Ceramic lid for sealing semiconductor element and method of sealing a semiconductor element in a ceramic package

Abstract: A ceramic lid (1) for sealing a semiconductor element and has a sealing surface for forming a seal with a ceramic package (6) housing a semiconductor (7) within a recess. A glass sealant is heated and hermetically seals said semiconductor element. The ceramic lid (1) is made of alumina of high purity and a glass sealing aid is applied to the sealing surface and is pretreated at a higher temperature than the heating temperature for sealing said semiconductor element, before the hermetic sealing of the semiconductor element in the ceramic package. This improves the strength of the seal and allows a smaller seal area.

Description: The present invention relates to a ceramic lid for sealing a ceramic package containing a semiconductor element and a method of sealing a semiconductor element in a ceramic package, and more particularly relates to an alumina ceramic lid having an excellent sealing strength with a ceramic package for housing a semi-conductor element. For example, the invention relates to a ceramic lid for sealing an EPROM (Erasable and Programmable Read Only Memory) semiconductor element for erasing once-stored memory by ultraviolet radiation. As a ceramic package for an EPROM semiconductor element, as described in JP-A-61-194,751(1986), there is known a structure in which an EPROM semiconductor element is mounted on a recess of a ceramic package, and said package is hermetically sealed to an ultraviolet-transmissible ceramic or glass lid by means of a sealing glass. That is, as shown in present Fig. 4, on a ceramic package 11 formed a recess 12 on the upper surface are secured a semiconductor element 13 and a lead frame 14 by a glass layer 15, 16, respectively and said semiconductor element 13 is bonded to the lead frame 14 by means of wires 17, and on the upper surface of the ceramic package 11 is sealed a plane ceramic or glass lid 18 through the glass layer 15 so as to seal said recess 12. Moreover, as shown in present Fig. 5, such a structure is also known in which a bottom portion 16, a wire bonding portion 20 and a conductor portion 19 of a recess 12 of a ceramic package 11 are metallized, respectively, to both end portions of said conductor portion 19 read frames 14 are brazed, ceramic frames 21 for supporting a lid 18 are laminated on the upper surface of said conductor portion 19, and a ceramic or glass lid 18 is soldered 15 or glass-sealed on the upper surface of said ceramic frames 21. As a ceramic package, use is made of alumina ceramics because of electrical properties such as thermal expansion, thermal conductivity, insulation resistance and the like and mechanical strength. Moreover, as a lid for sealing a semiconductor element, use is made of metal, plastic or ceramics, but when said package is alumina, it is preferable to use alumina ceramics for matching thermal expansion. Particularly, in case for an EPROM semiconductor element, use is made of translucent polycrystalline alumina in relation to ultraviolet ray permeability as disclosed in US-A-3026210, US-A-3792142 and JP-A-63-242,964(1988). However, in a ceramic package of the prior structure, when a sealant of low melting glass at 400-500°C is printed and dried on translucent polycrystalline alumina lid or an alumina lid of high purity of more than 98 wt%, a semiconductor element is mounted within the package, and thereafter the sealant is heated to more than the melting point thereof, and the ceramic lid is sealed to the ceramic package, the sealing strength is disadvantageously small. Moreover, for higher data processing and miniaturization of ceramic package, it is required to more increase memory amount of EPROM semiconductor element, while a ceramic package for memory semiconductor should be miniaturized. However, the high density technique of a semiconductor element is limited, and a size of the element becomes large with the increase of memory amount. Moreover, a size of a semiconductor ceramic package for memory is standardized for actually mounting on circuit boards, and it is difficult to change the size with the increase of said element size. An explanation is made in detail as follows. When the size of a ceramic package is 11 mm in width, 20 mm in length and 1.7 mm in thickness (except a lead portion), an ultraviolet irradiation portion of the semiconductor element requires about 9 mm x 9 mm at 1 Mega bit and about 9 mm x 14 mm at 2 Mega bit. In this case, the sealing portion of an ultraviolet permeable ceramic lid with a ceramic package becomes about 1 mm at 1 Mega bit and 2 Mega bit in the width direction, and 11 mm and 6 mm, respectively, at 1 Mega bit and 2 Mega bit in the lengthwise direction. That is, a sealing area is 139 sq.mm at 1 Mega bit and 94 sq.mm at 2 Mega bit. Therefore, in case of using a 2 Mega bit EPROM semiconductor element for the prior ceramic package structure, there are such disadvantages that sealing strength between a ceramic lid and a package is weak and that airtightness cannot be maintained. JP-A-58-044749 describes an alumina lid for sealing a ceramic package, the lid having a heat-treated glass layer having a thickness of 50mm present before the application of the glass sealant. The present invention seeks to solve the above disadvantages, and relates to a method and a ceramic lid for sealing a semiconductor element. The method of the present invention is set out in claim 1. Secondly, the invention relates to a ceramic lid as set out in claim 4. For a better understanding of the invention, reference is made to the accompanying drawings, in which: Fig. 1 is a plan view showing an embodiment of a ceramic lid for a semiconductor