Photoelastic Imaging of Process Induced Defects in 300mm-Silicon Wafers
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ABSTRACT Process induced crystal defects in silicon wafers can be detected by their stress fields. The nondestructive photoelasticity based on laser polarimetry is applied to visualize the stress fields of temperture gradient induced lattice defects like sliplines or extended defect areas around boat marks. The quantitative evaluation of the defects allows their characterization by a specific danger potential for further evolution causing upstream problems in IC manufacturing. INTRODUCTION Shrunk structural dimensions on enlarged wafer areas increase the influence of crystal defects on device function and integrity. Nondestructive and noncontact measurement techniques for rapid defect monitoring on the product wafer meet the demands of device integrity and cost efficiency. The introduction of 300mm wafers in IC manufacturing increases the effect some well known problems of uniformity and stability of process parameters. Temperature gradients during high temperature processing cause lattice defects like dislocation networks or slip lines, which can be detected by their local stress fields. The increased wafer dimension (radius, thickness, weight) complicate the wafer support. During each process step the high temperature equipment leaves its fingerprints at the support points of the wafer by creation of local temperature gradients. To prevent upstream problems in subsequent process steps a rapid defect control by measurement on product wafers is necessary. The visualization of local stress accompanying sliplines is realized by scanning infrared depolarization measurement (SIRD) of laser light transmitting the wafer [1]. The well known photoelastic effect in silicon is employed to measure stress by polarimetry. Stress causes anisotropy of the refractive index in the original optically isotropic cubic crystal The crystal becomes birefringent meaning that light changes its polarization state if it penetrates the anisotropic volume. A polarimeter measures the change of polarization of an infrared laser beam, the so called depolarization. A rapid scanning method gives the SIRD measurement tool the ability to record a full 300mm-wafer stress image in less than 4 minutes with lateral resolution of 1001am and allows to monitor crystal defects and their evolution during the technological process sequence. From the stress field evaluation critical defect parameters can be deduced by applying a defect modeL EXPERIMENTAL MEASUREMENT
SETUP
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PRINCIPLE
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PHOTOELASTIC
STRESS
The photoelastic measurement is carried out by use of the stress imager SIRDTm of the TePla AG. The opto-mechanical principle of this transmission polarimeter is shown in Fig.l. A defined polarized beam of a semiconductor laser diode with the wavelength of 1.3prm penetrates the wafer (perpendicular incidence) and will be analyzed by a stokes meter measuring the first stokes component [2]. The wafer is fixed at 3 points by an edge support in a scanning r-*-stage. The wafer is put into rotation and moved radially on the linear r-stage. Due to this move
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