Wafer Annealing Research Articles

The development of laser cutting and drilling processes for thick Lithium Niobate wafers for electronic sensor applications

Lithium Niobate (LiNbO3) is a CVD grown synthetic salt having a trigonal crystal structure consisting of niobium, lithium, and oxygen. Due to its variety of mechanical and optical properties, single crystal LiNbO3 finds end use applications in optical waveguides, piezoelectric sensors, optical modulators, and electronic oscillators.LiNbO3 wafers are traditionally mechanically processed using abrasive disks and wire saws, and as a result of the inherently low fracture toughness cut-edge chipping and fracture occur lowering process yields. This study aimed to develop a laser process for the cutting, profiling and drilling of thick whole LiNbO3 wafers, using a femtosecond pulsed 515 nm wavelength source. The stresses induced during processing were controlled through a combination of wafer annealing, crystallographic (012) and (010) cleavage plane orientation. Process generated thermal gradients were controlled using beam manipulation and a water spray liquid flow process. The developed processes substantially reduced material fracture and significantly enhanced cut-edge quality, elevating yields above 80%.

Development of high-power VCSEL emitter of size 30mm x 30mm, scalable in two dimensions and applicable to Si wafer annealing

Two-dimensionally scalable vertical cavity surface emitting laser (VCSEL) emitter of size 30mm×30mm was designed and fabricated for practical applications in the rapid heating processes of large-area targets. Nine emitters were arranged in a 3×3 array for an experimental VCSEL module, which was used for annealing of a 4” Si wafer. Each emitter was composed of 120 chips of size 1.26mm×1.26mm, and each chip was composed of 770 VCSEL diodes of 30μm in diameter, emitting a continuous beam with wavelength of 940nm and maximum optical power density of 47.6W/cm2. Since VCSELs generated a significant amount of Joule heat during operation, an effective cooling system was required for reliable thermal management of the emitter. A water cooling block of 126 microchannel flow passages with the heat transfer area-to-volume ratio approximately 1270m2/m3 was designed and fabricated for each emitter, resulting in the thermal resistance of 0.012K/W, equivalent to the overall heat transfer coefficient of 92,000W/m2K at water flow rate of 1L/min, which was the minimum value for the normal operation of the emitter. By comparing the experimental images on the 4” Si wafer irradiated by the single emitter and by the module with the numerical predictions based on the ray-tracing method, the laser beam emitted from a single VCSEL diode of 30μm in diameter was estimated to have the mode of TEM01* (doughnut profile, multimode) and a divergence angle of 20∘. These were two factors that affected the irradiation uniformity on the target. In addition, to validate the applicability of the high-power VCSEL system to anneal large-area targets, the heating process of 4” Si wafers using the module of nine emitters was experimentally carried out at atmospheric pressure, and was compared with numerical predictions.

Heat transfer analysis on wafer annealing process in semiconductor multi-wafer furnace using CFD simulation

Heat transfer analysis on wafer annealing process in semiconductor multi-wafer furnace using CFD simulation

Lifetime and degradation analysis of AgPt alloy thick film/AlN heater for semiconductor wafer annealing

In this study, a heater for 300-mm-diameter semiconductor wafer annealing was fabricated by high-temperature firing after screen-printing a silver–platinum (AgPt) paste on an aluminium nitride (AlN) substrate. For life testing, the changes in the electrical characteristics and thermodynamic properties of the Ag65Pt20-alloy thick film on AlN were analysed according to aging by varying the temperature. Ag65Pt20/AlN heater specimens were fabricated to have a resistance of 10 Ω for thermal life tests. The input power was adjusted in the range of 10%–50%, and the temperature was set between 200 °C–600 °C. The Ag65Pt20/AlN heater was aged for approximately 3000 h per unit input power to measure the changes in electrical characteristics. The resistance increased with aging at each temperature and decreased subsequently. The activation energy was 88.7 kJ/(mol·K) [0.92 eV/K]. Moreover, the resistance decreased from the electrode with AgPt metallization, where the bias is applied, to the inner part. Physical analysis of the Ag65Pt20/AlN specimen revealed that the modified Ag65Pt20 alloy exhibited Pt-rich dendrite phases. The separated Pt-rich dendrite phases increased the capacitance. Further, electrical characteristics and material thermodynamic characteristics were analysed through high-temperature aging of the Ag65Pt20/AlN heater. Through such analyses, prognostic health monitoring factors of the Ag-Pt/AlN heater were deduced, thus enabling service lifetime period prediction for predictive maintenance.

