M. S. Murali Sastry

Influence of residual gases and surface contaminants on the aging behavior of island silver films

The aging of island silver films deposited on glass at room temperature and at a pressure of 2×10−5 Torr was studied by monitoring the dc electrical resistance of the films. Films in the resistance range 1–11 MΩ/⧠ were studied under different conditions to ascertain the role of the residual gases and surface contaminants on the agglomeration rate. It was found that residual gases do not significantly affect the agglomeration rate, but a film of water vapor adsorbed on the glass surface retards the agglomeration to a great extent. An expression is arrived at for the functional dependence of the interisland spacing with time for the different conditions of study, assuming that the conduction in these island films is by quantum mechanical tunneling.

Effect of applied field and temperature on the aging of copper discontinuous films studied by the repeated deposition technique

Presented are the results of investigations on the effect of an applied DC electric field on the post-deposition resistance changes of island copper films on glass. Based on the functional dependence of the film resistance on time, an agglomeration rate is defined with the theory of mobility coalescence being invoked to explain the resistance increase of the films after deposition. Repeated deposition was employed until the film resistance became steady, after which measurements of resistance versus temperature were made. The agglomeration rate for film of a particular resistance decreased as the number of deposition cycles was increased, due to formation of large, less mobile islands. For one of the higher-resistance films, the agglomeration rate showed a drastic fall for a particular value of deposition cycle number indicating that large-scale coalescence (LSC) had occurred. It was found that the presence of a field does not alter the post-deposition coalescence process in island copper films, possibly due to the relative immobility of the copper islands. The initial resistances of the film with and without the field were, however, vastly different. The agglomeration rates of films of different initial resistances showed an oscillatory behaviour corroborating earlier findings on copper films. The resistance-temperature curves showed a minimum below which the film showed a negative temperature coefficient of resistance (TCR) and a positive TCR above the temperature of transition. The transition temperature is found to shift to higher values as the films approach the discontinuous-semi-continuous structure transition. The resistance-temperature behaviour is explained if one considers that the negative TCR region is due to an increase in the number of thermally activated charged islands while the positive TCR region is due to enhanced mobility of the copper islands at elevated temperatures leading to coalescence and thus to an increase in the average inter-island spacing and the resistance of the film.

Aging and field‐effect studies of Cu island films at near liquid‐nitrogen temperatures

We report the results of the investigation carried out on the aging and field effect of discontinuous copper films deposited on glass substrates held at around 125 K. Using the scheme adopted in our earlier investigations, it was found that even at this temperature there is considerable increase in resistance. However, the agglomeration rate is much less than for films deposited at room temperature. The I‐V characteristics of the films revealed an irreversible change in resistance following removal of the field. On stabilization, heating of the films revealed a decrease in resistance with temperature followed by an irreversible resistance increase beyond a transition temperature which was less than room temperature.

Ageing and field effect studies on discontinuous silver films at near liquid nitrogen temperatures

The post deposition resistance changes in discontinuous silver films deposited in a vacuum of 2 × 10−6 torr on glass substrates maintained at near liquid nitrogen temperatures have been studied. Reduced agglomeration rates in comparison with films studied at room temperature were obtained, supporting the thermally assisted mobility coalescence model explaining the post deposition resistance increase. The non-linearI-V characteristics of one of the films followed by observations of resistance changes before and after field effect measurements on the other films have been explained as arising due to field-induced structural changes. The investigations of the variation of film resistance with temperature revealed a transition temperature. A fall in resistance with increasing temperature below the transition temperature has been explained by an increase in the number of thermally charged islands. The increase in resistance with temperature above the transition temperature is due to an increase in the thermally assisted mobility coalescence.

Studies on the stability of Cu island films deposited on a softenable substrate

In this communication, we report the results of the investigations carried out on the stability of discontinuous Cu films deposited on polymethylmethacrylate coated substrate heated to a temperature above the glass transition temperature of the polymer. It was reported that evaporated materials on such substrates form a subsurface particulate structure. It was found that in Cu films post‐deposition instability persisted on a softenable substrate similar to the behavior of Cu on glass. On exposure to atmosphere, the resistance increased due to the interaction of atmospheric gases and water vapor. A film deposited at room temperature and and heated also behaved in a similar fashion when exposed to atmosphere.

Ageing studies of discontinuous copper and silver thin films

The results of experiments carried out on the post-deposition resistance changes in discontinuous films of copper and silver with and without overlayers of SiO and Al2O3 are presented. The changes in the sheet resistance of the films with time and pressure were studied for the above combinations. Mobility coalescence is assumed to be responsible for the resistance increase of an uncovered copper film of initial resistance 1.9 MΩ/□. On exposure to the atmosphere, it was found that an Ag/SiO combination of initial resistance of 0.1 MΩ/□ achieved stability in the sheet resistance much quicker than a Cu/Al2O3 combination of initial resistance 20 MΩ/□. The fall in resistance of the Cu/Al2O3 composite is attributed to the formation of Al2(OH)6 due to the interaction of Al2O3 with the water vapour in atmosphere. Copper films with and without overlayers of Al2O3 show an abrupt increase in the sheet resistance as a function of pressure at a pressure of about 5 × 10−2 torr with the maximum rate of change of resistance occurring at higher pressure for the higher resistance film. This indicates that the overlayer of Al2O3 is very porous in nature. Field effect studies were carried out on an uncovered copper film of initial resistance 10 MΩ/□ and the behaviour was found to be ohmic up to a field of 800 V cm−1.

Repeated deposition studies of the occurrence of large scale coalescence and effect of electric field on the ageing of island silver films

Abstract Presented in this paper are the results of an investigation into the influence of a dc electric field on the growth and post-deposition resistance changes in island Ag-films deposited on glass substrates at room temperature. Based on the functional dependence of the film resistance on time, an agglomeration rate is defined with the theory of mobility coalescence invoked to explain the resistance changes in these films. It was found that the magnitude of the electric field (9 V/cm) did not influence the post-deposition coalescence but did affect the initial resistance of the film. Films studied under relative humidity conditions of greater than 85% showed an interesting crossover of the functioanl dependence of the film resistance on time consistent with earlier findings in this laboratory. Repeated deposition studies revealed that the agglomeration rate fell drastically beyond a particular deposition cycle indicating that the percolation threshold had been reached with the occurrence of large scale coalescence (LSC).

Time variation of the tunnelling length in island Cu films studied by the repeated deposition technique

The post-deposition resistance increase (aging) in island Cu films on glass studied by the repeated deposition technique was quantified with the definition of an agglomeration rate in an earlier study. Using an analysis employed by Andersson, with some modifications, the change in the tunnelling length during aging (inter-island spacing) and the values of the tunnelling length before and after aging are determined. A new agglomeration rate is defined in terms of the change in the tunnelling length per unit time which enables easier comparison between films. As an illustration, the technique is applied to the Cu island films studied by the repeated deposition technique.

Influence of ion-bombardment cleaning on the aging rates in island copper films on fused quartz substrates

Presented in this communication are the results of aging experiments carried out on copper island films deposited on fused quartz substrates held at 380 K in a vacuum of 1×10−5 Torr. Ion‐bombardment cleaning of the substrates prior to deposition of the films was found to significantly alter the aging rates in the films. The mobility coalescence model was operative during the aging process, and it is inferred from the data that adsorbed gases on the substrate surface affect the nucleation and reduce the mobility of copper islands, which reduces the aging rate. The mean island sizes were determined for some of the films from activation energy measurements.