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contamination analysis for compound semiconductors ANALYTICAL SERVICES

B u r i e d d e f e c t s, E v a n s A n a l y t i c a l g r o u p h e l p s y o u C O N T R O L C O N T A M I N A T I O N Contamination control and defect reduction are critical issues for a wide range of industries, including compound semiconductors, optoelectronics, photonics, wireless and high-speed communications, and display technologies. In all of these areas, the ability to develop and manufacture devices with high yield and reliability is vital for companies to achieve rapid time to market. Surface analysis from Evans Analytical Group (EAG) can provide valuable insights to identify contaminants and characterize materials in areas as diverse as R&D, process development, process monitoring, QC and competitive analysis. From starting materials to finished products including packaging/mounting materials, leadframes and fiber optic bundles evaluating materials is essential to solving problems. Examples include: Identifying the chemical composition of contaminants is critical in tracing problems to a source. A first approach to evaluate particles and other residues is often SEM (Scanning Electron Microscopy), typically done in conjunction with EDS (Energy Dispersive X-ray Spectroscopy). SEM provides information about the topography and size of a defect, while EDS gives the elemental composition. This example shows an SEM image, as well as EDS maps, of a defect cross section. Cross sections can be prepared using standard polishing or cleaving techniques, as well as by FIB (Focused Ion Beam). secondary electron oxygen aluminum silicon Characterizing hazes on optics that can lead to degraded light transmission and efficiency. These can come from process problems, as well as product storage and transportation. Analyzing substrate materials such as GaAs for surface elemental impurities that can affect performance and epi growth. cross section of a 2 µm aluminum inclusion in a coating Assessing cleaning process efficacy and detecting residues pre- and post- cleaning. Cleaning is required throughout the manufacturing processes, and poor cleaning can leave potential dopants and/or new contaminants on the surface being cleaned. Identifying defects on devices. For example, metallic and polymer residues on laser diodes that lead to degraded beam profiles. 1

p a r t i c l e s a n d s m a l l a r e a a n a l y s i s Raman spectrum of particle. 12 1 Identification: Polyethylene ν CH Intensity 8 6 ν C-C 4 2 δ CH & 2 β CH 2 4 9 14 19 24 29 34 Wavenumber (cm) -1 sem image and raman spectrum of 1 µm polyethylene particle When a particle is composed of organic materials, it s important to go beyond the elemental composition information that EDS can provide. Because there are so many organic compounds, simply knowing that a particle is composed of carbon typically doesn t provide enough information to determine its source. Raman spectroscopy is often the best technique to analyze these particles, since it can be used to identify the molecular composition of particles as small as.8 µm. Some of the most difficult contamination problems to resolve are those involving extremely small particles or residues on microscopic features. The excellent spatial resolution (<15 nm), sampling depth (1-3 nm), and survey capabilities of Auger dn/de F Si Electron Spectroscopy make it the technique of choice C Al for elemental analysis of micron and sub-micron sized features and devices. Auger can also be used to evaluate etch residues, to verify layer sequences, Si O Si investigate migration, and evaluate adhesion/ delamination. 2 4 6 8 1 12 14 16 18 2 Kinetic Energy (ev) auger spectrum of 5nm particle of aluminum oxide 2

M e t a l c o n t a m i n a t i o n Ultra-clean surfaces are critically important to successful processing in many industries. Failures can often be attributed to surface contamination by inorganic contaminants. In order to control contaminants, it is necessary to identify and quantify them. As a survey technique, for planar samples Total Reflection X-Ray Fluorescence (TXRF) provides high sensitivity multi-element surface contamination measurements of elements from S to U. Whole wafers are analyzed non-destructively in a cleanroom environment. TXRF results for 3 GaAs wafers (units of 1 1 atoms/cm 2 ) S Cl Ti Cr Mn Fe Ni Cu Zn Wafer 1 56 12 11 <1.4 <1.4 1.2 1.7 11 8 Wafer 2 33 9 <4 5. 1.8 11.3 78 5.2 53 Wafer 3 27 78 <4 <1.6 <1.4 <1.2 <1.1 <2 <4 * Practical detection limits (indicated by <) are calculated for each spectrum and may vary between analysis. O r g a n i c c o n t a m i n a t i o n Identifying organic contamination the specific organic compound present is often essential if the source of a problem is to be found. FTIR (Fourier Transform Infrared Spectroscopy) is an established analytical tool well suited for the analysis of organic materials, since it is non-destructive and has extensive libraries to identify unknowns. Typical applications include the analysis of residues and organic particles on optics and wafers, as well as general material characterization. In the example shown here, a powder residue found on some electrical pins was analyzed by FTIR, and the resulting spectrum was found to match a solder flux reference. powder residue solder flux % Transmittance % Transmittance 4 35 3 25 2 15 1 Wavenumbers (cm) -1 Wavenumbers (cm) -1 3

