is connected with the metallized layer 2 or the lead frames 3 by bonding wires 5 . Further, a heat sink 6 may be attached to another major surface of the aluminum nitride base plate 1 as required. In addition if each lead frame 3 includes a three-layer composite metal portion 3a and a single layer metal portion 3b, a metal layer 23 of Kovar or the like out of the three layers is formed of the same low expansion metal material as that of the single-layer metal portion 3b and stress relieving layers 13 on both sides of the metal layer 23 are formed of the same soft metal material, as shown in an enlarged view of Fig. 3CI. The three-layer composite metal portion 3a can be formed in a manner in which the stress relieving layers 13 on both sides of the metal layer 23 to be soldered out of the lead frame 3 are formed as clad materials by a clad tape forming method. As. shown in Fig. 3CI, both surfaces of the portion to be soldered in the lead frame 3 may be made thin and the stress relieving layers 13 may be formed on those surfaces to cause the three-layer composite metal portion 3a to have almost the same thickness as that of the single-layer metal portion 3b. Fig. 3CII shows, in an enlarged view, another example of a lead frame 3 only formed of a three layer composite metal portion- 3a. The method for forming the metallized layer 2 on the aluminum nitride base plate 1 may be a method conventionally used for soldering lead frames to an insulating base plate, or any of the above described methods such as the method for coating metal paste including tungsten, molybdenum or the like on an aluminum nitride base plate and firing the paste together with the base plate or the method for forming a thin film of titanium, chromium or the like by vacuum evaporation or sputtering. The metal soldering material is preferably silver solder. However, it may be other metal soldering material insofar as it can ensure a good junction between the three-layer composite metal portion 3a of the lead frame 3 and the metallized layer 2 formed on the aluminum nitride base plate 1 by forming a thin coated layer of a metal having good wettability with the soldering material thereon. In addition, even in the case of using silver solder, if the metallized layer 2 includes tungsten as a major component, it is preferred to form, as required, a thin plated layer 7 of nickel or the like on the metallized layer 2 and a plated layer 8 of nickel or the like on the surfaces of the three-layer composite metal portion 3a of each lead frame 3 in advance thereby to ensure a stable wettability of metal solder 9. The plated layer may be formed on all the surfaces of each lead frame 3. In such a manner, adhesion and deposition of gold plating or the like applied in an aftertreatment can be improved and thus a uniform plated layer of gold or the like can be formed. Fig. 4 is a sectional view of an embodiment in which a connection structure between components for semiconductor apparatus is applied to a cap. According to Fig. 4, a metallized layer 2 is formed on peripheral side surfaces of a covering member 11 formed of a sintered body of aluminum nitride. A frame member 30 of a three-layer composite metal plate is joined to the metallized layer 2 by metal solder 9. The frame member 30 has a composite structure including a metal layer 230 of a low expansion metal material such as Kovar and stress relieving layers 130 of a soft metal material such as copper formed on both surfaces of the metal layer 230. The lower end of the frame member 30 is joined to a ceramic base plate 101 through another metallized layer 2 . A semiconductor device 4 is mounted on the ceramic base plate 101. Further, a heat sink 6 is provided on an upper surface of the covering member 11 of the sintered body of aluminum nitride, to dissipate heat generated from the semiconductor device 4 through the covering member 11 and thus the cooling effect is enhanced. The metallized layers 2 and the metal solder 9 are the same as described above. In addition, a thin plated layer may be formed on the frame member 30 or the junction surfaces of the metallized layers 2. The function of such thin plated layer is the same as described