Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Encapsulating
Reexamination Certificate
2000-06-29
2002-08-20
Sherry, Michael J. (Department: 2829)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Encapsulating
C438S106000
Reexamination Certificate
active
06436737
ABSTRACT:
BACKGROUND OF THE INVENTION
As the operating voltages and size of semiconductors continue to shrink to satisfy the demand for low power, high density semiconductor devices are being used more widely. However, high density semiconductor devices have a higher sensitivity to ionizing radiation. This phenomenon has been studied and is the subject of several papers, including several by Robert Baumann et al. The papers are “Neutron-Induced Boron Fission as a Major Source of Soft Errors in Deep Submicron SRAM Devices” and “Boron Compounds as a Dominant Source of Alpha Particles in Semiconductor Devices.” Ionizing radiation can be defined as any electromagnetic or particulate radiation that produces ion pairs when passing through a medium. An ion, generally speaking, is simply any atom or molecule which has a resultant electric charge due to loss or gain of electrons. The charge created by the formation of the ion pairs can, upon interaction with the semiconductor device cause a disruption in an electrical signal or can corrupt information stored in localized nodes on the device. Such a failure is referred to in the art as a “soft error” because only the data are corrupted while the circuit itself remains unaffected. Other sources of soft errors include electrical noise such as bit-line coupling, and other electronic effects such as cross-talk.
Prior research has focused on determining the sources of this ionizing radiation in an attempt to understand the effect of these sources on the soft error rate (SER). Three major sources of ionizing radiation have been determined. These sources are radiation from Alpha-particle emitting impurities, radiation from Cosmic Rays, and radiation from neutron capture by boron. Each source is discussed separately below.
Radiation from Alpha-particle Emitting Impurities
A significant source of ionizing radiation in semiconductor devices is the result of naturally occurring radioactive particles in semiconductor materials themselves. The major radioactive impurities are uranium (the elemental symbol for which is U) and thorium (the elemental symbol for which is Th), and their progeny.
238
U and
232
Th both decompose to isotopes of lead, which are stable. During the decay process,
238
U emits eight alpha particles, while
232
Th emits six.
238
U and
232
Th are found throughout the materials used in the fabrication of semiconductors. They can be found in the gold used for bond wires and lid plating, filler compounds, interconnect metals, various alloys, etc. Their presence has a small, but detectable effect on the occurrence of soft errors.
Radiation from Cosmic Rays
Cosmic rays are extremely energetic particles moving through the universe at a speed near the speed of light. As these extremely energetic particles move through the earth's upper atmosphere, they collide with other particles and form secondary particles. These secondary particles include neutrons (which are uncharged particles), protons (which are positively charged particles), and electrons (which are negatively charged particles). These secondary particles can cause disturbances in the semiconductor device directly or indirectly. Direct interference can occur when one or more of the secondary particles contacts the semiconductor device and has been found to be a minor effect on soft errors. Indirect interference occurs when the neutrons cause alpha particle emission indirectly, as is explained below.
Radiation from Neutron Capture by Boron
The third, and most significant source of ionizing radiation affecting soft errors is indirect radiation induced from the interaction of the neutrons generated by cosmic rays with boron (the elemental symbol for which is B). Boron is used extensively in semiconductor assembly as a p-type dopant. A p-type dopant is an atom introduced in small quantities into a crystalline semiconductor where the atom attracts electrons. In this way “holes” are produced which effectively become positive charge carriers. Boron is also used in the formation of borophosphosilicate glass (hereafter BPSG) that is deposited on the surface of the silicon wafer during processing.
Boron, as it occurs naturally, has two isotopes,
11
B and
10
B. In naturally occurring boron, approximately 80% of boron atoms are
11
B, and the remaining 20% are
10
B. Upon absorbing a neutron (such as the neutrons generated by the influx of cosmic rays into the upper atmosphere)
10
B fragments, or “fissions,” into an excited
7
Li nucleus and an alpha particle. Both the excited
7
Li nucleus and the alpha particle are capable of causing soft errors. It has been shown that, based on the amounts of boron commonly used in semiconductor manufacturing, neutron capture by
10
B can cause 0.02 soft error events per hour. This rate roughly corresponds to one event every two days. Thus, the neutron capture by
10
B can be a significant source of errors in computers. It has been shown that
11
B is significantly less likely to undergo a fission reaction similar to
10
B.
11
B does absorb the neutrons, however, it does so without undergoing the fission reaction.
In order to determine how to reduce the occurrence of soft errors due to ionizing radiation and electronic noise, some understanding of the semiconductor assembly process is needed.
Integrated Circuit Packages
Two common techniques for electronically coupling an integrated circuit to an integrated circuit package are wire bond connections and flip-chip connections. Wire bond connections are the most common used in the microelectronics industry. The wire bonding process starts by mounting an integrated circuit to a substrate with its inactive backside down. Wires are then bonded between an active front side of the integrated circuit and the integrated circuit package.
U.S. Pat. No. 5,972,736, issued to Malladi et al., and assigned to the assignee of the present invention, discloses an integrated circuit package using the wire bonding process. Prior art
FIG. 1
illustrates the components of an integrated circuit package. In
FIG. 1
, a package body
22
includes one or more land areas (not shown) having exposed electrical pads for wire bonding between a semiconductor die
28
and the land areas. The package body
22
may be formed from a variety of materials, such as alumina, glass-ceramic, and polymers with appropriate metal inter-connection layers. The land areas are connected by a conductive pattern to areas to which solder balls
42
are affixed allowing for the later surface mounting of the completed device to a printed circuit board or other substrate. Attached to the package body
22
is an attachment mechanism
146
. Various materials are used for the attachment mechanism
146
, including copper, aluminum, various alloys, and plastics. The attaching mechanism
146
is attached to the package body
22
by a low softening temperature adhesive
134
.
The next step in the assembly process is the die attach operation. In this step, semiconductor die
28
is attached to the attachment mechanism
146
utilizing a die attach adhesive
138
. Typical die attach adhesives
138
include epoxy, polyamides, metal filled polymers, ceramic filled polymers, diamond filled polymer, silver glass, or other suitable materials. After the semiconductor die
28
is attached to the attachment mechanism
146
, wire bonding is performed so that a plurality of wire bonds
30
are formed between the electrical interconnects on package body
22
and bond pads (not shown) on semiconductor die
28
. Next, the semiconductor die
28
and bond wires
30
are encapsulated by applying a suitable encapsulating material
49
such as an epoxy. The encapsulation material
49
serves to protect semiconductor die
28
and bond wires
30
as well as attaching semiconductor die
28
to package body
22
allowing the removal of the attachment mechanism
146
.
FIG. 2
illustrates the device
140
after removal of the attachment mechanism
146
and the addition of a heat sink (not shown). In
FIG. 2
, the attachment mechanism
146
has been removed by heating above the softening
Geyer Scott
Rosenthal & Osha L.L.P.
Sherry Michael J.
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