Low-melting point glass sealed semiconductor device and method of manufacturing the same.

0001The present invention relates to a low-melting point glass sealed semiconductor device containing a semiconductor chip which is die-bonded to a ceramic substrate by use of a thermosetting mounting agent.

0002As is shown in Fig. 1, a conventional low-melting point glass sealed semiconductor device comprises: a ceramic substrate 1 coated with low-melting point glass 3 and serving as a cap; and a ceramic substrate 2 coated with low-melting point glass 4 and having a chip 5 thereon. The substrates 1 and 2 are overlaid with each other, with their glass-coated sides overlapped, and are then subjected to heat treatment. After melted by this heat treatment, the glass coated on the substrates 1 and 2 sets or hardens, thereby sealing the chip 5 on the ceramic substrate 2.

0003The conventional low-melting point glass sealed semiconductor device shown in Fig. 1 will be described in more detail. The semiconductor chip 5 is located on the predetermined position of the ceramic substrate 2. The semiconductor chip 5 is adhered to the ceramic substrate 2 by use of a thermosetting mounting agent (not shown). Normally, a polyimide-based agent is used as this thermosetting mounting agent, since it enables efficient work and provides reliable adhesion and since it is lower in price than gold paste. A lead frame 6 is located on low-melting point glass 4. It is electrically connected to the semiconductor chip 5 by wires 7. The ceramic substrate 1 serving as a cap is adhered to the other ceramic substrate 2. Between these ceramic substrates 1 and 2, the lead frame 6 is sandwiched and hermetically sealed. It should be noted that the thermosetting mounting agent used in the semi­conductor device has a characteristic of decomposing into a gas at a high temperature.

0004The low-melting point glass sealed semiconductor device mentioned above is manufactured as follows. First, the ceramic substrate 2 is coated with low-­melting point glass 4 in a similar manner to that in a printing process, except for the portion on which the semiconductor chip 5 is to be mounted. Next, the lead frame 6 is mounted on the low-melting point glass 4, and the chip 5 is mounted on the portion which is not coated with the glass, by use of the thermosetting mounting agent. Thereafter, the lead frame 6 and the semiconduc­tor chip 5 are electrically connected together by wires 7. Next, the low-melting point glass 3 on ceramic substrate 2 and that 4 on ceramic substrate 3 are heated at a temperature higher than their melting points. In the state where the glass is melted, the ceramic sub­strates 1 and 2 are overlaid with each other and hermetically sealed.

0005However, the conventional semiconductor device men­tioned above cannot be fabricated at high yield and is not very reliable with respect to the sealed characte­ristic. This is because the thermosetting mounting agent it uses is likely to decompose into a gas at a high temperature. If the thermosetting mounting agent decomposes into a gas, voids are left in the low-melting point glass 3, 4. In addition, the gas spreads along the surface of the lead frame 6, so that a tunnel-like void extending from the interior of the glass to the outside of the semiconductor device is formed, resulting in defective adhesion.

0006Accordingly, the first object of the present inven­tion is to provide a low-melting point glass sealed semiconductor device, which has few voids (externally-­exposed ones and internal ones) in the glass portion thereof, is reliable in both sealed characteristic and operation and yet low in price, and can be manufactured at high yield. The second object of the present inven­tion is to provide a method of manufacturing such a semiconductor device.

0007To achieve the first object, the present invention provides a low-melting point glass sealed semiconductor device which comprises: a first ceramic substrate having a rectangular shape and curved around one of two axes thereof with a predetermined curvature; a semiconductor chip adhered to a central portion of an inwardly-curved surface of the first ceramic substrate by use of a polyimide-based thermosetting mounting agent; a second ceramic substrate having a rectangular shape and curved around one of two axes thereof with a predetermined curvature; and a filler member including: low-melting point glass which is coated on the inwardly-curved surface of the first ceramic substrate, except for a portion to which the semiconductor chip is adhered., and low-melting point glass which is coated on an inwardly-curved surface of the second ceramic substrate; wherein the first and second ceramic substrates are stacked upon each other, with their inwardly-curved sur­faces facing each other and with their straight side edges in contact with each other, and a space defined between the first and second ceramic substrates is filled with the filler member.