above. The following description is made of examples A to E using samples prepared with base members of aluminum nitride sintered body obtained by the above described method in accordance with the invention. Example A A metallizing process was applied to a base plate of an aluminum nitride sintered body obtained by the above described method. The metallizing process was applied by the steps of coating metal paste having a predetermined composition on a surface of each sample of an aluminum nitride sintered base plate, applying debindering thereto and then firing the base plate coated with the metal paste in a nitrogen atmosphere at a temperature of 1600°C for 60 minutes. As a result, a metallized layer was formed on predetermined regions of the aluminum nitride sintered base plate. The metal paste used in this case was prepared by adding calcium oxide powder and alumina powder to tungsten powder and mixing the powders with an organic binder such as a vehicle The amount of calcium oxide powder added was 14 wt. % and that of alumina powder was 4 wt.%. Further, a nickel plated layer of 2 µm in thickness was formed on the surface of the metallized layer. Then, as shown in Fig. 1C, a stress relieving member was interposed between each lead frame and the aluminum nitride sintered base plate and silver soldering was applied thereto at a temperature of 830°C. A dimensional relation in the junction between the aluminum nitride base plate and the lead frame is shown in Fig. 5. A lead frame 3 having a width of 8 mm and a length of 10 mm was joined to the aluminum nitride base plate 1 having a length of 20 mm by silver solder 9. The area of junction between the lead frame 3 and the aluminum nitride base plate 1 was 2 mm x 8 mm. In order to evaluate a junction strength between the lead frame and the aluminum nitride base plate, tensile tests were carried out by pulling the lead frames 3 in the direction shown by the arrow, so as to measure the soldering strength. The results are shown in Table 1. More specifically, the soldering strength for each sample in Table 1 is an average value obtained based on five measured values. For comparison, an example of junction of lead frames to a beryllia base plate is shown Table 1 (as the sample No 10 ). According to Table 1, the soldering strengths of the samples not using any stress relieving member were extremely low, i.e, 0.5 to 2 kg. In the case of the samples not using the stress relieving members, the start point of breaking existing in each aluminum nitride base plate and cracks due to thermal stress were exhibited apparently. In the case of the samples using stress relieving members for junction between the lead frames and the aluminum nitride base plate in accordance with the present invention; it was found that a desired soldering strength can be obtained if the thickness of the stress relieving member is in the range from 0.01 to 1 mm with the lead frames having a thickness of 0.1 mm and a width of 8 mm. In addition, after a metailized layer having the above described composition was formed on specified regions of the surface of each sample of the aluminum nitride base plate, a nickel plated layer of 2 to 3 µm in thickness and a gold plated layer of 2 to 3 µm in thickness were formed thereon. After that, a high power FET was soldered to the surface of the plated layer by solder material of gold-silicon. Then, by using a method of ΔV BE, a change amount ΔV BE caused by application of a forward drop voltage V BE between the emitter and the base of the transistor was measured and a thermal resistance of the unit including the transistor and the aluminum nitride base plate was evaluated. The measured thermal resistance values of all the samples were less than 1.9°C/W. Consequently, it is understood, that the aluminum nitride base plate is suited as a base plate on which a semiconductor device is to be mounted, from a view point of thermal conductivity. No. Lead Frame Stress Relieving Member Soldering Strength (kg) 1 Kovar (t = 0.05 mm) 4µm Ni-plated Cu (t = 0.1 mm) 13 2 ditto ditto (t = 0.05 mm) 11 3 ditto not provided 1.3 4 Kovar (t = 0.1 mm) 4µm Ni-plated Cu (t = 0.1 mm) 13 5 ditto ditto (t = 0.05 mm) 12 6 