Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2000-06-01
2001-10-16
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S411000, C257S678000, C264S493000
Reexamination Certificate
active
06303907
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of semiconductor manufacturing, more particularly to “back end” assembly, and in particular to furnaces used in ceramic packaging.
BACKGROUND OF THE INVENTION
The manufacturing of packaged integrated semiconductor devices occurs generally in two stages known as “front end” processing and “back end” processing. “Front end” processing deals with formation of various devices such as transistors, resistors, and capacitors on a semiconductor wafer. “Back end” processing deals with assembly and test wherein after formation of the various devices on the semiconductor wafer, the wafer is sliced into semiconductor dies, the dies are assembled into packages, and the packaged dies are tested. Although various packaging techniques exist, the two main techniques are plastic packaging and ceramic packaging.
Ceramic packaging is discussed on pages 455-522 of chapter 7 of
Microelectronics Packaging Handbook
by Rao R. Tummala and Eugene J. Rymaszewski, copyright 1989 and on pages 727-777 of chapter 10. Although different types of ceramic packages exist, most ceramic packages have a lid covering a semiconductor die (chip) mounted to a ceramic base. Prior art
FIG. 1
shows a typical ceramic dual in line (CERDIP) packaged device
10
. In
FIG. 1
, a semiconductor die
12
is mounted to a ceramic base
14
by a chip bond material
13
. Semiconductor die
12
represents generically many types of semiconductor devices, such as, for example, dynamic random access memories (DRAMs), electrically erasable programmable read only memories (EEPROMs), and microprocessors. Three broad categories of chip die attach bond materials are solders, organic adhesives and glass. Exemplary of solders are Au—Si, Au—Sn, Pb—Ag—In, and Pb—Sn metallic compositions. Exemplary of organic adhesives are epoxies, polyimides (most frequently filled with silver) and thermoplastics such as acrylics, polyester or polyamides filled with metal. Exemplary of glasses are silver-filled glass materials as discussed in “A Critical Review of VLSI Die-Attachment in High Reliability Applications” by Shukla and Mancinger appearing in
Solid State Technology
, July 1985 page 67 et seq. The die attach material must be heated to bond the chip to the ceramic base
14
.
Continuing with reference to prior art
FIG. 1
, packaged device
10
includes wire bonds
16
connecting semiconductor die
12
to a lead frame
18
that is adhered on ceramic base
14
by a seal material
20
. Seal material
20
is typically a glass that must be heated to embed lead frame
18
. A lid
22
is attached by a seal
24
and covers semiconductor die
12
to seal ceramic package
10
. Lid
22
is typically alumina oxide. Seal
24
is typically a glass and must be heated to secure lid
22
to base
14
.
Prior art
FIG. 2
illustrates an assembly work cell
26
as may exist in a semiconductor manufacturing facility for packaging a ceramic device as illustrated in prior art FIG.
1
. Work cell
26
includes a base and lead frame loader station
28
. At station
28
, ceramic base
14
is placed upon a cerdip metal tray holder, illustrated in prior art FIG.
3
. Lead frame
18
is then placed on ceramic base
14
. Cerdip tray
30
of
FIG. 3
is made of stainless steel, is about 12 inches long, is about 3 inches wide, and holds
10
ceramic bases. At die bonder station
32
, the die attach material
13
is placed onto base
14
and the die
12
is placed onto base
14
. At furnace station
34
, the die attach material and seal material are heated to firmly adhere die
12
to base
14
and to embed lead frame
18
.
Furnace station
34
of prior art
FIG. 2
unfortunately occupies a large amount of expensive manufacturing floor space; it is up to 30 feet long and about 4 feet wide. While multiple cerdip trays
30
travel through furnace
34
on a conveyor belt, it unfortunately takes about 1 hour for a tray
30
to pass through furnace
34
. Furnace time needs to be reduced to reduce cycle time. Heat is typically provided by electricity passing through metal heater filaments or by gas flames that are disposed over the conveyor furnace belt. The furnace
34
unfortunately has a problem with particle contamination as it is a dirty process. As device geometries continue to shrink (for example, present 16 megabit dynamic random access memories and electrically programmable read only memories are manufactured using 0.5 micron design rules while 256 megabit dynamic random access memories are expected to utilize 0.25 micron design rules), particulates that were formerly acceptable become unacceptable because their size may approximate that of the design rules of the manufactured device. Carbon particulates from furnace
34
may cause about a 10% product loss. Another problem is incomplete lead frame embed; about 2 to 3 thousand packages per million are lost due to incomplete lead frame attach.
After die attach and lead frame embed in prior art
FIG. 2
, the ceramic bases
14
on cerdip trays
30
go to bonder stations
36
where wire bonds
18
are connected to semiconductor die
12
and to lead frame
18
. Several bonding stations
18
are typically provided in a work cell. Next, trays
30
go to cap loader station
38
. Here, lids
22
with seal material
24
thereon are placed onto bases
14
. Thereafter, not shown in prior art
FIG. 2
, the devices are heated in a conveyor furnace similar to furnace
34
to seal the lids. Prior art
FIG. 4
, taken from page 752 of
Microelectronics Packaging Handbook
, shows a typical sealing/glazing profile for cerdip ceramic packages.
A proposed lid sealing apparatus and method by Bokil would replace a metal heater filament with a beam of focused infrared light to reduce heat transfer into the ceramic package. See U.S. Pat. No. 4,481,708 issued Nov. 13, 1984 and U.S. Pat. No. 4,685,200 issued Aug. 11, 1987, and, the article by D. E. Erickson “Hybrid Circuit Sealing-Problem Prevention Clinic”, Electronic Packaging and Production, 22(11): pp. 133-137, November 1982. The Bokil system unfortunately appears relatively large and complicated due to the spacing design required to focus the infrared beams towards the glass for the lid seal and the required number of infrared beams (one on each side of the package).
It is thus an object of the invention to provide a new apparatus and method for lid seal on ceramic packages.
Further objects and benefits of the invention will be apparent to those of ordinary skill in the art having the benefit of the description and drawings following herein.
SUMMARY OF THE INVENTION
A halogen lamp furnace substantially reduces the time required for lid seal in ceramic packages. The furnace chamber has a lamp assembly including a reflector that is disposed above the ceramic package. Reflection profile is used to optimize the distance from the top of the ceramic packages such that the light is unfocused and temperature across the lid and package is uniform. Glass lid seal may be accomplished in about 5 minutes.
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