0008To achieve the second object, the present invention provides a method of manufacturing a low-melting point glass sealed semiconductor device, which comprises the steps of: preparing first and second ceramic substrates each of which has a rectangular shape and is curved around one of two axes thereof with a predetermined curvature; coating low-melting point glass on an inwardly-­curved surface of the first ceramic substrate, except for a portion to which a semiconductor substrate is to be adhered; adhering the semiconductor substrate to a central portion of the inwardly-curved surface of the first ceramic substrate by use of a polyimide-based thermoset­ting mounting agent; coating low-melting point glass on an inwardly-­curved surface of the second ceramic substrate; bringing straight side edges of the first and sec­ond ceramic substrates into contact with each other, with the inwardly-curved surfaces thereof facing each other; and gradually heating the first and second ceramic substrates, such that a gas generated from part of the thermosetting mounting agent is first guided to the out­side of the device through a space defined between the curved ceramic substrates and then the low-melting point glass melted by heat fills the space and hardens.

0009In the low-melting point glass sealed semiconductor device of the present invention, the low-melting point glass does not contain a void arising from a gas gener­ated by the thermosetting mounting agent. In addition, the semiconductor device does not have a tunnel-like void extending along the lead frame, i.e., a a tunnel-­like void extending from the interior of the glass to the outside of the semiconductor device. Therefore, the semiconductor device is reliable in its sealed characteristic.

0010In the manufacturing method of the present invention, the gas produced by the decomposition of part of the thermosetting mounting agent escapes to the outside the device through the spaced defined between the first and second ceramic substrates. After the gas is completely expelled from the device, the low-melting point glass is melted, thereby filling the space and adhering the first and second ceramic substrates to each other.

0011This invention can be more fully understood from the following detailed description when taken in con­junction with the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Fig. 1 is a perspective view of a conventional low-­melting point glass sealed semiconductor device;</li><li>Fig. 2 is a perspective view of a low-melting point glass sealed semiconductor device of the present invention;</li><li>Figs. 3A to 3C illustrate the process in which the low-melting point glass sealed semiconductor device of the present invention is manufactured;</li><li>Fig. 4 is a graph showing how the temperature and weight of a thermosetting mounting agent vary in rela­tion to heating time;</li><li>Fig. 5A is a graph showing the distribution of sat­isfactory and defective products in relation to the shapes of ceramic substrates; and</li><li>Figs. 5B to 5D are views illustrating curved states of ceramic substrates.</li></ul>

0012The low-melting point glass sealed semiconductor device according to one embodiment of the present inven­tion and the manufacturing method thereof will now be described, with reference to the accompanying drawings.

0013Fig. 2 is a perspective view of the low-melting point glass sealed semiconductor device according to one embodiment of the present invention.

0014As is shown in Fig. 2, a ceramic substrate 1 (which is used as a cap) and a ceramic substrate 2 (on which a semiconductor chip is to be installed) are both rectangular. They are curved around one of the two axes thereof with a predetermined curvature and look like arches. That is, they are warped when they have been manufactured. Low-melting point glass 3 is coated on the inwardly-curved surface of ceramic substrate 1. Likewise, low-melting point glass 4 is coated on the inwardly-curved surface of ceramic substrate 2. A semi­conductor chip 5 is adhered to a predetermined portion on the inwardly-curved surface of ceramic substrate 2 by use of a thermosetting mounting agent 9. As in the prior art, this thermosetting mounting agent is a polyimide-based agent.

0015A lead frame 6 is attached to low-melting point glass 4. More specifically, the lead frame 6 is first placed on low-melting point glass 4, and then the resul­tant structure is made to pass through a furnace, so as to heat the lead frame 6 to a predetermined temperature. Accordingly, the lead frame 6 is melted and adhered to the low-melting point glass 4. Thereafter, the lead frame 6 is electrically connected to the semiconductor chip 5 by means of wires 7. After this connection, the ceramic substrate 1 serving as a cap is mounted on the ceramic substrate 2, with the inwardly-curved surfaces overlapped with each other, in such a manner as to pro­vide a space between the two ceramic substrates. In this state, the two ceramic substrates are subjected to a bonding step, thereby obtaining the low-melting point glass sealed semiconductor device of the present invention.