3D system-on-a-chip design with through-silicon-via-less power supply using highly doped silicon via

A low-cost power supply technique for a 3D system-on-a-chip (SoC) is experimentally confirmed by using highly doped silicon via (HDSV). A 100 × 100 μm2 HDSV in a 2 μm thick silicon wafer exhibits a resistance of 2.7 Ω after wafer thinning, bonding, and annealing. A 9-stack 3D SoC is designed with an HDSV power supply, and its area overhead is calculated as 11%. Moreover, technology computer-aided design simulations show that the resistance of the same HDSV could be as low as 22 mΩ, which turns into an area overhead of 0.09%. This work shows detailed experimental results of wafer bonding and current–voltage (IV) characteristics and discusses the applied 3D SoC design.

Data-assisted physical modeling of oxygen precipitation in silicon wafers

A quantitative continuum model for oxide precipitation in silicon is presented that accounts for vacancy absorption and shape change as mechanisms of precipitate stress relief. All model parameters except one, the Si/SiO2 interface free energy, are fixed at values established in prior studies of microdefect formation. The interface free energy is described by an 8-parameter function, whose functional form and dependencies were based on an analysis of electronic structure calculations of small oxide cluster thermodynamics. The interface energy function parameters are regressed, using global optimization, to an experimental benchmark consisting of 13 wafer thermal anneals, with different temperature-time histories and resulting in widely varying measured final oxide precipitate densities. We demonstrate that the model is able to capture the benchmark features well with multiple parameter combinations and that additional constraints are required to fully specify a unique solution. We also show that a simple, single-parameter, constant interface free energy model cannot fully capture the diverse experimental benchmark, highlighting the complexity of oxide precipitation. The precipitation model is used to analyze the mechanisms responsible for several features of oxide nucleation and growth during wafer annealing.

Growth of large diameter ZnTe single crystals by the double crucible liquid encapsulated pulling method

AbstractIn our work, large diameter ZnTe single crystals were grown by various melt growth methods. By a liquid encapsulated vertical gradient freezing method, crystals with large grains and low dislocation density were obtained. By a liquid encapsulated Kyropoulos method, 80 mm diameter crystals were successfully grown from ZnTe seed crystals. However, crystals grown by the both methods had large strain and cracks owing to difference in coefficient of thermal expansion between an encapsulant and ZnTe. Therefore, a new crystal growth method was developed. Two crucibles were used in the new method. It was named double crucible liquid encapsulated pulling method. Crystals up to 100 mm in diameter and 40 mm in length were reproducibly obtained. Both <100> and <110> crystals were successfully grown. The etch pit density of the grown crystal was lower than 10,000 cm–2 and any large strain or cracks were not observed. In order to reduce Te inclusions, wafer annealing was examined. Most Te inclusions were eliminated with suitable Zn pressure and annealing temperature. Optical transparency of the ZnTe crystal increased after annealing. This is because scatter of the incident light by Te inclusions decreased. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantitative evaluation of gettering efficiencies in device process after p-well formation

In this study, we developed a method for evaluating the gettering efficiencies between the gettering sites in p well region and the internal gettering sites in the wafer after p-well formation using implantation and evaluated the efficiencies of copper and nickel gettering. The bulk microdefects (BMDs) introduced by heat treatment caused gettering of copper and nickel, which were introduced by intentional spiking of the device with 65Cu and 60Ni solutions.For evaluating the gettering efficiencies, we performed trace analyses using 65Cu and 60Ni stable isotopes. This study demonstrates the phenomenon of competitive gettering between the p-well region and the internal gettering sites in the wafer. Moreover, the fact that silicon-integrated circuit devices act as efficient metal traps should be taken into account when evaluating the actual gettering ability of silicon wafers. The gettering efficiencies evaluated for the wafers in the no-implantation and boron-implantation groups. The gettering efficiencies of the wafers in the no-implantation and boron-implantation groups are in the range 99.78–100% and 68–100%, respectively. This implies that the gettering efficiency of the wafers is sufficiently high when the competitive gettering between the device and the internal gettering sites is not considered. However, in the competitive-gettering model, 100% gettering is observed only in the p−/p+ epitaxial wafer. The results of the present study indicate that p−/p+ epitaxial wafers have better competitive gettering efficiency than normal polished wafer or rapid thermal annealing (RTA) wafer designed with proximity gettering ability.

Interface morphology, chemistry and dislocation structures of wafer-bonded compound semiconductors

The interface morphology, chemistry and dislocation structures of a few typical wafer-bonded compound semiconductors were characterized. The relations between the interface dislocation structures and electrical performances were discussed. The interface chemistry of these semiconductors was analysed by using electron energy loss spectra. In general, different wafer annealing conditions could result in varied interface crystalline to amorphous real ratios. It has been found that the interface oxides start to segregate and form the scattered interface amorphous nanoinclusions upon wafer thermal annealing. A mismatched wafer-bonded heterointerface is accommodated by the elastic strain relief through the combination of Lomer edge dislocations and interface amorphous nanoinclusions. Different annealing temperatures may result in different morphologies and oxygen concentrations of the nanoinclusions. It is believed that the formation of these interface nanoinclusions with possibly highly concentrated oxygen is a result of the interface atomic diffusion and oxide relocation processes.