ditto ditto (t = 0.02 mm) 9 7 ditto ditto (t = 0.005 mm) 4.5 8 ditto ditto (t = 1.5 mm) 0.8 9 ditto not provided 1.5 10 ditto not provided 5.5 11 Kovar (t = 0.2 mm) 4µm Ni-plated Cu (t = 0.2 mm) 10 12 ditto ditto (t = 0.1 mm) 8.5 13 ditto not provided 1.2 14 Kovar (t = 0.07 mm) Cu (t = 0.1 mm) 14 15 ditto Cu (t = 0.05 mm) 12.5 16 ditto Cu (t = 0.03 mm) 11 17 ditto not provided 2 18 Kovar (t = 0.1 mm) Cu (t = 0.1 mm) 11 19 ditto Cu (t = 0.05 mm) 8 20 Kovar (t = 1.4 mm) Cu (t = 0.01 - 1.0 mm) 1.5 21 ditto not provided 1.5 22 Kovar (t = 0.1 mm) Ni (t = 0.1 mm) 7 23 ditto Ni (t = 0.05mm) 5 24 Fe-Ni 42 alloy 4µm Ni-plated Cu (t = 0.1mm) 10.5 (t = 0.1mm) 25 ditto ditto (t = 0.05 mm) 9 26 ditto not provided 1 27 Fe-Ni 42 alloy 4µm Ni-plated Cu (t = 0.2 mm) 8.3 (t = 0.2 mm) 28 ditto ditto (t = 0.1 mm) 7.3 29 ditto not provided 1 30 Fe-Ni 42 alloy Cu (t = 0.03 mm) 9.5 (t = 0.07 mm) 31 ditto not provided 2 32 Fe-Ni 42 alloy Ni (t = 0.1 mm) 8.5 (t = 0.1 mm) 33 ditto Ni (t = 0.05 mm) 7 Example B A metallized layer was formed on specified regions of a surface of a base plate of an aluminum nitride sintered body by the following metallization: processing. The metallization processing of samples was performed by applying tungsten paste of prescribed composition to the surfaces of respective samples of the aluminum formed base plates shaped in a configuration such as a green sheet by the aforementioned method with screen printing of prescribed patterns, drying, performing debindering and then sintering, the same in a nitrogen atmosphere at a temperature of 1850°C for 5 hours. Thus, metallized layers were formed on prescribed portions of aluminum nitride sintered base plates. Nickel plating layers of 2 µm in thickness were formed on the metallized layers, and sintered in a hydrogen atmosphere at a temperature of 750°C for 20 minutes. Lead frames were joined on the metallized layer in the same manner as in the example A, whereby samples were prepared. The junction strength between the lead frames and the base plate of each sample was evaluated in the same manner as in the example A. The results are shown in Table 2. From Table 2, it is found that the soldering strengths of the samples not using any stress relieving member were as low as 0.5 to 2 kg. The start point of breaking of each of such samples existing in the aluminum nitride base plate and cracks due to thermal stress were found for each of such samples. No. Lead Frame Stress Relieving Member Soldering Strength (kg) 1 Kovar (t = 0.1 mm) 4µm Ni-plated Cu (t = 0.1 mm) 13.5 2 ditto ditto (t = 0.05 mm) 10:5 3- ditto Cu (t = 0.1 mm) 13 4. ditto ditto (t = 0.05 trim) 10.5 5 ditto Ni (t = 0.1 mm) 9.5 6 ditto ditto (t = 0.05 mm) 7 7 ditto not provided 1.5 8 Fe-Ni 42 alloy (t = 0.1 mm) 4µm Ni-plated Cu (t = 0.1 mm) 14 9 ditto ditto (t = 0.05 mm) 10.5 10 ditto Cu (t= 0.1 mm) 12 11 ditto ditto (t = 0.05 mm) 10.5 12 ditto Ni (t = 0.1mm) 8.5 13 ditto ditto (t = 0.05 mm) 7 14 ditto not-provided 1.5 Example C Lead frames were soldered without interposition of a stress relieving member by silver solder at a temperature of about 830°C to be joined onto a metallized layer including tungsten as a major component formed on a surface of an aluminum nitride base plate. In this case, the lead frames used had the shape shown in Fig. 5 with the thickness thereof being 0.1 mm. As shown in Fig. 2C or 3C, the junction portion of each lead frame was formed of a three-layer composite metal portion or a two-layer composite metal portion, the composition thereof being shown in Table 3, and each lead frame was formed to have a uniform thickness. The junction strength between the lead frames and the aluminum nitride base plate of each of ten samples prepared was measured in the same manner as in the Example A. The results of the measurement of the junction strength are shown in Table 3 which indicates ranges of the different measured values of the ten samples. As is evident from Table 3, in the case of using lead frames having three-layer composite metal portions and two layer composite metal portions for junction with the respective aluminum nitride base plates, the junction strengths were remarkably improved compared with the cases