Controllability Of Dopant Ion Number In Single Ion Implantation

Abstract

In the single ion implantation(SII), which enables us to implant dopant ions one by one in order for suppressing fluctuation in dopant number in a fine semiconductor region, extraction of single ions by chopping a focused ion beam and detection of secondary electrons(SEs) emitted from the target upon each ion incidence are the key technology for the precise control of the ion number. There are at least three factors which deteriorate the advantage of the SII. They are the less than one probability of SE detection, smaller number of ions which actually stay in a fine semiconductor region due to range straggling, and insufficient electrical activation of the implanted ions. Since the latter two factors are common to the conventional ion implantation, we have investigated the influence of SE detection efficiency in this work. 2. Definition of quantities used in this work First , we define the quantities used in this work as schematically shown in fig. 1. Nion is the number of ions to be implanted, NSE the number of pulses counted in a PMT by detecting SEs emitted upon each ion incidence, NT total number of ions actually implanted due to the less than one efficiency ( t ) of SE detection, NI the number of ions which stay in the top-Si region and n the number of ions electrically activated after annealing. SE detection efficiency t is defined as NSE/Nion, ratio of ions implanted in top-Si region as Nl/NT, and electrical activation ratio rj as n/NI, respectively. By using these quantities, n is expressed as n= rj N,=Q 5 NT=( v 5 15 INIon. 3. Experimental 60 keV P2+ single ions were implanted into test specimens. The number of ions to be implanted was set to be 990 and 19180 u.m2. The detection efficiency 5 was chosen to be 56 and 91% in order to investigate the influence of ,E on the controllability of ion number. 91% is the highest value of t obtained for SiO, in our system. The lower & can be easily achieved by decreasing the gain of a PMT. After single ion implantation and the subsequent annealing, the sheet electron concentration was evaluated by Hall measurement at room temperaturie. 4. Results and discussion The results are summarized in table 1. For the number of ions to be implanted, 990 and 1980 [ m2], the sheet electron concentration is estimated to be 770-809 and 2423-2555, respectively, by taking all the factors as shown in fig. 1 into account. NI was calculated by using the process simulator "SUPREM-IV". The value of 92-97'3, had been obtained for v beforehand by comparing the electron concentration in a bulk-Si implanted with P and the P concentration measured with SIMS. The sheet electron concentration was 798 and 2530 for the Nion of 990 and 1980, respectively. Although there is small discrepancy between Ni, and nmeas., the latter coincide quite well with the estimated values. This verifies the advantage of SI1 in controlling the number of dopant atoms in a laterally confined fine semiconductor region. We previously assessed the relation between and the fluctuation in the implanted ion number by SII, and found that the fluctuation by SI1 dramatically degraded at lower t '1. Under low detection , the deviation between the measured and estimated sheet electron concentration become large. Therefore, the higher E is essential in the SII.