Morphology Control of Cu Added Electrodeposited Bi-Sb-Te Thick Films for Micro Thermoelectric Devices
Authors: Chen Chuyi , Saito Mikiko , Homma Takayuki 
Published: May 2020

Micro thermoelectric devices, which transfer thermal energy to electric energy by Seebeck effect, have aroused great attention these years [1]. Among the thermoelectric materials, Bi-Te based ones are considered to have promising performance near room temperature with the respective ZT values of approximately 0.8 [2]. Electrodeposition, as one of the ways to prepare thermoelectric materials for the micro devices, has varies of advantages such as deposition selectivity for pattern formation in combination with lithography and low process cost. We have successfully fabricated micro thermoelectric devices using electrodeposited Bi-Te materials [3] and are continuously working on the further improvement of the thermoelectric performance. One of the problems which negatively affects the performance is the dendritic and rough surface of p-type Bi-Sb-Te material [4]. The protrusions at the film surface can lead to unreliable contact with upper electrode and difficulty in fabricating thicker devices to obtain higher temperature difference. Hence, in this study, morphology control for Bi-Sb-Te film surface was investigated. Growth behavior of Bi-Sb-Te continuous and patterned films was observed, and the effects of the thickness of the photo resist were studied. The possibility of two-step electrodeposition based on the idea of “resetting” the film surface was also explored. Finally, the application of the improved process to the device was attempted.
Electrodeposition of Bi-Sb-Te was carried out using a three-electrode system with a stirring paddle at room temperature. A glass substrate coated with 100 nm Au/10 nm Cr layers was used as working electrode, Pt mesh as counter electrode, and Ag/AgCl as reference electrode, respectively. Electrolyte composition is shown in Table 1. Deposition potential was -120 mV and annealing condition was 250 ℃, 2 hours in Ar+H2 atmosphere. The thermoelectric performance of the films was evaluated by electric resistivity and maximum output. The size of p-n legs in the device is 200 μm in diameter and 30 μm in height. Patterned substrates for the selective deposition were formed using photolithography, and the details of fabrication procedure were described in our previous study [2].

Table 1 Electrolyte composition

Bi(NO3)3・5H2O

1.3 mM

TeO2

6.0 mM

Sb2O3

10 mM

C4H6O6

0.10 M

Cu(NO3)2・3H2O

1.0 mM

HNO3

1.0 M

Firstly, in order to understand the growth behavior of the protrusions for the 30 μm thick films, in-plane and cross sectional morphology was observed by SEM. It turned out that the films were compact and smooth at the initial stage of the deposition and protrusions appeared in the halfway. Thus, a two-step electrodeposition process was explored. The film surface was “reset” by CMP or annealing, working as a “buffer layer” substrate. The morphology turned out to be improved greatly by the two-step process in continuous films, which indicated a new possible process to fabricate patterned films over 30 μm thick with compact and flat surface. By studying the deposition process of the patterned films, the effects of the thickness of the photo resist were investigated. Thicker photo resist tends to cause faster growing speed in the middle part of the pattern, which lead to the out-of-flatness of the upper surface of the patterned films. It was suggested that the diffusion condition and current density distribution were more uniform in thinner photo resist, which offered a closer environment to continuous film condition. Thus, a two-step electrodeposition process was explored, in which a 10 μm-thick film was firstly deposited in 10 μm photo resist with annealing followed and then another 20 μm-thick film was electrodeposited in 20 μm photo resist, forming a 30 μm-thick patterned film. The upper surface turned out to be flat with improved morphology compared to one-step electrodeposition. Finally, the thermoelectric device was fabricated by the improved process and thermoelectric performance was analyzed to be compared with our previous results.
References:
[1] G.J. Snyder, J.R. Lim, C.K. Huang, J.P. Fleurial, Nature Materials, 2, 528-531 (2003).
[2] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater., 7 (2008), pp. 105-114.
[3] K. Uda, Y. Seki, M. Saito, Y. Sonobe, Y-C. Hsieh, H. Takahashi, I. Terasaki, T. Homma, Electrochimica Acta, 153, 515-522 (2015).
[4] D. D. Frari, S. Diliberto, N. Stein, C. Boulanger, and J.-M. Lecuire, J. Appl. Electrochem., 36(4), 449 (2006).