Metrology
being used, the ultrasonic frequency of the it will narrow the range considerably.
echoes (and of the pulse itself) will include In practice, the user of VRM will take
ultrasound over a range from roughly 270 MHz advantage of his own knowledge of the device
to 330 MHz. A single return echo may be geometry to isolate a crack even more closely.
anywhere in this range. In making a Time In addition to the metallization and the ILD, the
Domain Image, the amplitude is what matters; VRM images may contain other features whose
the precise frequency is unimportant. relative depth is known. In some instances the
field of view is shifted or enlarged in order to
Speeding the search incorporate such features into the VRM images.
A technique developed at Sonoscan and known The information from the VRM images can then
as the Virtual Rescanning Module (VRM) makes be used in conjunction with the known depth of
no image. Instead, it collects all echoes of all related features to zero in on the depth of the
frequencies at all locations, and stores this data. crack. In some instances it has been possible to
The area and depth of interest can then be identify the specific layer in which a crack lies by
virtually rescanned to make a great variety of using these two techniques – VRM imaging and
Issue IV 2009
images. To locate ILD cracks, FDI (Frequency local landmarks – together, and to verify the
square4
Domain Imaging) is used instead of Time identify of the cracked layer by subsequent
.com
Domain Imaging. FDI takes advantage of the destructive physical analysis.
fact that multiple echoes representing multiple
frequencies are reflected at each x-y location,
and that all of these echoes are available in the
VRM data. The VRM data can then be used to
make a number of different images, each of
which represents a narrow frequency range –
oasiasemiconductor
306 MHz, for example. The acoustic data
.eur
collected from one metallization stack might
www
yield 30 different images, each displaying the
depth of interest as it was imaged by a
43
particular frequency. These successive
dissections of the echoes make it easier to place
a given feature such as a crack at or near a
given depth. This technique will not usually
pinpoint a crack as being in a specific layer, but
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56