Implant dose monitoring by MOS C–V measurement

Abstract

The most critical parameter for deep sub-micron MOS field effect transistors is the threshold voltage, which is highly dependent on processing specifically, the ion implanted channel dose. Monitoring the channel doping on product wafers is highly desirable and is a major issue for process engineers. MOS C–V methods are widely used for process ramp up and monitoring and MOS C–V doping profiling is an introduced method for monitoring of low dose implants. However, the failure of the depletion approximation in the near surface region implies that conventional MOS C–V measurements yield erroneous doping profiles in that region. Integrating MOS C–V doping profiles yields only a partial implant dose excluding the important near surface dose portion. Here, we report a new approach, which enables the determination of the entire implant dose, taking into account the crucial surface region. Moreover, the MOS threshold voltage can be obtained self-consistently. The method is also applicable to MOS structures with ultra thin gate oxides.

Introduction

Highly scaled CMOS technologies require the precise control of the channel implant dose during processing. The threshold voltage, the transconductance, and the effective channel length are all strongly influenced by the implanted channel dose. MOS capacitors with metal, polysilicon, or mercury gate electrodes are ideal test elements for the development, ramp up and routine monitoring of CMOS technologies. MOS structures are widely used to qualify and monitor important process modules, such as cleaning, epitaxy, oxidation, implantation, metallisation and anneals. Once the first polysilicon layer in a CMOS process is structured, the curriculum vitae of MOS structures, usually located in the scratch line area of process wafers, yields valuable information on irregularities in the process flow. For production monitoring, rapid and accurate determination of low and medium dose implants in the range <2×10¹² cm⁻² is still a challenging task. Four-point probe measurements lack the sensitivity for the middle and low dose range, and cannot be used on actual product wafers. Secondary ion mass spectroscopy and spreading resistance are destructive and time consuming, and therefore not suitable for in-line production monitoring. Thermal wave techniques quantify the lattice damage in the implanted wafer after ion implantation as a measure for the implanted dose, but cannot detect the electrically active dopant dose after anneals. MOS C–V is non-destructive, and yields a direct measurement of the majority charge carrier profile, but not the dopant profile. In the near surface region, i.e. in a depth range lower than about three Debye lengths from the Si–SiO₂ interface, the depletion approximation, a prerequisite for MOS C–V profiling, fails. In this depth range, the approximately obtained carrier density profile differs significantly from the dopant profile. Therefore for the surface region a doping profile with reasonable accuracy cannot be determined by means of C–V.

C–V profiling is based on the measurement of the deep depletion C–V curve [1], [2]. A general prerequisite for the applicability of MOS C–V profiling is a low density of interface traps (<10¹⁰ cm⁻² eV⁻¹) to avoid an additional gate voltage stretch out in the C–V curve. Fig. 1 illustrates the strong deviation of C–V profiles from the corresponding dopant profiles in the near surface region. For several implant doses in the low and medium dose range, both the dopant profiles and the corresponding C–V profiles are shown. The surface Debye length limit implies that dose determination by integrating of MOS C–V profile is not applicable in the near surface region. Thus, this approach can be used only to determine a partial doping dose in the deeper bulk, neglecting the important surface contribution [3].

Here, we present a new approach, which overcomes this restriction. The new method enables the accurate determination of the entire dopant dose D from the surface into the bulk. The maximum dopant dose that can be measured amounts to approximately 2×10¹² cm⁻². This limitation results from the onset of avalanche breakdown at higher fields. Moreover, the method enables a self-consistent determination of further important MOS parameters, such as surface potential φ_s(V_g) characteristic and threshold voltage V_th.