Atomistic-to-Continuum Modeling of Defect-Related Phenomena in Silicon Crystals

Continuum process modeling of point defect and impurity aggregation during silicon crystal growth and wafer annealing has led to significant contributions toward understanding and improvement of industrial processes. Key inputs to these models are thermophysical properties of point defects and their clusters, which may be strong functions of temperature and cluster size. In this paper, a theoretical framework is presented for probing the high-thermodynamic properties of vacancy and self-interstitial clusters in crystalline silicon at elevated temperature. In particular, configurational and vibrational entropy are shown to be significant in both types of defect clusters. In both cases, configurational entropy leads to the existence of a wide distribution of possible cluster configurations that collectively lowers the free energy of formation relative to the energetic ground state configuration. Moreover, certain self-interstitial cluster sizes are additionally stabilized by configurations that possess anomalously high vibrational entropy.

Possibility of SOI Fabrication Using Plasma Source Ion Implantation with High-Power Pulsed RF Plasma

Silicon-on-insulator (SOI) structure fabricated by plasma source ion implantation (PSII) with high-power pulsed RF plasma has been studied. Oxygen ions were implanted into p-type silicon wafer and high temperature annealing was subsequently used to form SOI structure. The top silicon and the buried oxide (BOX) layer were formed with 500 Å and 400 Å, respectively, in the sample implanted with the dose of 2.5 × 1017 #/cm2 at the ion energy of -75 kV and annealed at 1350 °C for 30 min in Ar+O2 (0.5 %) ambient. This study showed the possibility of SOI fabrication using the PSII with pulsed ICP source.

Microscopic underpinnings of defect nucleation and growth in silicon crystal growth and wafer processing

Accurate quantitative modeling of point defect and impurity aggregation during silicon crystal growth and wafer annealing requires a detailed understanding of the underlying atomic scale mechanisms involved in defect formation, diffusion, and clustering. Examples are presented that demonstrate the utility of atomic scale studies for generating a complete picture of defect aggregation in crystalline silicon. In the first example, an approach for computing the thermodynamic properties of point defect clusters at high temperature is presented that accounts for cluster configurational entropy. In the second example, a lattice kinetic Monte Carlo model is applied to the direct simulation of vacancy clustering in the presence of oxygen atoms, which are known to act as reversible vacancy traps. The simulations are able to capture complex aggregate morphologies that have been observed experimentally, in particular the cloud-like distribution of small clusters around voids, and the double-void structures frequently observed in Czochralski crystal growth.

Investigation of acceptor states in ZnO by junction DLTS

Abstract We have realized a p-type ZnO surface layer by N + ion implantation of a high quality ZnO wafer and subsequent annealing. The conduction type of this surface layer was revealed by scanning capacitance microscopy. Rectifying current–voltage characteristics for processed devices were coherent with the existence of an internal pn junction. Deep donor- and acceptor-like defects were investigated by junction deep level transient spectroscopy. The donor-like levels correspond to those commonly observed for E1 and E3 defects. The acceptor states resolved have thermal activation energies of about 150 meV and 280 meV, respectively.

Electromigration Reliability in Nanoscale Cu Interconnects

AbstractElectromigration behavior in Cu damascene wires was studied for various Cu grain struc-tures. The grain size was modulated by Cu linewidth and thickness, and by adjusting the wafer annealing process step after Cu electroplating and before Cu chemical mechanical polishing. A larger variation of Cu grain size between the samples was achieved on CMOS 65 nm node tech-nology than previous nodes which was due to the finer line width and thinner metal thickness. The Cu lifetime and mass flow in samples with bamboo, near bamboo, bamboo-polycrystalline mixture, and polycrystalline grain structures were measured. The effects of a Cu(2.5 wt.% Ti) alloy seed, Cu surface pre-clean, and selective electroless CoWP deposition techniques on Cu electromigration were also observed and a significantly improved Cu lifetime was found. The electromigration activation energies for Cu in Cu(Ti) alloy, along Cu/amorphous a-SiCxNyHz in-terface and grain boundary were found to be 1.3, 0.95 and 0.79 ± 0.05 eV, respectively. In addition the Cu line size effect on the Cu conductivity for Cu area less than 4×104 nm2 was found to be a linear function of the Cu line area.

An Atomically Accurate Model for Point Defect Aggregation in Silicon

An atomically accurate model for vacancy aggregation is presented. The model is based on two recently developed elements: (1) an accurate set of point defect properties derived from extensive model regression to multiple types of experimental data, and (2) a new description of point defect clusters at elevated temperature, which includes previously neglected entropic contributions. The latter are found to substantially alter both the cluster thermodynamics and the cluster structures, leading to modified clustering dynamics during crystal growth and wafer annealing. The model is tested extensively against experimental measurements of void densities, sizes, and aggregation temperatures, and is found to provide excellent agreement with experimental data across a wide range of crystal growth conditions.

