X-ray or gamma ray systems or devices – Beam control – Filter
Reexamination Certificate
2002-08-08
2004-06-15
Glick, Edward J. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Filter
C250S505100
Reexamination Certificate
active
06751294
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to x-ray inspection of semiconductor devices, and more particularly, to prevention of changes to semiconductor device properties while proper x-ray inspection is achieved.
2. Background Art
In order to maximize the quality of printed circuit board manufacturing process, semiconductor devices, in particular surface mounted devices with “hidden” solder joints to a printed circuit board, typically undergo an x-ray in on. In f thereof, and with reference to
FIG. 1
(rotated 90 degrees clockwise from a conventional orientation of elements therein), a semiconductor device
20
is placed on an inspection tray
22
of for example polymer material. Such a typical semiconductor device
20
includes silicon body
24
having a protective coating
25
of molding compound (In
FIG. 1
shown lying on the tray
22
), the silicon body
24
having active region
24
A and inactive region
24
B secured to a substrate
26
by a silver-organic material adhesive
28
(wire bonds connecting silicon body
24
and substrate
26
not shown). The substrate
26
includes organic portions
30
,
32
(dielectric layers) and patterned copper layers (one shown at
34
), which copper layer
34
communicates with the active region
24
A of the silicon body
24
(active region
24
A approximately one micron (1&mgr;m) in thickness and oriented most adjacent the tray
22
) and lead/tin solder balls
36
which connect to a layer of copper traces
38
on an organic material (for example polyimide, epoxy, polyethylene, or glass fiber) board
40
, i.e., a printed circuit board or printed wiring board. It will be understood that the particular configuration of the semiconductor device
20
shown is for purposes of illusion, and that such device
20
may be configured in a wide variety of ways, i.e., for example, such semiconductor device may cam a number of levels of copper layers
34
and dielectric layers
30
,
32
, with appropriate vias connecting the copper layers.
In this example, for purposes of illustration, the following typical thickness values are given:
Tray 22:
400
&mgr;m
Molding Compound 25:
300
&mgr;m
Silicon body 24:
200
&mgr;m (including 1 &mgr;m active region)
Silver-organic adhesive 28:
30
&mgr;m
Dielectric layer 30:
200
&mgr;m
Copper layer 34:
50
&mgr;m
Dielectric layer 32:
200
&mgr;m
Solder balls 36:
400
&mgr;m
Copper layer 38:
50
&mgr;m
Printed circuit board 40:
200
&mgr;m
During the x-ray inspection, x-rays of a wide range of energies are provided from a source
42
through the tray
22
and into and through the semiconductor device
20
, with substantial absorption of x-rays taking place in the copper layers
34
,
38
and lead/tin solder balls
36
, as compared to the rest of the device, so that proper contrast been the images of the copper layers
34
,
38
and solder balls
36
on the one hand, and the rest of the device on the other hand, is provided at image detector
44
. In this way, flaws in the copper and/or solder balls can be observed.
During the x-ray inspection process, radiation damage can occur in the silicon body
24
. That is, an x-ray beam passing through the silicon body
24
may ionize the silicon, forming electron/hole pairs in the active region
24
A, the region approximately 1 &mgr;m thick most adjacent the tray
22
. These electrons/hole pairs in the active region
24
A can cause undesirable changes in device operating characteristics, and can cause changes to stored charge on device internal nodes or within dielectrics, causing improper operation.
For the following discussion, reference is made to pp. 1-17 of ELEMENTS OF X-RAY DIFFRACTION by B. D. Cullity, Addison-Wesley Publishing Co. Inc., published 1956, which material is herein incorporated by reference.
FIG. 2
is a graph showing x-ray absorption coefficient vs. x-ray energy for silicon, copper, tin and lead, with both axes on a logarithmic scale. As will be sen, and as described in that text, for each material, the general trend of the magnitude of absorption coefficient is downward for increasing levels of x-ray energy, varying as the inverse cube of the energy. In addition, as also described in the text, abrupt, distinctive “edges” occur for each element, corresponding to the characteristic K, L, M, etc. lines of the material. As indicated in the graph of
FIG. 2
, silicon has a high coefficient of absorption in the x-ray energy range of about 3 KeV (and is therefore highly vulnerable to the problem described above). As illustrated in
FIG. 2
, the absorption coefficient of silicon drops off significantly as x-ray energy increases, so that the vulnerability of the silicon to this problem decreases substantially with increase in x-ray energy.
FIG. 3
is a graphical representation of the structure of
FIG. 1
, showing x-ray absorption at the 3 KeV energy level (intensity axis on a logarithmic scale, distance axis on a linear scale). As will be seen, after some absorption by the tray
22
and the molding compound
25
, the silicon body
24
, including the active region
24
A thereof, is exposed to x-ray energy of a high intensity and absorbs a substantial amount of x-ray energy at this energy level (the actual absorption of a body is indicated by the change in intensity of the x-ray entering and passing through the body in accordance with the formula.
I
x
=I
0
e
&mgr;x
where
&mgr;=linear absorption coefficient, dependent on material considered, its density, and the wavelength or energy of the incident x-rays
I
0
=intensity of incident x-ray beam, and
I
x
=intensity of transmitted beam after passing through a thickness x (see the above cited text at page 10).
Even though the active region
24
A is only approximately 1 &mgr;m thick, as pointed out above, silicon has a high coefficient of absorption at this energy, and there is only the tray
22
and molding compound 25 between the source of x-rays
42
and the active region
24
A to absorb x-rays as they travel toward the image detector
44
, leading to the problems described above.
With reference to
FIG. 4
, m the event that the semiconductor device
20
is in a “flipped over” state on the tray
22
, with substantial absorption of x-ray energy by the material between the active region
24
A of the silicon body
24
and the source of x-rays
42
, the problem described above is generally avoided (see
FIG. 5
, even for x-rays passing between the solder balls
36
). However, complete structures commonly include semiconductor devices
20
on both sides of a printed circuit board
40
, combining the orientation of
FIG. 1 and 4
in a single structure, so that the problem described above with regard to the orientation of
FIG. 1
continues to exist.
While x-ray inspection system suppliers mention use of a filter in the x-ray process, no systematic approach is indicated for dealing with this problem.
What is needed is an x-ray system wherein the silicon body of a device being x-rayed absorbs minimal x-ray energy, while copper layers and solder balls of the device are highly absorbent of x-ray energy so that proper imaging of the device is provided.
DISCLOSURE OF THE INVENTION
The present apparatus for irradiating a device with x-rays comprising a first material and a second material associated therewith includes a source of x-rays, a filter for receiving x-rays from the source of x-rays and allowing transmission of x-rays therethrough to the device, the filter having an atomic number greater than the atomic number of the second material of the device, and an x-ray imager for receiving x-rays from the device.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable
Black J. Courtney
Blish II Richard C.
Darling Don C.
Lehtonen David S.
Li Susan Xia
Advanced Micro Devices , Inc.
Glick Edward J.
Thomas Courtney
LandOfFree
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