of using conventional lead frames formed of only Kovar or 42 alloy. It is further understood that in the cases of using lead frames having three layer composite metal portions, the ranges of difference in junction strength were smaller than those in the cases of using lead frames having two-layer composite metaL portions. Further, bending strength tests were conducted with a period required for a free end of a lead frame bent at right angles to be restored to the initial state being regarded as one cycle. As a result, as for the lead frames having three-layer composite metal portions and, the conventional lead frames formed of only Kovar or 42 alloy, more than 24 cycles were required until they were broken. On the other hand, the lead frames entirely formed of a two-layer composite metal portion were broken by 5 to 10 cycles and the lead frames formed of only a soft metal material were broken by 2 to 6 cycles. No. Stress Relieving Layer on Soldered Surface (µm) Central Metal Layer (µm) Stress Relieving Layer on Opposite Surface (µm) Junction Strength (kg) Breakage 1 Cu (20) Kovar (60) Cu (20) 9.4~10.8 In Lead Frame 2 Cu (40) Kovar (60) none 2.4~10.1 ditto 3 Cu (30) 42 Alloy (40) Cu (30) 7.6~9.9 ditto 4 Cu (40) 42 Alloy (60) none ~7.1 ditto 5 Fe (20) Kovar (60) Fe (20) 5.6~7.7 ditto 6 Ni (20) Kovar (60) Ni (20) 6.0~6.9 ditto 7 Fe (40) 42 Alloy (60) none 3.1~5.8. ditto 8 Ni (40) 42 Alloy (60) none 5.2~6.1 ditto 9 Kovar (100) 0.9~1.4. In AIN 10 42 Alloy (100) 0.8~1.1 ditto Values in () represent thickness of the respective layers. Example D In the same manner as in the Example C, lead frames having the same shape and the same thickness of 0.1 mm were joined to an aluminum nitride base platen by soldering as shown in Fig. 5. Each lead frame was prepared to have a portion to be soldered including a three-layer composite metal portion or a two-layer composite metal portion as shown in Table 4. Each lead frame was joined to a metallized layer including tungsten as a major component formed on specified region of a surface of the aluminum nitride base plate as in the Example A, by a soldering process at a temperature from 450 to 600°C using a low melting metal soldering material having a composition of gold-silicon, gold-silver-germanium or the like. The junction strength was measured in the same manner as in the Example A with regard to ten samples of each kind thus obtained. The results of the measurement of the junction strength are shown in Table 4, which indicates ranges of difference of the measured values. As can be seen from Table 4, in the cases of using lead frames having three-layer composite metal portions and lead frames having two-layer composite metal portions, the junction strengths were remarkably improved compared with the conventional lead frames formed of only Kovar or 42 alloy. It is further understood that in the cases of using the lead frames having the three layer composite metal portions, the ranges of difference in the junction strength were small. No. Stress Relieving Layer on Soldered Surface (µm) Central Metal Layer (µm) Stress Relieving Layer on Opposite Surface (µm) Junction Strength (kg) Breakage 1 Cu (30) Kovar (40) Cu (30) 9.4~10.8 In Lead Frame 2 Cu (40) Kovar (60) none 3.9~10.1 ditto 3 Cu (20) 42 Alloy (60) Cu (20) 8.6~10.2 ditto 4 Cu (40) 42 Alloy (60) none ~8.0 ditto 5 Fe (20) Kovar (60) Fe (20) 8.8~11.0 ditto 6 Fe (40) 42 Alloy (60) none 2.4~7.6 ditto 7 Ni (20) Kovar (60) Ni (20) 6.6~8.1 ditto 8 Ni (40) 42 Alloy (60) none 1.0~7.1 ditto 9 Al (20) Kovar (60) Al (20) 10.1~11.4 ditto 10 Al (40) Kovar (60) none 11.2~12.2 ditto 11 Al (20) 42 Alloy (60) Al (20) 9.2~10.8 ditto 12 Al (40) 42 Alloy (60) none 8.8~11.0 ditto 13 Kovar (100) 1.6 In AlN 14 42 Alloy (100) 1.8 ditto Values in () represent thickness of the respective layers. Example E A metallized layer including molybdenum as a major component was formed in the same manner as in the Example A on a surrounding portion of a covering member formed of a sintered body of aluminum nitride having a thickness of 3 mm and an area of 10 mm x 10 mm. After nickel plating was applied to the metallized layer, a frame member of 0.1 mm in thickness was soldered thereon by silver