Reduction of Te inclusions in ZnTe single crystals by thermal annealing

ZnTe crystals grown from the melt have Te inclusions. The wafer annealing under Zn pressure was carried out to reduce Te inclusions in ZnTe wafers. It was found that the cyclic annealing process, where heating and cooling was done cyclically over a temperature range of several tens of degrees, was effective to annihilate the Te inclusions. We obtained the inclusion-free ZnTe crystals by optimising the temperature cycling condition. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Wafer Cleaning and Annealing on Glass/Silicon Wafer Direct Bonding

We studied cleaning and annealing effects in glass/Si direct bonding using 4 inch Pyrex glass and silicon wafers. SPM cleaning (sulfuric-peroxide mixture, H2SO4:H2O2=4:1,120°C), RCA cleaning (NH4OH:H2O2:H2O=1:1:5,80°C), and combinations of these two methods were examined to investigate the wafer cleaning effect. When wafers were cleaned by RCA after SPM cleaning, maximal bonding quality at room temperature was gained. Surface roughness, as measured by AFM (atomic force microscope), was found to correspond with bonding quality at room temperature. Bonding strength increased as the annealing temperatures increased to 400°C, but debonding occurred at 450°C. The difference in thermal expansion coefficients of the glass and the Si wafer used led to this debonding. When wafers bonded at room temperature were annealed at 300 or 400°C, the bonding strength increased for 28 hours, and then it decreased with further annealing. The decrease in bonding strength caused by further annealing was due to sodium ion drift through the glass/Si interface.

Relaxation of Strained SiGe on Insulator by Direct Wafer Bonding

ABSTRACTStrained SiGe was bonded to silicon dioxide by direct wafer bonding and Smart-cut technique. Strained SiGe with a graded 20%-30% Ge concentration was deposited by RTCVD on (100) Si to a thickness between 300nm to 350nm. H2+ for wafer splitting was implanted at an energy varied from 40 keV to 150 keV and a dose between 2.5E16 and 4.5E16 ions/cm2. SiGe relaxation was found to depend on wafer split temperature, and on post-split annealing temperature. SiGe relaxation of greater than 80% was observed after wafer splitting and annealing. Was it from gliding of a SiGe film weakly bonded to an oxide surface? In order to determine the relaxation mechanism, samples with different film structure were prepared. The films were then annealed at various temperatures. Some film showed a high degree of relaxation, and some showed minimal relaxation, depending mostly on hydrogen implant depth. The results indicated that the generation of dislocation is the possible cause of SiGe relaxation.

Computer Simulation for Morphology, Size, and Density of Oxide Precipitates in CZ Silicon

A computer simulation model for oxygen precipitation during crystal growth and wafer annealing of Czochralski silicon is constructed. In this model, the precipitate morphology is determined by minimizing the excess free energy. This model is connected with the formation models of grown-in defects, such as crystal-originated particles (COPs) in the vacancy-rich region or extrinsic stacking faults in the self-interstitials rich region. That is, the simulation of oxygen precipitation starts at 1000 or 900°C with the input data of residual point defect concentrations after COP or stacking-fault formation. The model well simulates oxygen precipitation behavior. For example, the simulated results of isothermal annealing at 700 and 800°C show that the precipitate morphology is initially spherical, then changes to oblate spheroidal with an increase in annealing time, and the aspect ratio β is almost constant at for longer than 200 h. The simulated β agrees approximately with experimentally determined of platelet precipitates reported in the literature. The values of both the density and the growth rate of precipitates agree for the simulations and experiments. The model also shows that oxygen precipitation during crystal growth strongly depends on the residual point defect concentrations after grown-in defect formation. © 2003 The Electrochemical Society. All rights reserved.

Thermally stimulated current study of electron-irradiation induced defects in semi-insulating InP obtained by multiple-step wafer annealing

Electron-irradiation induced defects in semi-insulating (SI) InP wafers with Fe concentration ranging from 1.5×10 15 to 2.5×10 15 cm −3, which have been obtained by multiple-step wafer annealing (MWA) under phosphorus vapor pressure, were studied using a thermally stimulated current (TSC) method. New traps, e 1, e 2, e 3, e 4 and e 5, with activation energies of 0.22, 0.28, 0.37, 0.44 and 0.46 eV, respectively, were observed. Based upon the annealing behavior of traps and the calculated defect levels, traps e 1 and e 5 produced by the irradiation with electron doses above 1×10 15 cm −2 were linked to In P and P In antisite defects, respectively, that probably form complexes. Traps e 3 and e 4 produced by the irradiation with doses above 1×10 14 cm −2 were associated with In and P vacancy related defects, respectively.