Czochralski Wafers Research Articles

Improved phosphorus injection synthesis for bulk InP

High purity, stoichiometric InP is being produced in crucible-shaped, 3-kg charges by the phosphorus injection method in a high-pressure magnetic liquid encapsulated Czochralski (MLEC) crystal growth system. Dedicated heaters in the phosphorus injector assembly are used to heat and controllably inject the phosphorus vapor into the liquid encapsulated indium melt. Glow discharge mass spectroscopy and van der Pauw measurements of the polycrystalline charges and the Czochralski wafers confirmed the low background levels of impurities.

Imaging of the lateral GOI-defect distribution in silicon MOS wafers with lock-in IR-thermography

Yield and reliability of MOS devices are strongly affected by crystal-originated particles which may generate gate oxide integrity (GOI) defects. For the semiconductor industry it is highly desirable not only to measure the density, but also to image the lateral distribution of GOI-defects. A novel technique to image GOI defects across large gate areas has been developed. First, a low-ohmic bias pulse is used to break down nearly all GOI defects in a large-area MOS structure. Then a periodic bias of typically 2V is applied and the local temperature variation caused by the leakage current through the broken GOI defects is imaged by lock-in IR-thermography. This technique has been used to image the GOI defect distribution across 8′′ Czochralski wafers. Various lateral variations of the defect distribution have been confirmed.

ウエハ上微粒子およびウエハ内結晶空洞（ＣＯＰ）からの散乱光の実験的解析

In this study, an experiment to detect the small particles adhering to a wafer and the crystal originated particles (COPs) existing in the wafer, whose diameters are approximately 0.1μm, is carried out successfully. An experiment to separate COPs from detected particles is also carried out. Both particles and COPs are detected when an Ar-ion laser is illuminated perpendicularly downward onto the wafer, and the obliquely scattered light is detected using a photomultiplier tube No. 1. Only particles are detected when a YAG laser is illuminated obliquely with an incident angle of 77° and the light scattered perpendicularly upward is detected using a photomultiplier tube No. 2. On the basis of the two detected signals, the particles and COPs can be recognized independently. The experiment was conducted using two types of wafers. One is a wafer made by a Czochralski (CZ) method, which is known as a wafer with COPs, and the other is a wafer made by a floating zone (FZ) method, which is known as a non-COP wafer. COPs are detected on the CZ wafer but not on the FZ wafer. On the basis of this experimental result, it is experimentally verified that COPs can be detected by this detection method.

Quasipersistent change in Hall sensitivity after illumination

A quasipersistent change in the magnetic sensitivity of nonplate-like Hall sensors after light exposure has been discovered. The recovery time constant is about 10 min. The observed effect is very similar to the persistent photoconductivity (PPC) which has been described some time ago in silicon bulk material. Surprisingly, the new effect is about 1 order of magnitude stronger than the PPC. The similar recovery behavior of both effects suggests a common origin in precipitated oxygen clusters in the active zone. The amplitude of the variations is highly dependent on the cluster concentration in the substrate. The presented Hall sensors give a sensitive means to characterize the oxygen-denuded zone in Czochralski wafers.

Accurate measurements of the intrinsic diffusivities of boron and phosphorus in silicon

All activity in modeling transient diffusion behavior relies on knowledge of the inert intrinsic diffusivities of dopants in Si. The measurements upon which these values are based were conducted over 15 years ago. Since then, the quality of wafers used in industrial applications has significantly changed. This will affect the effective diffusivity through changes in trap concentrations. The reliability of measurement techniques has also changed dramatically from tracer and staining methods to secondary ion mass spectrometry (SIMS) measurements that are dominant today. Finally, our understanding of diffusion behavior has changed significantly. For example, we now understand that the extraction of diffusivities from implanted samples with no pre-anneal includes a significant transient effect. We have measured the inert intrinsic diffusivities of As, B, P, and Sb in different substrates in defect-free Czochralski and float zone wafers and epitaxially grown layers. All samples underwent a 30 min anneal at 1000 °C in dry oxygen in order to grow a cap oxide and eliminate transient enhanced diffusion. We performed SIMS analysis on an initial batch of samples to evaluate the different factors that may affect the diffusivity in a nonideal manner and concluded that there are no transient effects but that surface effects are important. Hence, for the fast moving dopants (B, P) we restrict our data extraction to the deep implants. Our data show that B and P diffusivities are different than the values commonly assumed in the literature at low temperatures. We compare our results to previously published data in light of the factors mentioned here.

