Page 97 - Sami Franssila Introduction to Microfabrication

76 Introduction to Microfabrication

microstructure (temperatures are normalized to melting sputtered under different conditions can end up in
point temperatures, T /T m , in K). Zone 1 is small- either body centred cubic (bcc) structure or as
grained and porous. Zone 2 has larger columnar grains tetragonal β-Ta. Resistivity of bcc-Ta is ca. 20 µohm-
and Zone 3 exhibits still larger grains. The intermediate cm with temperature coefficient of resistivity (TCR)
◦
region is termed Zone-T (for transition). 3800 ppm/ C. Values for β-Ta are ca. 160 µohm-cm
◦
Z1 is the region where the low momentum of and 178 ppm/ C, respectively (see Figure 2.8 for
the impinging specie is combined with slow chemical another tantalum deposition experiment). In Chapter 19,
processes due to low temperature: the film atoms come TiSi 2 phase transformation upon annealing will be
to rest almost immediately and do not move. This discussed.
leads to a porous structure with columnar grains (see Grains in polycrystalline films can have any crystal
Figure 3.6 for simulated columnar-grain structure). Such orientation, but in practice, films are often strongly
a structure is under moderate tensile stress. The voids textured: the distribution of grain orientations are along
between the grains are nanometre-sized, which leads to one or two main crystal planes. For example, aluminium
measurable density reduction and poor stability because films usually have a (111) texture, that is, (111) planes
of the absorption of moisture and oxygen. Impurities are parallel to the wafer surface. For undoped LPCVD,
such as oxygen can change the intrinsic stress from polysilicon (110)-orientation crystals dominate, but for
tensile to compressive and complicate the simple model in situ phosphorus doped poly (311) is the dominant
described above. orientation.
At lower pressure, ion bombardment induces densifi- The texture is established during deposition, and it
cation of the film, and the film stress is highly tensile. A is not much affected by subsequent annealing steps
further increase in ion bombardment (at lower pressure below (2/3) T m even though the grain size is. Texture
or higher sputtering power) leads to the disappearance inheritance is common: subsequent films easily acquire
of voids and conversion to compressive stress. Higher the same texture as the underlying film. Thin seed layers
temperature leads to enhanced surface diffusion that can can therefore be used to modify the thick layers. This is
be calculated from Equation 7.1: true for CVD and electrodeposition too.
√
2
x = 4Dt (7.1)
7.2.1 Characterization of PVD films
where D = D 0 exp (−6.5T m /T ) and surface diffusion
2
constant D 0 is of the order of 10 −7 m /s and t is the PVD films, especially sputter-deposited films, can be
time it takes to deposit the next atomic layer. For atoms modified by a number of parameters. System configu-
to diffuse distances similar to void sizes (∼nanometre), ration and geometry come to play via target-substrate
Equation 7.1 can be used to estimate temperatures where distance, base pressure/gas phase impurities and power
transition from Z1 to Zone T takes place. coupling scheme/bias voltage; and process parame-
Z2 occurs at T /T m > 0.3, so the surface diffusion is ters such as pressure and power affect the momen-
significant. The grains grow larger, and the defects are tum of the impinging atoms and ions, and substrate
eliminated. Z3 occurs at T /T m > 0.5, and the diffusion temperature is important for desorption, diffusion and
process is very fast. Elimination of the voids enhances reactions.
diffusion. The films are annealed during deposition. The Collimated sputtering is a technique in which a
grains are more isotropic and the films ‘lose memory’ mechanical grid is placed between the anode and the
of the deposition-process details. cathode, and off-angle atoms do not contribute to the
The final grain size is determined by subsequent flux arriving at the wafer, but are deposited on the
annealing steps. The sputtered aluminium grain size collimator walls. Collimated sputtering is better in filling
is ca. 0.5 µm, similar to a typical film thickness. In the bottoms of holes and trenches. In Table 7.1, a
3 µm lines, there are always many grains across the collimated system is compared with a conventional
line, but in 0.5 µm lines, the situation changes dramat- system, and analysed for an extensive range of film
ically: there are practically no three-grain boundaries parameters. These characterization measurements relate
and the grains are end-to-end, known as bamboo struc- to R&D phase, and in manufacturing sheet resistance
ture. All processes that depend on grain boundaries, will be used for quick monitoring.
such as diffusion and electromigration, are strongly Electrical characterization described in Chapter 2 and
affected. above has been DC, but circuits that operate at gigahertz
Film structure can change not only continuously frequencies must be measured at proper frequencies. The
as described above but also abruptly. Tantalum films same applies to dielectric films too.