WO1995008842A1

WO1995008842A1 - Integrated circuit package having a lid that is specially adapted for attachment by a laser

Info

Publication number: WO1995008842A1
Application number: PCT/US1994/010552
Authority: WO
Inventors: Mohammad Akhavain; Timothy James Perhrm
Original assignee: Unisys Corporation
Priority date: 1993-09-22
Filing date: 1994-09-16
Publication date: 1995-03-30

Abstract

An integrated circuit package (10) comprises a body (11) with a cavity (14), at least one integrated circuit chip (15) mounted in the cavity, and a lid (16) which has a novel laminated structure. A bottom layer (16a) of this laminated lid forms a fused metallurgical seal (21) with the package body around the cavity to thereby stop contaminants from entering the cavity; and this bottom layer has a predetermined small thickness which optimizes its ability to be fused to the body without cracks through the seal. By comparison, an overlying top layer (16b) of the lid has a large thickness which protects the bottom layer and the chips in the cavity from damage due to handling. During the packages fabrication, the thin bottom layer is fused by itself to the body of the package with a laser welding step; and after that step is completed, the thick top layer is attached to the thin bottom layer with an adhesive (16c).

Description

INTEGRATED CIRCUIT PACKAGE HAVING A LID THAT IS SPECIALLY ADAPTED FOR ATTACHMENT BY A LASER

BACKGROUND OF THE INVENTION:

This invention relates to integrated circuit packages; and more particularly, it relates to the structure of lids for such packages and methods of attaching the lids to the packages .

In the prior art, integrated circuit chips have been enclosed in integrated circuit packages in order to protect the chips from damage due to handling and/or contamination. Commonly, the integrated circuit package is comprised of a ceramic body that has a cavity in which one or more chips are mounted, and that cavity is covered with a thin-flat single piece of material called a lid.

To attach a metal or ceramic lid to the body of the package, various epoxies have been used. However, epoxies do not form hermetic seals; and thus contaminants can migrate into the cavity and corrode the chip and/or its electrical connections. Alternatively, solder has been used to attach a metal lid to metal seal ring on the body of the package. However, to properly melt and resolidify the solder, the entire package must be heated with a certain temperature profile in a furnace; and this heating of the entire package can cause other previously soldered connections between the chips and the body of the package to soften and/or outgas contaminants into the package cavity.

As yet another alternative, a metal lid has been attached to a metal seal ring on the package body by an electron beam welding process. However, that process requires the lid to be attached in a vacuum chamber; and that is too expensive to be suitable for mass production.

To avoid the above drawbacks, a metal lid can be attached to a metal seal ring on the integrated circuit package body by a laser welding process. There, a portion of the lid and the seal ring are heated and melted with a pulse of a laser beam; and then the melted metal is resolidified to form a fused metallurgical joint. However, this laser welding process has its own drawback in that microscopic cracks can form through the resolidified metal; and these cracks then enable contaminants to enter the package cavity.

Accordingly, a primary object of the invention is to provide a novel structure for the lid of an integrated circuit package, and a novel process by which lid is attached, whereby the above cracking problem is avoided.

BRIEF SUMMARY OF THE INVENTION:

In accordance with the present invention, an integrated circuit package comprises: 1) a body with a cavity, 2) at least one integrated circuit chip mounted in the cavity, and 3) a lid which has a novel laminated structure. With this laminated lid, only a bottom layer forms a fused metallurgical seal with the package body around the cavity; and this bottom layer has a predetermined small thickness which optimizes its ability to be fused to the body without cracks through the seal. By comparison, an overlying top layer of the lid has a large thickness which protects the chips in the cavity from damage due to handling. During the packages fabrication, the thin bottom layer is fused by itself to the body of the package with a laser welding step; and after that step is completed, the thick top layer is attached to the thin bottom layer with an adhesive. If the thickness of the bottom layer is increased in an attempt to protect the chips in the cavity from damage due to handling and thereby eliminate the need for the top layer, microscopic cracks through the fused metallurgical seal with the package body occur.

BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is an enlarged cross-sectional view of an integrated circuit package which is structured as one preferred embodiment of the present invention;

Fig. 2A shows the integrated circuit package of Fig. 1 at an intermediate stage of its fabrication wherein a bottom layer of a lid is attached to the package body with a laser;

Fig. 2B shows the integrated circuit package of Fig. 1 at another stage of fabrication wherein a top layer of the lid is attached to the bottom layer with an adhesive;

FIG's. 3A-3B are a set of drawings which show how cracks are minimized in a fused metal seal between the lid and body of the Fig. 1 integrated circuit package; and,

FIG's. 4A-4B are a set of drawings which show how cracks penetrate the fused seal between the lid and package body as the thickness of the lid is increased.