memory according to the invention; Fig. 2 is a side view of the ceramic lid shown in Fig. 1; Fig. 3 is a cross-sectional view showing the condition of sealing the lid according to the invention to a ceramic package; Figs. 4 and 5 are cross-sectional views of a prior ceramic package; Fig. 6 is a graph showing shearing torque values of the present example and the reference example; and Figs. 7 and 8 are graphs showing shearing torque values of the present example. The ceramic package for a semiconductor memory and the method of manufacturing the same according to the invention will be explained in detail by referring to Figs. 1 to 3. Fig. 1 shows that a glass sealing aid 2 is applied to a sealing surface of an ultraviolet ray permeable ceramic lid 1 for a semiconductor memory with a ceramic package and treated at a higher temperature than the temperature for sealing a semiconductor element. The ceramic lid 1 is a high-purity (e.g. at least 98%) translucent polycrystalline alumina which can transmit ultraviolet rays and consists of a central ultraviolet ray permeable window portion 1A and a circumferential frame portion 1B, and the sealing aid 2 of the first layer is screen-printed, dried and heat treated at the frame portion 1B at the rear of the lid 1. It is not illustrated, but the glass sealing aid 2 may be formed in the whole region of the frame portion 1B. The glass sealing aid may be a low melting point glass sealant which will be explained in the next step, but crystalline glass is preferable from the standpoint of adhesive strength of the lid with the package. As crystalline glass, use may preferably be made of SiO₂-CaO-ZnO series, MgO-B₂O₃-SiO₂ series and the like having a sintering temperature 100-250°C higher than the sealing temperature of a semiconductor element. The reason why the temperature is limited is because less than 100°C above the sealing temperature can hardly obtain sufficient sealing strength or requires more than 30 minutes for sintering so as to give rise to trouble in mass production, and the sintering temperature exceeding 250°C above the sealing temperature hardly contributes to sealing strength. When using a low melting point glass sealant as a glass sealing aid, the heat treating temperature may preferably be 100-200°C higher than the sealing temperature. The temperature of less than 100°C higher also may lower sealing strength, and the temperature exceeding 200°C higher makes the viscosity at the time of sealing glass small so as to spread the sealing aid beyond the sealing portion or does not show predetermined thermal expansion properties and the like by evaporation, diffusion and the like. As a method of applying a glass sealing aid, in a glass sealing aid is kneaded a vehicle dissolved 5% of low molecular acryl resin in terpineol or a 5-10% of ethylcellulose in butylcarbitolacetate to prepare printing paste, and printed with the screen of 80-100 mesh made of stainless steel. The printing thickness is 10-30 µm. If the printing thickness is less than 5 µm, depending on the condition of a ceramic lid, such as surface coarseness and the like, a glass sealing aid layer becomes too thin or almost no layer is existent for contributing to improvement of strength, and in order to form a thickness exceeding 30 µm, it is necessary to repeat printing and there is no contribution to improvement of strength. Fig. 2 shows the formation of a glass sealant 3 of the second layer on the sealing surface of a ceramic package including a glass sealing aid 2 formed on a ceramic lid 1. The composition of a glass sealant may be commonly used glass for sealing IC package, and mention may be made of PbO₂-B₂O₃-SiO₂ series low melting point glass, or low melting point glass added with components such as Aℓ₂O₃, TiO₂, SnO, Na₂O, K₂O and the like to this borosilicate glass series and the like. For selecting a glass sealant, it is necessary to give consideration to a sealant temperature and time for reducing thermal damage on a semiconductor element as possible, so that use is made of low melting point glass usually having a sealing temperature of about 410°C-600°C and a time of about 10 minutes. Moreover, the ceramic package and the ceramic lid are made of alumina, so that in order to minimize a sealing strain, a coefficient of thermal expansion of the glass sealant is selected as about 50-70×10⁻⁷/°C within the range of 30-250°C. The composition of a representative glass sealant contains 60-85% by weight of PbO, 7-10% by weight of B₂O₃, 10-30% by weight of SiO₂, and 3-10% by weight of one or more than two of Aℓ₂O₃, TiO₂ and SnO₂. The above glass sealant is kneaded with the same vehicle as that in the method of applying said glass sealing aid to prepare printing paste, printed on the sealing surface of a ceramic package including the glass sealing aid 2 of the ceramic lid 1 according to a screen printing method and thereafter dried. Printing and drying are usually repeated 3-4 times to obtain a predetermined thickness of 200-300 µm. Drying is carried out for about 10-20 minutes at 120°C. After printing and drying, in order to remove an organic binder (low molecular acrylic resin liquid and the like) of the vehicle contained in the glass paste for sealing, there is carried out calcination for thermal decomposition by slowing a temperature rising rate within the temperature range of 320-380°C lower than the airtight sealing temperature of a semiconductor element in air or nitrogen. The thus formed ceramic lid 1 is sealed in a projected portion 6A of an IC package 6 through glass sealing layers 4 previously formed in the same manner as the ceramic lid, after a semiconductor element 7 is mounted on the bottom portion of a recess 8 of a package 6 as shown in Fig. 3 and wires 