Metrology
Right: Figure 1:
Crack in interlayer
dielectric (ILD). Pores
may have shapes
other than circular
Far Right: Figure 2:
Planar acoustic image
showing cracks (white
features within circles)
in ILD. Other cracks
of various sizes are
present as well
somewhat more likely to be cracked by Narrow vertical
differences in the CTE. Cracking can also be To date, acoustic micro imaging of ILDs has
induced by mechanical stresses created during merely demonstrated the x-y location and the
various handling steps, including probe testing, size of a crack, but not its precise depth. Since
dicing of the wafer, wire-bonding and the the ultrasound of an acoustic micro imaging
attaching of solder bumps. system is reflected from the whole thickness of
The ILD itself is typically less than 2.0 the layers of metallization and ILD, there has
microns thick. A crack in the ILD’s nanoporous been no way to identify which of several layers a
structure [Figure 1] can be large or small in crack resides in. More recently, however,
relation to the layer itself. Only a crack that is techniques have been developed that make it
large enough to create an electrical open will be possible in many devices to define more
revealed by electrical tests. Many ILD cracks do narrowly the vertical location of a crack within
42
not attain this size during fabrication, and thus the layers. Isolating the crack to a single ILD
escape detection. But they are still capable of layer by acoustic micro imaging alone is still
www
degrading the electrical insulation properties of usually not possible, but it is possible to
.eur the ILD, and if they grow larger during service ascertain the approximate vertical location – to
oasiasemiconductor
as a consequence of thermal cycling or other know, for example, that a crack is near the top
factors, they may cause an unexpected failure. of a stack of 8 layers, and that it is probably in
The problem that engineers face is that ILD layers 1 or 2.
cracks are very difficult to detect by In making an acoustic image, the ultrasonic
nondestructive methods. They are poor subjects pulse emitted by the scanning transducer above
for x-ray or IR imaging, but the fact that they the wafer or device is focused on the depth of
.com
contain a gap makes them good subjects for interest. As higher ultrasonic frequencies are
acoustic micro imaging. Even though ILD cracks used, the spatial resolution of the acoustic
are small, thin, and located in the midst of image increases, while the depth of field
square4
Issue IV 2009
multiple layers of material, they have been decreases. But even at very high ultrasonic
successfully imaged within the last three to four frequencies (230 MHz, 300 MHz, 400 MHz), the
years, as demonstrated by the Sonoscan C-SAM depth of field is on the order of tens of microns,
acoustic microscope image in Figure 2. Their a distance greater than the total thickness of
high visibility is due to the interface between the several layers of metallization and ILD. Focusing
air in the gap and the solid material above the alone, then, will not isolate a feature such as an
gap. The sharp difference in the acoustic ILD crack in the z dimension.
properties of these two materials at this A Time Domain Image (TDI), also known as
interface causes more than 99.99% of the a planar image, is made at the depth at which
ultrasound to be reflected. The high amplitude the ultrasound has been focused. This image
of the signal makes the cracks in the planar uses echoes within a defined time window
acoustic image in Figure 2 appear bright. corresponding to the depth of interest, and
Finding ILD cracks nondestructively gives an consists of thousands or millions of pixels. Each
overall appraisal of production processes. More pixel’s level of brightness is determined by the
important, since both wafers and individual amplitude of the echo at that specific x-y
devices can be imaged, acoustic imaging can location. Data for thousands of pixels is
show, for example, that cracks that are present collected per second as the transducer scans
after dicing did not exist in the intact wafer. the wafer or device. If a 300 MHz transducer is
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