O x i d a t i o n & g e n e r a l compound semiconductor surfaces. In the c o n t a m i n a t i o n example shown here, high energy resolution In many cases, suspected contamination problems can turn out to be changes in the oxidation state or surface chemistry brought about by processing steps such as heating in the presence of an oxidizing gas. XPS (X-Ray Photoelectron Spectroscopy, also known as ESCA, Electron Spectroscopy for Chemical Analysis) is an excellent technique to determine the chemistry of the top 3-7 Å of a surface, with detection limits in the.5 to.5 atomic % range. XPS is also the only technique available Ga and As spectra from a GaAs wafer show the relative amounts of both alloy and oxide forms of the elements at the surface of the wafer. This type of analysis can be used to measure the percentage of each species present as an oxide and to determine which species in a compound semiconductor preferentially oxidize. XPS is also useful to evaluate residues or hazes, compare cleaning processes, or determine the stoichiometry of elements in thin films (depth profiling). that provides chemical state information from 3 Ga 3 As 25 25 2 2 As as GaAs c/s 15 Ga as GaAs c/s 15 As 2 O 3 1 Ga 2 O 5 1 As 2 O 5 5 5 23 22 21 2 19 18 47 46 45 44 43 42 41 4 39 Binding Energy (ev) Binding Energy (ev) high resolution Ga and As XPS spectra from a GaAs wafer 4

L o w l e v e l c o n t a m i n a t i o n Finding and identifying contamination present at low concentrations can be difficult. TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) is an extremely sensitive surface analytical technique that can provide molecular and elemental information from the top few monolayers of a surface. This makes it a good tool to analyze residues and hazes on surfaces such as optics, to detect ppm levels of metals on GaAs, and to examine defects down to a few microns in size. In the example shown here, very low (sub-monolayer) levels of silicone contamination transferred from packaging material are observed on a GaAs wafer. 5 69 Ga ion counts 4 3 2 71 Ga 1 C 4 H 7 C2 H 5 C3 H 7 AsO 5 1 15 2 25 3 8 * ion counts 6 4 2 Si * * 69 Ga * * * Silicone Contaminant * * * * * 5 1 15 2 25 3 m/z Tof-sims spectra from clean and contaminated GaAs 5

n-type DBR active region p-type DBR 1. µm 1. 2. 1. C a r r i e r p r o f i l e s a n d s u r f a c e r o u g h n e s s Characterizing new products or comparing competing processes is an important part of R&D. Scanning Capacitance Microscopy in conjunction with Atomic Force Microscopy (SCM/AFM) has been widely used to investigate the 2D carrier profile of semiconductors. This example shows AFM and SCM images from a VCSEL cross section. 1. 2. µm AFM (top) and Scm (bottom) images from a VCSEL cross section µm In the AFM image, the blue bands are the GaAs layers and the yellow bands are the AlAs layers (because Al is easily oxidized, the AlAs layers are topographically higher than the GaAs layers). In the SCM image, the green bands are n-type regions and the red bands are p-type regions. Light-blue bands indicate that no carrier exists in these bands. Overlapping the two images reveals that AlAs is in the no carrier regions. Whether it s R&D, process development, process monitoring, or QC, the Evans Analytical Group has a range of surface analysis and metrology tools, along with teams of highly qualified staff, to address your characterization needs. The EAG global network of labs is committed to providing the best analytical services to our worldwide customer base. Our experience over more than 25 years has made us responsive to issues that you care about, including quality, accuracy, timeliness and consistency. If you have questions regarding a problem or a specific analysis, please call us. 6

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