Implementing Potentiostat for Electrochemical Capacitance Voltage Dopant Profiling

In this work, we have developed an algorithm to use potentiosatat for acquiring data required for ECV calculations. The results have been compared with the results of ECV instrument in the literature. Dopant distribution characterization of semiconductor devices is critical for obtaining optimum performance [1]. Conventional methods for obtaining depth profile are spreading resistance profiling(SRP), secondary ion mass spectroscopy (SIMS), capacitance-voltage (CV), electrochemical capacitance voltage (ECV) [2]. The SIMS method is expensive in comparison to the other methods and are limited to measuring chemical doping concentration[3]. In addition SIMS has difficulty in characterizing textured wafers. SRP method required that the junctions be less than 100 nm deep. Moreover SRP accuracy depends on the proper probe conditioning[3]. Conventional CV technique is useful in the measurements of the carrier concentration profiles. In this technique a metal Schottky contact is formed on the sample and a stepped reverse bias is applied to slowly depleting thin regions of the semiconductor. Using differential capacitancevoltage the carrier profile can be deduced. Main limitation in this method occurs during applying a high field to low-doped regions which will be create an electrical breakdown. Electrochemical capacitance voltage (ECV) method overcomes on this limitation by using an electrolyte solution instead of a metal for creating a schottky contact. By frequently etching the sample and measuring the capacitance at every etch step, one can calculate concentration in any depth without the need of applying high bias voltages [4]. While secondary ion mass spectroscopy (SIMS) has difficulty in characterizing textured wafers, ECV can practically measure them by assuming the area factor ([surface area] / [projected area]). Another advantage of ECV method over other methods is that this method provides only the electrically active dopants and has a depth resolution in the sub-nm range, which makes it a very powerful measurement tool for the use in device simulations and for device optimization in the PV industry. Unfortunately ECV instrument is expensive instrument and is not a conventional analysis instrument in laboratories, but potentiostat is commonly available in 116 labs. This method can be used for obtaining concentration profile for variety of semiconductors such as p and n-type silicon, GaAs, InSb by using appropriate electrolyte[5]. Developed algorithm began by differentiating C-V and using formula 1 and 2 to obtain carrier concentration and depletion regions depth. Where q is charge of electron, 0 is permittivity of vacuum, r is relative permittivity and A is the measurement area[2]. (1) N(W) = c3 qε0εrA2( dc dv ) (2) W = ε0εrA c By step by step etching the wafer and calculating the etch thickness by integrating Current–time, and adding etched thickness to W the accurate profile to any depth can be calculated. Figure 1 shows C-V curve measured at 20KHz[6]. Implementing the algorithm for the C-V, we have calculated dopant profile as shown in figure 2. As it can be seen in figure 2 at width 19nm a dopant profile smoothly decreases and will be fixed in width 46nm which reached to p-n junction. Our result is different from the reported values by about 10% which could be improved by using correction factors in the algorithm and implementing repeated etching we will obtain more accurate profile. Fig.1. Capacitance-voltage curve (reference [6]) Fig.2. Concentration dopant by ECV method