Tutorial #22
K. O’Donnell, J. Kostetsky, and R.S. Post
September 2002
Adapted from “Stress Control in NiV, Cr and TiW Thin Films used in UBM and Backside Metallization,” a paper presented at IMAPS Flipchips 2002, authored by K. O’Donnell, J. Kostetsky, and R.S. Post of NEXX Systems LLC, 90 Industrial Way, Wilmington MA 01887-4610.
INTRODUCTION
Two packaging processes which require reliable metallurgical interfaces are UBM (Under Bump Metallurgy) and Backside Metal. The UBM provides the critical interface between the metal pad of the integrated circuit and the solder ball. Backside metal is used in high power devices as a heat sink. UBM requires excellent adhesion and a low stress metal stack on aluminum while backside metal requires excellent adhesion and a low stress stack on silicon. High intrinsic stress in thin films can cause adhesion problems and compromise long-term reliability.
Causes of Stress
Stress in thin films results from differences in thermal expansion (thermal stress) or from the microstructure of the deposited film (intrinsic stress). At substrate temperatures less than 20% of the melting point, intrinsic stress due to incomplete structural ordering dominates.
Thermal stress occurs because film depositions are usually made above room temperature. Upon cooling from the deposition temperature to room temperature, the difference in the thermal expansion coefficients of the substrate and the film cause thermal stress.
Intrinsic stress results from the microstructure created in the film as atoms are deposited on the substrate. Tensile stress results from microvoids in the thin film, because of the attractive interaction of atoms across the voids.
Figure 1. Tensile stress, conceptual diagram. The film wants to be “smaller” than the substrate because it was “stretched” to fit.
Compressive stress results when heavy ions or energetic particles strike the film during deposition. The impacts are like hitting the film with a hammer, packing the atoms more tightly.
Figure 2.Compressive stress, conceptual diagram. The film wants to be “larger” than the substrate, because it was “compressed” to fit.
Stress Control
Stress control is achieved by varying the degree of energetic particle bombardment during sputtering. Compressive stress is usually attributed to an ‘atomic peening’ mechanism [1] in which reflected neutral atoms bombard the growing film at low sputtering pressures.
An increase in sputtering pressure increases the frequency of gas phase collisions, reducing the kinetic energy of sputtered neutral atoms and reflected neutrals bombarding the growing film. This reduction in ‘atomic peening’ reduces compressive stress. The sputtering pressure at which a stress reversal from compressive to tensile occurs increases with the atomic mass of the metal being sputtered [1].
The majority of metal films deposited by sputtering are in tensile stress. RF Bias of substrates during deposition increases bombardment of the film with energetic ions and causes a stress reversal from tensile to compressive.
This tutorial shows the effects of sputtering pressures and bias voltages in stress control. Data are presented for stress and resistivity in chromiun (Cr), nickel-vanadium (NiV, 7 wt.% V) and titianium-tungsten (TiW, 10 wt.% Ti) deposited at different sputtering pressures and with different bias voltages applied to the substrate during deposition. Films were deposited with a NEXX NimbusTM sputtering system.
Microstructural analysis of the films was performed using Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) to determine the correlation between the microstructure and the macroscopic film properties (stress and resistivity).
RESULTS AND DISCUSSION
TiW Films
We first review the results from sputtering high mass TiW, which as deposited is compressive. Figure 3 shows resistivity and stress in TiW (3000 Å) deposited at sputtering pressures from 2 to 20 mTorr.
Figure 3. Stress control in TiW film by varying sputtering pressures.
The grain size does not appear to change with increasing pressure but the grain morphology changes dramatically. The surface roughness increased from 2.91 nanometers (rms) at 2 mTorr to 5.24 nanometers (rms) at 20mTorr.
The surface morphology indicates a dense microstructure in the TiW sputter deposited at 2 mTorr which is reflected in the large compressive stress measured for this film (- 1800 MPa). The microstructure changes from a dense to a voided microstructure in the TiW film deposited at 20 mTorr which is reflected in the tensile stress measured for this film (+700 MPa). The increase in resistivity is consistent with the development of such a voided structure.
The energetic particles bombarding the substrate are the sputtered atoms of TiW and working gas ions that are neutralized and reflected at the cathode. No argon was detected using EDXS analysis of TiW samples, which is consistent with sputtered atoms having the greatest effect. Wehner and Anderson showed that sputtering with argon and krypton yielded similar stress variations as a function of process pressure and concluded that sputtered atoms dominate [2].
NiV and Cr Films
We now review sputtering of lower mass NiV and Cr films, which as deposited are in tensile stress. Figure 4 shows film stress in NiV (4000Å) with a Titanium (200Å) seed layer and Cr (4000Å) deposited directly on SiO2. NiV deposited directly on SiO2 showed a similar variation in stress with RF Bias. The Ti (200Å) seed layer was used to improve adhesion and to reflect more accurately metal stacks used for UBM or backside metal.
Figure 4. Stress control in NiV and Cr films by varying RF bias voltage.
The resistivity of Cr (4000Å) does not vary significantly with increasing substrate bias.The NiV resistivity also does not vary with substrate bias. Energetic ions bombarding the growing film at increased substrate bias results in a high density of defect clusters, which modifies the film stress but not the resistivity.
The increase in resistivity seen above for TiW films deposited at increased process pressure is associated with a voided microstructure and increased tensile stress. The ‘atomic peening’ mechanism evident in TiW is not significant in the case of Cr and NiV due to the lower atomic mass of these elements. Reduction of tensile stress in Cr and NiV films is accompanied by reduced surface roughness and more uniform grain size.
CONCLUSIONS
Stress in TiW is controlled by varying gas pressure during deposition. TiW sputter deposited at pressures ranging from 2 to 20 mTorr shows a structure change from a dense fibrous microstructure to a voided structure of isolated columns which is associated with the stress reversal from compressive to tensile.
Reduction of tensile stress in Cr and NiV is achieved by applying substrate bias during deposition. Reduced surface roughness of films deposited with substrate bias together with a more uniform grain size distribution are consistent with a more dense microstructure in films with reduced tensile stress.
ACKNOWLEDGEMENTS
TEM and AFM analysis courtesy of Dr. Changmo Sung of the Center for Advanced Materials at the University of Massachusetts-Lowell. Thanks to graduate students Xianglin Li (AFM), Carolina Ospina (Image Processing), and Bongwoo Kang (TEM sample preparation) for their assistance.
Additional TEM analysis provided by Professor Kevin S. Jones of the Department of Materials Science and Engineering at the University of Florida-Gainesville
Technical contributions from staff at Nexx Systems include Johannes Chiu for RF bias processing and Jon Roberts for material analysis.
REFERENCES
[1] J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol. 14, 164 (1977)
[2] G. K. Wehner and G.S. Anderson, Handbook of Thin Film Technology, edited by L. I. Maissel and R. Glang (McGraw Hill, New York, 1970), p.3-23
FOR MORE INFORMATION
The complete paper with 12 figures was presented at the IMAPS Flip Chip Technology Workshop & Exhibition, June 24-26 2002, Austin, Texas. For more information, contact NEXX Systems as listed below.
NEXX Systems, LLC
Wilmington MA 01887-4610
978-284-4980
www.nexxsystems.com