Applications
Tungsten (Wolfram for European types) is an extremely hard and
refractory transition metal. W is fairly inert to wet chemical attack but
readily etched in fluorine plasmas. It has several important applications
in semiconductor fabrication.
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Deposition of pure W can be used to fill the holes that make
contact to the transistor source and drain ("contact
holes") and also to fill vias between successive layers of
metal. This approach is known as a "tungsten plug"
process. Tungsten is used because of the extraordinarily good
conformality of CVD from WF6. It is necessary to provide an
adhesion/barrier layer such as Ti/TiN, to protect the underlying Si
from attack by fluorine and to ensure adhesion of W to the silicon
dioxide. |
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The silicide can be used on top of polysilicon gates to increase
conductivity of the gate line and thus increase transistor speed.
This approach is popular in DRAM fabrication,
where the gate is also the word line for the circuit. WF6 and SiH4
can be used, but dichlorosilane (SiCl2H2) is more commonly employed
as the silicon source, since it allows higher deposition
temperatures and thus results in lower fluorine concentration in the
deposited film. |
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Deposition of W is almost always by CVD from tungsten hexafluoride,
WF6. WF6 is a quite volatile liquid at room temperature; the vapor may be
delivered directly delivered through a metering device (mass flow
controller) from the reservoir. WF6 reacts readily with any moisture
present to produce tungsten oxides and hydrofluoric acid, HF. It is
imperative to avoid moisture in the plumbing: purged high quality
stainless steel and metal seals are required. The liquid will produce
copious HF vapor if exposed to air and thus must be considered quite
hazardous; toxic monitoring and protective equipment are necessary.
Dichlorosilane is a volatile liquid at room temperature, and will react
readily with air to produce hydrochloric acid (HCl); thus, similar
precautions in plumbing and handling are appropriate. We have discussed
the issues associated with handling silane previously.
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The direct exposure of a silicon substrate to WF6 at pressures of
a few Torr results in the self-limiting formation of a thin layer of
W metal. The reaction with silicon involves a large change in free
enthalpy and is highly favored, whereas silicon dioxide is
substantially inert to reaction with WF6. Thus, thin layers of W
metal can be selectively grown on exposed silicon. |
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Reduction of WF6 by hydrogen can proceed readily on any metallic
surface, but proceeds slowly or not at all on clean thermal silicon
dioxide. This approach was examined in the 80's as the basis for
selective growth of W in contact vias, but commercialization was
extremely difficult due to sensitivity to trace moisture in the
plumbing and reactor, and to damage to or contamination of the oxide
surface by previous process steps. It was abandoned in favor of
blanket deposition followed by etch back or removal by CMP (chemical
mechanical polishing) in the 90's.
To get reasonable deposition rates from this reaction, fairly
high pressures are required: 10-60 Torr. Typical temperatures are
300-500 C. |
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The use of tungsten in metal interconnects has important
reliability implications. Tungsten atoms do not drift readily under
the influence of electric currents (electromigration) or mechanical
stress. However, they form a barrier for the movement of aluminum
atoms. |
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As a consequence, long aluminum runs connected at the ends with
tungsten vias may fail preferentially at the vias under extended
high current stress. |
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Adhesion of CVD tungsten to sputtered Ti, TiN, TiW and related
materials is excellent. Adhesion to CVD silicon dioxide is generally poor.
When sputtered adhesion layers are employed, coverage at and under the
wafer edge is generally poor, whereas CVD W readily deposits on all
exposed surfaces. This excessive coverage results in W deposited directly
on underlying oxides at the wafer edges; the W metal there readily spalls
off as metal particles in cleaning baths or upon thermal cycling.
Therefore, it is important to make provisions to exclude deposition of W
on the wafer edge in design of W CVD reactors. A specialized wafer chuck
design providing gas flow around the wafer edges is used for this purpose.
Much of the development of WF6-based CVD was reported in a series of
symposia "Tungsten and Other Refractory Metals for VLSI
Applications" I through (at least) V, from about 1985 to 1990,
sponsored by the Materials Research Society (MRS).
Some other specific references:
"Surface Chemistry of the WF6-based Chemical Vapor Deposition of
Tungsten" M. Yu, K. Ahn, and R. Joshi IBM J Res Dev 34 875 (1990)
"Thermodynamic modeling of selective chemical vapor deposition
processes in microelectronic silicon" R. Madar and C. Bernard, J Vac
Sci Technol A8 1413 (1990)
"Highly Manufacturable Process Technology for Reliable 256 Mbit and 1
Gbit DRAMs" H. Kang, K. Kim, Y. Shin, I. Park, K. Ko, C. Kim, K. Oh,
S. Kim, C. Hong, K. Kwon, J. Yoo, Y. Kim, C. Lee, W. Paick, D. Suh, C.
Park, S. Lee, S. Ahn, C. Hwang and M. Lee [Samsung] IEDM 1994 p. 635
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