0016The method of manufacturing the low-melting point glass sealed semiconductor device will be described, with reference to 3A through 3C.

0017Figs. 3A to 3C are perspective views schematically illustrating the process in which the low-melting point glass sealed semiconductor device according to one embodiment of the present invention is manufactured.

0018First of all, two ceramic substrates 1 and 2 are prepared. As is shown in Fig. 3A, each of the ceramic substrates is rectangular in shape and is curved around one of the two axes thereof with a predetermined curvature. In the curved state, the ceramic substrates are baked. The inwardly-curved surface of ceramic substrate 1 is entirely coated with low-melting point glass 3. The inwardly-curved surface of ceramic sub­strate 2 is also coated with low-melting point glass 4, but this coating is performed except for the portion on which a semiconductor chip is to be mounted. These coating steps are carried out in a manner similar to that of a printing process. Next, a lead frame (not shown in Fig. 3A) is mounted on the low-melting point glass 4 of ceramic substrate 2, and in this state the substrate 2 is heated, to thereby cause the lead frame to adhere to the glass. Further, a semiconductor chip (not shown in Fig. 3A) is adhered to the installation portion on the inwardly-curved surface by use of a ther­mosetting mounting agent (not shown in Fig. 3A). Next, wire bonding is performed, wherein the lead frame is connected to the semiconductor chip by means of wires (not shown in Fig. 3A).

0019Next, an adhesion step is carried out. As is shown in Fig. 3B, the two ceramic substrates 1 and 2 are stacked upon each other in such a manner as to define a space therebetween. That is, they are stacked, with their inwardly-curved surfaces overlapped. In this state, the ceramic substrates 1 and 2 are placed in a furnace (not shown), for heating. In the initial stage of the adhesion step, the low-melting point glass 3, 4 does not melt, so that there is a space between the two substrates, as is indicated by "8" in Fig. 3B. As the thermosetting mounting agent becomes higher in tempera­ture with time, it begins to decompose into a gas. However, this gas is discharged from the semiconductor device through the space 8 mentioned above, and does not remain inside the device. In other words, the space 8 continues to exist at temperatures at which the thermo­setting mounting agent decomposes and generates a gas. This condition will be explained, referring to the graph shown in Fig. 4.

0020Fig. 4 is a graph showing how the temperature and weight of a thermosetting mounting agent vary in rela­tion to heating time.

0021In Fig. 4, curve <u style="single">a</u> represents a weight-decreasing ratio of the thermosetting mounting agent, while curve <u style="single">b</u> represents a temperature increase of the thermosetting mounting agent.

0022How to use the graph shown in Fig. 4 will be briefly explained. At time t3, the weight-decreasing ratio of the mounting agent is about 82.5%, as is read­ily understood from the graph. In this case, a line segment is drawn in parallel to the ordinate from that point on curve <u style="single">a</u> which corresponds to time t3 until it intersects line <u style="single">b</u>. From this intersection, a line seg­ment is drawn in parallel to the abscissa until it intersects the ordinate, and the temperature value at this intersection is read. In this way, it is under­stood that a weight-decreasing ration of 82.5% corre­sponds to a temperature of about 450°C.

0023As can be understood from curve <u style="single">a</u> in the graph shown in Fig. 4, the weight of the mounting agent abruptly decreases at the point where the temperature exceeds 350°C. This means that the mounting agent is decomposed into a gas from this point. The decomposi­tion of the mounting agent (i.e., the generation of a gas) continues until the point where the temperature is about 450°C, as may be understood from curve <u style="single">b</u> in Fig. 4.

0024At 450°C, low-melting point glass 3, 4 begins to change from a solid state into a liquid state. The low-melting point glass in this transition state has a certain degree of viscosity, but can keep the shape which it has in the solid state. In other words, the space 8 between the ceramic substrates 1 and 2 is main­tained in the transition state of the low-melting point glass.