DETAILED DESCRIPTION:

Referring now to Fig. 1, it shows an enlarged cross-section of a multi-chip integrated circuit package 10 which constitutes one preferred embodiment of the invention. This embodiment 10 has a body 11 which includes a rectangular shaped ceramic base 12 and a Kovar seal ring 13 which is brazed to the perimeter of the base 12. Together, the base 12 and seal ring 13 form a cavity 14 in the integrated circuit package 10; and in that cavity, integrated circuit chips 15 are mounted on a surface 12a of the base 12. On the opposite surface 12b of the base 12 are dozens of electrical power and signal pins 12c which are selectively connected to corresponding bump leads 15a on the chips 15 by microscopic conductors in the base 12; and an example of such a conductor is shown as 12d.

Now in accordance with the present invention, the cavity 14 of the integrated circuit package 10 is covered by a lid 16 which has a laminated structure. In particular, the lid 16 includes a bottom layer 16a and a top layer 16b which are held together by an adhesive 16c. Also, the bottom layer 16a, but not the top layer 16b, is metallurgically fused to the seal ring 13.

When the integrated circuit package is fabricated, the bottom layer 16a by itself is placed on the seal ring 13. Then the perimeter of the bottom layer 16a is heated with a laser beam 20, as shown in Fig. 2A, to thereby form a fused metal seal 21 between the layer 16a and the seal ring 13. In order to prevent cracks from penetrating the fused metallurgical seal 21 which is formed by the Fig. 2A step, the bottom layer 16a has an accurately controlled thickness t. If the layer 16a is too thin, then at spots, it will melt too long and flow away; whereas if the layer 16 is too thick, then at spots, cracks will form which penetrate the seal. In either case, the result is a defect in the seal 21.

In one actual process which fabricated the integrated circuit package 10, the laser beam 20 was pulsed while the perimeter of the bottom layer 16a was moved beneath the laser beam; and, the particular process parameters were as shown below is Table 1. TABLE 1

Laser type C0₂ laser

Laser beam power 105 watts average

Laser beam pulse rate 2 milliseconds ON, 5 milliseconds OFF

Laser beam diameter 0.020"

Thickness of layer 16a 0.005" ± 0.0008"

Movement of layer 16a under laser beam 0.106"/Sec Layer 16a material Kovar

Seal ring 13 material Kovar

With the above process parameters, the fused metallurgical seal 21 between the bottom layer 16a and the seal ring 13 had no leaks. By comparison, when the bottom layer 16a was decreased in thickness to just 0.004", or increased in thickness to just 0.0075", leaks would occur.

Now the above ideal thickness of 0.005" for layer

16a which results in the leak free seal is too thin to adequately protect the chips 15 from damage during the normal handling of the IC package. Consequently, to shield against such damage, the top layer 16b is attached to the bottom layer 16a via the adhesive 16c. This attachment step is shown in Fig. 2B wherein the arrows 30 indicate that the top layer 16b is moved and held against the adhesive until it hardens. Table 2 below shows the particular parameters of the layer 16b and adhesive 16c that were used to complete the package of Table 1.

TABLE 2

Thickness of layer 16b 0.010" minimum Layer 16b material Kovar

Adhesive 16c epoxy

As was pointed out above, the layer 16a cannot simply be made thicker in order to protect the chips 15 from damage due to handling because increasing it's thickness causes cracks to occur in the metallurgical seal 21. Why these cracks occur will now be described in greater detail in conjunction with FIG's. 3a-3c and 4a-4c. In FIG's. 3a-3c, the layer 16a has its ideal thickness t; whereas in FIG's. 4a-4c, the layer 16a has a much larger thickness t' . In either case, to form the metallurgical seal between the layer 16a and the seal ring 13, the laser beam 20 must be directed against the lid 16a for a time interval which is long enough to melt through a portion of the lid 16a plus melt a portion of the underlying seal ring 13. This is illustrated in FIG's. 3b and 4b wherein the melted portions are respectively identified by reference numerals P21 and P21' . Since the thickness t' is substantially larger than the thickness t, it necessarily follows that the volume of the melted material P21' is substantially larger than the volume of melted material P21.

After a pulse of the laser beam 20, the melted material P21 and P21' resolidifies. This is shown in FIG's. 3c and 4c wherein reference numeral 21 identifies the resolidified material P21 and reference numeral 21' identifies the resolidified material P21'. During the resolidifying process, the outside of the material P21 and P21' cools faster than its inside; and thus the outside solidifies and contracts faster than the inside. This uneven contraction produces stress which results in microscopic cracks 21a and 21a', and those cracks get larger as the volume of the melted material P21' increases. Eventually, as Fig. 4c shows, the cracks get large enough to penetrate completely across the seal. In addition to providing a leak free seal and a shield against mechanical damage, a third feature of the disclosed laminated lid 16 is that it is relatively inexpensive to manufacture. This third feature arises because both the bottom layer 16a and the top layer 16b can simply be stamped from uniformly thick sheets of material.