solder at a temperature of 830°C. This frame member was formed by a three-layer composite metal shown in Table 5. In the case of using copper as an outer layer portion- of- the frame member, a nickel plating process was applied before the soldering. Warping and airtightness of each of the caps thus obtained were measured. The warping was measured as the value a as shown in Fig. 7 in junction between a covering member 11 and a frame member 30. Airtightness was measured by- placing each cap in a helium gas atmosphere and evacuating the cap as shown in Fig. 8. The junction portion between a covering member of an aluminum nitride sintered body and a frame member was examined by a scanning type electronic microscope of a magnification 5000 or a stereoscopic microscope of a magnification- 40 to examine cracks or defects. Samples each having a frame member of a two-layer composite metal or a single-layer composite metal were also prepared for comparison and warping and airtightness thereof were measured. The results of the measurements are shown in Table 5. The values of warping indicated in Table 5 are average values each obtained from five measured values. As can be clearly seen from Table 5, the caps having the connection structures in accordance with the present invention have excellent characteristics in resistance to warping and airtightness. In addition, no cracks were found in the junction portions of the caps in accordance with the present invention.

Claims: A connection structure between components of a semiconductor package comprising: a base member (1) formed of aluminum nitride having a major surface on which a semiconductor device is to be mounted, a metallized layer (2) disposed on a bonding surface of the base member (1) a lead frame (3) to be joined to the base member (1) via the metallized layer (2), the lead frame including, as a main material, any of iron-nickel alloys and iron-nickel-cobalt alloys, a stress relieving member (13) interposed between the metallized layer (2) of the base member (1) and the lead frame (3), the stress relieving member being formed of copper based or nickel based materials which materials cause the stress relieving member to become softened near a temperature of soldering and to become readily plastically deformable so that a thermal stress caused by a difference between a thermal expansion coefficient of the base member (1) and that of the connecting member (3) at the time of soldering is relieved and a soldering material (9) for joining the base member (1) with the stress relieving member (13) and the stress relieving member (13) with the lead frame (3). A connection structure in accordance with claim 1 wherein the stress relieving member (13) has a thickness in the range of 0.01 to 1 mm, whereas the lead frame (3) has a thickness of 0,1 mm and a width of 8 mm. A connection structure in accordance with claim 1, wherein the metallized layer (2) contains at least a metal selected from tungsten and molybdenum; at least an aluminum compound selected among the group of aluminum nitride, aluminum oxide and aluminum oxynitride; and calcium oxide. A connection structure in accordance with claim 1, further comprising a plated, thin Ni-layer (7), formed on a surface of the metallized layer (2) to be joined to the soldering material (9) and/or on a surface of the lead frame (3) to be joined to the soldering material (9) and/or on the surface of the stress relieving member (13) for improving wettability of the respective surface(s) with respect to the soldering material. A connection structure between components of a semiconductor package comprising: a base member (1) formed of aluminum nitride having a major surface on which a semiconductor device is to be mounted, a metallized layer (2) disposed on a bonding surface of the base member (1), a lead frame (3) to be joined to the base member (1) via the metallized layer (2), the lead frame including a lead frame layer (23) of any of iron-nickel-alloys and iron-nickel-cobalt-alloys and at least one additional layer as a stress relieving layer (13) formed of a material selected from the group comprising copper, copper alloy, iron and aluminum which stress