0025As may be understood from the above, time t3 at which the low-melting point glass changes into the liq­uid state should be after time t1 at which the gas gen­eration is completed. As long as this requirement is satisfied, all gas is discharged from the semiconductor device through the space 8.

0026After time t3, the low-melting point glass 3, 4 changes into a liquid state, thus filling the space 8 between the two ceramic substrates 1 and 2, as is shown in Fig. 3C. More specifically, the low-melting point glass 3, 4 in the fluid state flows into the space 8. After the space 8 is filled with the low-melting point glass, the heating is stopped, so as to change the low-­melting point glass back into the solid state.

0027In the above process, the low-melting point glass sealed semiconductor device of one embodiment of the present invention is manufactured.

0028Fig. 5A is a graph showing the distribution of sat­isfactory and defective products, the graph being obtained by examining low-melting point glass sealed semiconductor devices whose ceramic substrates have such a shape as is determined in the present invention and by examining low-melting point glass sealed semiconductor devices whose ceramic substrates do not have such a shape as is determined in the present invention.

0029In Fig. 5A, the abscissa represents the maximum displacement by which the ceramic substrate 1 serving as a cap is located away from the contact plane in which substrate 1 and substrate 2 contacts each other, while the ordinate represents the maximum displacement by which the ceramic substrate 2 having a semiconductor chip thereon is located away from the same contact plane.

0030The displacements shown in Fig. 5A will be explained in more detail, with reference to the cross sectional views shown in Figs. 5B to 5D.

0031The ceramic substrate illustrated in Fig. 5B has such a shape as is determined in the present invention. The maximum displacement of this substrate with refer­ence to the contact plane is represented by a negative value.

0032The ceramic substrate illustrated in Fig. 5C also has a shape as is determined in the present invention, but low-melting point glass is coated on the outwardly-­curved surface of this substrate. Therefore, the maxi­mum displacement of the substrate with reference to the contact plane is represented by a positive value.

0033The ceramic substrate illustrated in Fig. 5D is corrugated. In the case of this substrate, the dis­placements of points located on the glass-coated surface are represented by positive values.

0034Referring to Fig. 5A, all satisfactory products indicated by are in the third quadrant, where both the substrates 1 and 2 have negative displacements. As is seen in Fig. 5A, the negative displacements of the subs­trates 1 and 2 are concentratedly distributed in the region from 20 µm to 120 µm. The upper limit of these negative displacements is determined by the size of the low-melting point glass sealed semiconductor devices, namely, the sizes of substrates 1 and 2. For example, if the substrates 1 and 2 have a surface of approxi­mately 52 mm × 15 mm, the upper limit of the displace­ments is 120 µm or so.

0035Defective products indicated by and × are either in the first quadrant, where both the substrates 1 and 2 have positive displacements, or in the second and fourth quadrants, where one of the substrates has a positive displacement.

0036Basically, the defective prooucts indicated by are a semiconductor device wherein each of the paired substrates has such a shape as is illustrated in Fig. 5C, while the defective products indicated by × are a semiconductor device wherein at least one of the paired substrates has such a shape as is illustrated in Fig. 5D.

0037In the low-melting point glass sealed semiconductor device of the present invention, the gas which is gener­ated from the thermosetting mounting agent in the adhe­sion step hardly remains inside the device. Since the formation of voids noted above is suppressed, the semi­conductor device is satisfactory in operation. Further, the method of the present invention enables such satis­factory semiconductor devices to be manufactured at high yield.

0038In the low-melting point glass sealed semiconductor device of the present invention, the two ceramic subst­rates have substantially the same shape. Therefore, the space produced when such ceramic substrates are stacked upon each other is located in the center of the semicon­ductor device. Thus, the formation of voids can be effectively suppressed.

0039As mentioned above, the low-melting point glass sealed semiconductor device of the present invention is very reliable in its sealed characteristic. In addition, the method of the present invention enables such semiconductor devices to be manufactured at high yield, thus contributing to reduction in the manufac­turing cost.

0040Reference signs in the claims are intended for better understanding and shall not limit the scope.