By comparison, if a lid is a single non-laminated piece of material, then that single piece of material should be made thin at its perimeter and thick elsewhere in order to provide a leak-free seal and mechanical damage protection. However, to thin the perimeter of such a one piece lid necessarily takes extra process steps such as: 1) coating all of the lid except its perimeter with a photo resist mask, 2) chemically etching the exposed lid perimeter to the desired thickness, and 3) removing the mask from the lid. But those steps add to the lid's manufacturing cost. Further, a fourth feature of the disclosed laminated lid 16 is that it seals the cavity 14 hermetically. Atmospheric contaminants cannot migrate into the cavity 14 through the joint between the seal ring 13 and the bottom lid layer 16a because they are blocked by the sidewalls of fused metallurgical seal 21. Also, the adhesive 16c acts as a barrier which prevents atmospheric contaminants from directly reaching and interacting with the top of the seal 21.

By comparison, if the lid 16 was a single non- laminated piece of material with a thin perimeter and thick interior as described above, then to make the seal in that thin perimeter be hermetic, the lid would require a corrosion resistant plated coating - such as Nickel. However, to add the corrosion resistant coating, a plating step would be needed, and a high temperature heat treatment step would be needed to eliminate corrosive residuals from the plating step. These two steps, together with the steps 1-3 above, make the cost of a single piece lid about $7; whereas the cost of the laminated lid 16 is only about $1. One preferred embodiment of the invention, as well as a preferred process for making that embodiment, has now been described in detail. In addition however, many changes and modifications can be made to those details without departing from the nature and spirit of the invention.

For example, as one particular modification, the lid layer 16a can be attached to the seal ring 13 by heating the perimeter of layer 16a via a resistance seam welding step rather than with the laser beam 20. In a resistance seam welding step, a pair of spaced apart electrodes make a high resistance contact to the layer 16a, and an AC current is passed through those electrodes to thereby heat and melt a portion of the layer 16a and the underlying seal ring 13. For this melting step, the thickness of the layer 16a must be carefully controlled in order to avoid cracks in the resulting seal for the same reason that was described above in conjunction with FIG's. 3a-3c and 4a-4c.

Also, as another modification, the lid layer 16a and the seal ring 13 can be made of various materials other than the Kovar of Table 1. For example, layer 16a can be can be made of aluminum or stainless steel or a copper alloy or an iron-nickel alloy; and the seal ring 13 can be made of molybdenum or an iron-nickel alloy or a copper- tungsten alloy. For all of these materials, the thickness t of layer 16a preferably is between 0.004" and 0.007". Also, the layer 16a and/or the seal ring 13 can be plated or non-plated with another material such as, for example, nickel or gold.

In addition, as another modification, the integrated circuit package 10 can be increased in size to enclose any desired number of chips; or it can be decreased in the size to enclose only a single chip. Likewise, the shape of the integrated circuit package 10 can be circular instead of rectangular. Also, the base 12 can be extended past the seal ring 13 to enable the power/signal pins 12c to be on the base surface 12a instead of surface 12b. Also the bump leads 15a to/from the chips 15 can be replaced with bonding wires. Further, the top lid layer 16b need not cover the entire cavity 14, but instead it can be patterned - as, for example, a grid - to cover only portions of the cavity. Also, the top layer 16b can be made of any material, can have any thickness, and can be attached to the bottom layer 16a by any adhesive - as, for example, solder.

Accordingly, it is to be understood that the present invention is not limited to the illustrated preferred embodiments but is defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of fabricating an integrated circuit package, including the steps of: providing a body portion of said package which has an open cavity that contains at least one integrated circuit chip; covering said open cavity with a thin bottom layer of metal; fusing said thin bottom layer of metal to said body portion of said package around said cavity; and thereafter. attaching a thick top layer to said thin bottom layer.

2. A method according to claim 1 wherein said fusing step is performed by heating said thin bottom layer of metal with a laser.

3. A method according to claim 1 wherein said fusing step is performed by heating sid thin bottom layer of metal with a seam resistance weld.

4. A method according to claim 1 wherein said thin bottom layer is made with a predetermined thickness which optimizes its ability to be fused to said body without cracks .

5. A method according to claim 1 wherein said thin bottom layer is made with a uniform thickness of 0.004" to 0.007".

6. A method according to claim 1 wherein said thin bottom layer is made less than half as thick as said thick top layer.

7. A method according to claim 1 wherein thin bottom layer is selected from the group of Kovar, aluminum, stainless steel, copper alloy, and iron-nickel alloy.

8. A method according to claim 1 wherein said thick top layer covers all of said thin bottom layer.

9. A method according to claim 1 wherein said thick top layer covers only portions of said thin bottom layer.

10. A method according to claim 1 wherein said thin bottom layer is attached to said thick top layer with an epoxy.

11. A method according to claim 1 wherein said thin bottom layer is attached to said thick top layer with solder.

12. A method according to claim 1 wherein said cavity holds multiple integrated circuit chips and said thin bottom layer covers all of said chips.