relieving layer (13) is plastically deformed when soldered so that thermal stress caused by a difference between a thermal expansion coefficient of the base member (1) and that of the lead frame member (3) at the time of soldering is relieved, and a soldering material (9) for joining the base member to the stress relieving layer (13) of the lead frame (3). A connection structure in accordance with claim 5, wherein a portion (3a) of the lead frame (3) which is to be soldered to the base member (1) comprises two layers, namely the stress relieving layer (13) to be soldered to the base member (1) and the lead frame layer (23) prepared of any iron-nickel alloys and iron-nickel-cobalt alloys. A connection structure in accordance with claim 5, wherein a portion (3a) of the lead frame (3) which is to be solded to the base member (1) comprises three layers, namely the stress relieving layer (13) to be solded to the base member, the lead frame layer (23) and another stress relieving layer (13) in this order. A connection structure in accordance with claim 5, wherein a portion (3b) of the lead frame (3) not to be connected with the base member (1) is formed completely of any of iron-nickel alloys and iron-nickel-cobalt alloys. A connection structure in accordance with claim 5, wherein the metallized layer (2) contains at least a metal selected from tungsten and molybdenum; at least an aluminum compound selected among the group of aluminum nitride, aluminum oxide and aluminum oxynitride; and calcium oxide. A connection structure in accordance with claim 5 to 9, wherein a thin plated Ni-layer is formed on a surface of the metallized layer (2) to be joined to the soldering material and/or on a surface of the lead frame to be joined to the soldering material for improving wettability of the metallized layer and/or the lead frame with respect to the soldering material. A cap for airtightly enclosing a semiconductor device mounting on an insulating base plate comprising: a covering member (11), formed of aluminum nitride provided over the semiconductor device (4) to protect the same, a metallized layer (2), formed on a bonding surface of the covering member (11), a frame member (30) to be joined to the covering member (11) to surround the semiconductor device located under the covering member, the frame member (30) being formed by a three-layer composite metal including, as a main material, iron-nickel alloys and iron-nickel-cobalt alloys, and, as stress relieving layers, outer layers formed of any materials selected among the group of copper, copper alloys, nickel, nickel alloys, iron and aluminum, and a soldering material (9) for joining the covering member (11), the stress relieving layer (130) and the frame member (30), whereby the stress relieving layer (130) is to relieve by plastic deformation of itself, a thermal stress caused by a difference between a thermal expansion coefficient of the covering member and of the frame member at the time of soldering. A cap according to claim 11, wherein the metallized layer (2) contains at least a metal selected from tungsten and molybdenum, at least an aluminum compound selected among the group of aluminum nitride, aluminum oxide and aluminum oxynitride, and calcium oxyde. A cap according to claim 11, wherein a thin plated Ni-layer is formed on a surface of the stress relieving layer (130) of the frame member (30) to be joined to the soldering material (9) for improving wettability of the stress relieving layer with respect to the soldering material.

Inventors: AKAZAWA HITOSHI, MIYAKE MASAYA, SAKANOUE HITOYUKI, SASAME AKIRA, TEKEUCHI HISAO, YAMAKAWA AKIRA, YUSHIO YASUHISA, AKAZAWA, HITOSHI, MIYAKE, MASAYA, SAKANOUE, HITOYUKI, SASAME, AKIRA, TEKEUCHI, HISAO, YAMAKAWA, AKIRA, YUSHIO, YASUHISA, Akazawa, Hitoshi c/o Itami works of Sumitomo, MIYAKE, MASAYA C/O ITAMI WORKS OF SUMITOMO, SAKANOUE, HITOYUKI C/O ITAMI WORKS OF SUMITOMO, Sasame, Akira c/o Itami Works of Sumitomo, Tekeuchi, Hisao c/o Itami works of Sumitomo, YAMAKAWA, AKIRA C/O ITAMI WORKS OF SUMITOMO, Yushio, Yasuhisa c/o Itami works of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1988
Main classification: H01L23
IPC codes: H05K3, H01L23, H01L21, H05K1