Contactless Characterization of Silicon Wafers

Contactless measurements are attractive and more commonly used because they do not contaminate the sample and generally do not require sample preparation. After an outline of the more common contactless characterization techniques, I will discuss a few of these in more detail. In particular resistivity or doping density profiling, minority carrier lifetime, stress, temperature, layer thickness, and critical dimension will be more fully described. Introduction A summary of the major contactless characterization techniques is shown here. A few of these techniques are discussed in more detail. Resistivity Eddy current, capacitance-voltage, corona charge, photoluminescence Wafer Flatness Capacitance, optical interferometry, AFM Epi Layer Thickness FTIR, capacitance-voltage Junction Depth Carrier illumination, modulated photoreflectance Implant dose Modulated photoreflectance (thermawave) Surface Impurities Total reflection X-ray fluorescence, time-of-flight-SIMS, inductively coupled plasma-mass spectrometry (ICP-MS) Surface Particles Surface scattering, SEM/EDS, Raman Bulk Impurities Microwave-photoconductance decay, surface photovoltage, photoluminescence Bulk Lifetime μW-PCD, surface photovoltage, corona charge, quasi-steady-state photoconductance, free carrier absorption Surface/Oxide Charge Corona charge, surface charge analyzer Insulator Thickness Ellipsometry, optical reflectivity, X-ray photoelectron spectroscopy, corona charge/Kelvin probe Critical Dimensions SEM, scatterometry, spectral ellipsometry, X-ray scattering Crystal Defects X-ray topography, polariscopy Stress Raman spectroscopy, polariscopy Temperature Raman spectroscopy, liquid crystal, scanning thermal probe, pyrometer Dielectric/Metal Voids Positron annihilation spectroscopy, acoustic microscopy Electromigration X-ray microscopy Metal Thickness Picosecond ultrasonics, X-ray reflectometry, X-ray fluorescence, Rutherford backscattering, optical illumination, impulsive stimulated scattering Dielectric Porosity X-ray scattering, ellipsometry, neutron scattering, positron annihilation spectroscopy Some Techniques Resistivity, Doping Density Capacitance-Voltage While doping density in polished wafers is not commonly measured, profiling of epitaxial wafers is routinely carried out. It is advantageous to use contactless techniques for this purpose so that the wafer does not have to be sacrificed. A contactless capacitance doping profiling measurement technique uses a contact held in close proximity to the semiconductor wafer. An independently biased guard electrode surrounds the 1 mm diameter sensor electrode, coated with a high dielectric strength thin film. The sensor electrode is held above the wafer by a porous ceramic air bearing, which provides for a very stable distance from the wafer as long as the load on the air bearing does not change, shown in Fig. 1. Pressurizing a bellows provides the controlled load. As air escapes through the porous surface, a cushion of air forms on the wafer that acts like a spring and prevents the contact from touching the wafer. The porosity and air pressure are designed such that the sensing contact floats approximately 0.5 μm above the wafer surface. A stainless steel bellows acts to constrain the pressurized air and to raise the porous disk when the air pressure is reduced. If the air pressure fails, the disk moves up, rather than down to prevent damaging the wafer.1 To prepare the wafer, it is placed in a lowconcentration ozone environment at a temperature of about 450C. The treatment reduces the surface charge on the wafer, especially critical for n-Si, makes it more uniform, reduces the surface generation velocity and allows deeper depletion.2 A recent comparison of epitaxial resistivity profiles by this contactless method with Hg-probe C-V measurements compared very favorably.3 The system measures the capacitance from the wafer chuck to the electrode. The capacitance of the air gap is measured by biasing the semiconductor surface in accumulation. Light is used to collapse any possible space-charge region due to surface charge while the sensor is lowered and while the air gap modulation due to the electrostatic attraction is determined to eliminate any series space-charge capacitance. Assuming that the air gap does not vary with changing electrode voltage, the capacitance of the air gap is the measured capacitance at its maximum value. The doping density profile is determined from the conventional C-V expressions.4 Figure 1 shows a resistivity profile, determined from C-V data, of a p-type Si epitaxial layer on a p+ substrate. Minority Carrier Lifetime Quasi-Steady-State Photoconductance An important addition to the array of minority carrier lifetime characterization tools, is the quasi steady-state photoconductance (QSSPC) instrument.5 In this method the sample is illuminated with a “slow” flash lamp having a decay time of several ms and an illumination area of several cm2, which can be reduced to several mm2 with a light pipe. Due to the slow decay time, the sample is under quasi steady-state conditions during the measurement as the light intensity varies from its maximum to zero. The steady-state condition is maintained as long as the flash lamp decay time is longer than the effective carrier lifetime. For a p-type semiconductor with steady state or transient light incident on the sample, solving the continuity equation for uniform electron-hole pair (ehp) generation and zero surface recombination gives the effective lifetime dt t n d t G t n n eff / ) ( ) ( ) ( ) ( ∆ − ∆ = ∆ τ (1) where ∆n(t) is the time-dependent excess minority carrier density and G the ehp generation rate. In the transient photoconductance decay (PCD) method, with G(t) <<