9 are bonded between said semiconductor element 7 and a lead frame 5 formed in the projected portion 6A of the package 6. The sealing condition is the same explained with respect to said sealing glass. In addition, the lid according to the invention may be sealed in the portion where the lead frame is formed as shown in Fig. 3, or sealed in a ceramic frame provided on elongated conductor portions 19 of wire bonding portions 20 as shown in Fig. 5. Moreover, the lid can be applied to the case of having no metal lead frame, that is, the so-called leadless package, as not shown in Figs. 3 and 4. Example There were prepared ceramic lids of permeable high purity alumina consisting of 99.9% of Aℓ₂O₃ and 0.05% of MgO and black alumina consisting of 90% of Aℓ₂O₃, 2.9% of Mn, 2.3% of TiO₂ and 2.3% of Fe₂O₃, their dimensional shape is 20 × 11 × 0.65 (thickness). There was also prepared a ceramic package made of the same black alumina as that of the ceramic lid which external shape is 20 × 11 × 0.05 (thickness), sealing portion with the lid is 20 × 11 and having a window of 1.3 × 8 at the center. On the sealing surfaces of the high purity alumina lid and the black alumina lid 1 with the ceramic package were printed and heat-treated with the use of crystalline glass and low melting point non-crystalline sealing glass as glass sealing aid layers under the condition shown in Table 1. Then, on the glass sealing aid layer was further printed low-melting point non-crystaline sealing glass in thickness of 200 µm, heat-treated at 380°C for 10 minutes, formed a glass sealing layer, and prepared a ceramic lid for sealing. On the other hand, the portion corresponding to the sealing surface of the ceramic lid with the black alumina package was formed a glass sealing layer under the same condition as that of the lid. The thus obtained lid and the package were heat-treated at temperatures shown in Table 1 for 10 minutes, and the lid and the package were sealed. Then, the thus obtained sealed body was measured at the shearing torque of the sealed surface by lid torque for glass-frit-sealed packages of American Military Standard MIL-STD-883 C 2024.2. The average value of ten test pieces as a result of measurement is shown in Table 1, and the maximum, average and minimum values are shown in a graph of Fig. 6. The ceramic lids formed with glass sealing aid layers according to the invention show high shearing torque values. An example of Table 2 is to examine an influence of a printing thickness of a glass sealing aid layer formed on the ceramic lid, and the other conditions are the same as in Table 1. With respect to shearing torque values, No. 1 of Table 1 and Nos. 1-5 of Table 2 are shown in a graph of Fig. 7. It is understood from this graph that a preferable printing thickness of the glass sealing assistant layer is 5-30 µm. An example of Table 3 is to examine an influence of a heat treating temperature of the glass sealing aid layer formed on the ceramic lid, and the shearing torque values are shown in a graph of Fig. 8 together with No. 3 data of Table 1. From this graph, in case of low melting point non-crystalline glass, it is preferable to apply heat treatment at a 100-200°C higher temperature than the sealing temperature. As apparent from the above examples, according to the invention, sealing strength of the package lid of the ceramic package is improved. The reason is because the glass sealing aid layer is previously heat treated at higher temperature than the sealing temperature, so that the glass sealing aid layer is diffused on the high purity alumina lid surface to improve affinity and to improve wettability with the glass sealing layer. As above, the ceramic lid according to the invention is explained by referring to examples of the translucent alumina lid for EPROM semiconductor element, but the ceramic lid is not limited to translucent alumina from the essential feature of the invention, and the invention is effective for high purity alumina, such as more than 96% of an alumina lid. As apparent from the above explanation, the ceramic lid and the method of manufacturing the same according to the invention are industrially useful with excellent sealing strength, particularly in case of ceramic lid for ultraviolet ray erasing type semiconductor memory, with a remarkable effect by offering a lid simply meeting with the increase of memory amount of semiconductor element.

Claims: A method of sealing a semiconductor element (7) within a recess of a ceramic package (6) using an alumina ceramic lid (1) and a heated glass sealant (3) to seal said semiconductor element hermetically in the ceramic package, comprising applying a glass sealing aid layer (2) onto at least part of a sealing surface of said ceramic lid (1), heat-treating said glass sealing aid layer (2) at a higher temperature than the sealing temperature used for sealing the semiconductor element by means of the lid (1), thereafter applying said glass sealant (3) on said glass sealing aid layer (2), characterised in that said glass sealing aid layer (2) has a thickness in the range 10 to 30 µm. A method as claimed in claim 1, wherein the ceramic lid is translucent alumina. A method as claimed in claim 1 or claim 2 wherein the glass sealing aid is crystalline glass. An alumina ceramic lid (1) for use in sealing a ceramic package containing a semiconductor element in a recess thereof, said lid (1) having a heat-treated glass sealing aid layer (2) of thickness in the range 10 to 30 µm on at least a part of a sealing surface thereof.

Inventors: KAJIHARA TAKEHIRO, MAEKAWA KOICHIRO, OHASHI TOSHIO, KAJIHARA, TAKEHIRO, MAEKAWA, KOICHIRO, OHASHI, TOSHIO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: H01L23
IPC codes: H01L23, H01L21