The general concepts governing the electrochemical deposition of metal films onto semiconductors are discussed. Deposition onto semiconductor surfaces is complicated due to the band structure of the semiconductor, which affects both the thermodynamics and the kinetics of metal deposition processes. The influence of the potential distribution at the semiconductor/solution interface on the charge transfer mechanisms involved in deposition of metals is discussed. Models for electrochemical nucleation and growth are described and the influence of the unique physical properties of semiconductors is analysed. Finally, we present recent results for electrochemical deposition of gold, copper and platinum onto n-type silicon.

For systems where deposition occurs through Volmer-Weber island growth, the structure and properties of thin films are critically dependent on the mechanism of nucleation and growth. 1 For example, high nucleus densities are essential for achieving island coalescence at low coverages. For complex structures with small length scales, such as trenches and vias in integrated circuits, 2 a detailed understanding of nucleation and growth during electrodeposition is critical for designing deposition processes for obtaining void-free features.The mechanism of nucleation and growth of copper on TiN from noncomplexing, acidic fluoroborate solution involves instantaneous nucleation of hemispherical clusters followed by diffusion-limited growth, over a wide potential range. 3 The nucleus density is in the range 10 5 -10 9 cm Ϫ2 and is dependent on the applied potential. In this paper we report on the deposition of copper on TiN from pyrophosphate solution and show how deposition from the pyrophosphate complex influences the nucleation and growth process. In particular, we show that the deposition mechanism also follows instantaneous nucleation of hemispherical clusters followed by diffusion-limited growth, however, the nucleus densities are in the range 10 8 -10 11 cm Ϫ2 , about two orders of magnitude larger than for fluoroborate solution.Copper pyrophosphate solutions are used in the electronics industry for through-hole plating. 4,5 For solutions where [P 2 O 7 4Ϫ ]/ [Cu 2ϩ ] > 1, more than 99% of the copper ions are present in the form of the Cu(P 2 O 7 ) 2 4Ϫ complex. Deposition of copper from a complex is kinetically slower than for uncomplexed solutions, generally resulting in better deposition in complex geometries. Common additives include NH 4 ϩ , NO 3 Ϫ (cathode depolarizer), and organic brighteners (e.g., dimercaptothiadiazole). 4,5 Experimental The substrates for deposition were prepared by sputter deposition of 30 nm TiN on n-Si(100), N D ϭ 1 ϫ 10 15 cm Ϫ3 (Wacker Siltronic, AG). The TN layer was rf sputtered at room temperature for about 1 min (V rf ϭ 620 V). In all cases ohmic contacts were made to the back side of the silicon wafer using InGa eutectic. Since the nSi/TiN contact is ohmic, this method avoids limitations associated with the sheet resistance of the TiN layer. The aqueous 50 mM Cu(II) solution was prepared from 25 mM Cu 2 P 2 O 7 и3H 2 O with 0.2 M K 4 P 2 O 7 . The pH of the solution was adjusted to pH 8.5 with pyrophosphoric acid (H 4 P 2 O 7 ). From the equilibrium constants, 6,7 we determine that greater than 99% of the Cu(II) is present in the form of Cu(P 2 O 7 ) 2 6Ϫ . 8 Common additives such as NH 4 ϩ , NO 3Ϫ , and organic brighteners (e.g., dimercaptothiadiazole) were not investigated in this work. NO 3Ϫ , which serves as a cathode depolarizer, is redox active and hence represents an additional contribution to the deposition current. 4 The experiments were performed under ambient conditions using a conventional three-electrode cell with a Ag/AgCl (3 M NaCl) reference electrode connected...

The deposition of copper metallization is achieved from acidic copper sulfate solution containing small concentrations of chloride, suppressor, accelerator, and leveler. Void-free filling of trenches and vias results from the complex interplay between the additives in the solution. Here we report on the adsorption/desorption of the copper-suppressor-chloride complex in solutions without accelerator or leveler. We show that desorption of the suppressor complex occurs at a critical potential that is dependent on the pH, chloride concentration, cupric ion concentration, and suppressor concentration. We derive an expression for the critical potential from which we determine the composition of the complex. In addition, the solubility product of the complex is determined to be log K s = −14.4.

The general concepts governing the electrochemical deposition of metal films on semiconductors are discussed, and recent results on the fabrication of Schottky junctions consisting of silicon electrodeposited with platinum, copper and gold are presented. In order to obtain good adherent metal films, the density of nuclei should be high and the films should be grown at low current densities where the charge transfer process is rate limiting. This situation can be realized using potential controlled electrochemical deposition. For metal deposition on silicon, the surface should be pretreated in HF to dissolve the oxide layer. Furthermore, the surface should be stable during deposition which can be achieved by tailoring the deposition solutions and by using electrochemical deposition at negative potentials. It is shown that by using this approach, n-type silicon / Pt, Au, and Cu Schottky junctions can be fabricated of a quality comparable to that of junctions prepared by sputter and vapor deposition techniques.

We describe a case of a 5-year-old girl with onchocerciasis. The patient was recently adopted from Ethiopia and presented with a firm, raised nodule on the midportion of the forehead. Initially, Langerhans cell histiocytosis with bone involvement was suspected; however, histopathologic analysis of the excised nodule revealed the presence of a young-adult, female Onchocerca volvulus worm. This case exemplifies the importance of recognizing the key morphologic characteristics of adult O. volvulus worms isolated from pediatric patients in nonendemic areas to ensure adroit clinical management.