Defect observation on a 12-in. silicon wafer using large sample atomic force microscopy

As semiconductor devices have become more highly integrated and miniaturized, defects originating within single-crystal silicon occur more frequently. Defect observation of silicon wafer surfaces using atomic force microscopy (AFM) is an analysis technique that reduces defect density on silicon surfaces and, thereby, increases yield (Ref. ). We have developed an AFM that can directly observe the entire surface of next-trend, 12-in. wafers. We observed the defects of 12-in. Czochralski (CZ) wafers and compared the results with those of 8-in. wafers. We also observed the defects of 12-in. epitaxial Si wafers as a reference. The defects of the 12- and 8-in. CZ wafers are similar, and the 12-in. wafers display two types of defects in the silicon crystals: (1) Crystal originated particle, which is a shape surrounded by a {111} facet, and (2) dislocation loops. These defects are caused when crystals are lifted, and reflect the lifting conditions (Ref. ). Although, there were fewer defects on the epitaxial Si wafers than on the CZ wafers, defects like hillock were found on the epitaxial Si wafers.

New developments in silicon Czochralski crystal growth and wafer technology

An overview of recent progress in large-diameter crystal pulling and associated new wafer preparation technologies is given. A clear trend can be observed that, with increasing crystal diameter, the average oxygen concentration and the density of intrinsic grown in defects is reduced. Oxygen reduction and control is achieved by using magnetic fields, while bulk defect reduction requires sophisticated hot zone engineering. New growing methods currently under development result in new crystal types with a high density of interstitials in the outer area and a lower density of vacancies in the center. In addition, crystals are grown under near-equilibrium conditions so the super-saturation of the intrinsic point defects remains below the critical value required for clustering of the point defects into larger agglomerates . Geometry and light point defects/localized light scatter requirements for future wafer generations are becoming more stringent with each decrease in design rule. This leads to the necessity of novel wafer surface preparation methodologies such as advanced single side polishing, double side polishing and plasma assisted chemical etching with local wafer surface treatment. Finally, the trend toward low thermal budget integrated circuit processes leads to the necessity of having oxygen precipitation tuning and gate oxide integrity tuning, as well as the creation of a denuded zone by the silicon wafer manufacturer. New approaches to meet these requirements are annealing at high temperature and/or rapid thermal processing. The development of the new epitaxial wafer types is highlighted.

Silicon substrate effects on the current–voltage characteristics of advanced p–n junction diodes

An in-depth analysis of the forward and reverse current–voltage characteristics allows determination of the different geometrical (area, perimeter and corner) and physical (diffusion and generation) current components. This is a powerful technique to assess the silicon substrate quality. In this paper it is shown that the diffusion current of a good quality silicon p–n junction is significantly lower for an epitaxial (Epi) wafer compared to a Czochralski (Cz) wafer. This can be explained by a correction factor, F, which depends on the parameters of the highly doped substrate. The impact of the substrate is less pronounced when the leakage current is dominated by the peripheral component. Furthermore, for low reverse bias, current transients occur for large area diodes in Cz substrates. These are related to the presence of generation centres, which are absent in epitaxial wafers.

Annealing kinetics of {311} defects and dislocation loops in the end-of-range damage region of ion implanted silicon

The evolution of both {311} defects and dislocation loops in the end-of-range (EOR) damage region in silicon amorphized by ion implantation was studied by ex situ transmission electron microscopy (TEM). The amorphization of a (100) n-type Czochralski wafer was achieved with a 20 keV 1×1015/cm2 Si+ ion implantation. The post-implantation anneals were performed in a furnace at 750 °C for times ranging from 10 to 370 min. After annealing the specimen for 10 min, the microstructure showed a collection of both {311} defects and small dislocation loops. The evolution of a specific group of defects was studied by repeated imaging of the same region after additional annealing. Quantitative TEM showed that {311} defects followed one of two possible evolutionary pathways as annealing times progressed; unfaulting to form dislocation loops or dissolving and releasing interstitials. Results indicate that in this temperature regime, {311} defects are the preferential site for dislocation loop nucleation.

ウエハ上微粒子およびウエハ内結晶空洞（ＣＯＰ）からの散乱光の理論的解析

In this study, an experiment to detect the small particles adhering to a wafer and the crystal originated particles (COPs) existing in the wafer, whose diameters are approximately 0.1μm, is carried out successfully. An experiment to separate COPs from detected particles is also carried out. Both particles and COPs are detected when an Ar-ion laser is illuminated perpendicularly downward onto the wafer, and the obliquely scattered light is detected using a photomultiplier tube No. 1. Only particles are detected when a YAG laser is illuminated obliquely with an incident angle of 77° and the light scattered perpendicularly upward is detected using a photomultiplier tube No. 2. On the basis of the two detected signals, the particles and COPs can be recognized independently. The experiment was conducted using two types of wafers. One is a wafer made by a Czochralski (CZ) method, which is known as a wafer with COPs, and the other is a wafer made by a floating zone (FZ) method, which is known as a non-COP wafer. COPs are detected on the CZ wafer but not on the FZ wafer. On the basis of this experimental result, it is experimentally verified that COPs can be detected by this detection method.

Junction Depth Reduction of ion Implanted Boron in Silicon Through Fluorine ion Implantation

AbstractThe interaction between boron and excess silicon interstitials caused by ion implantation hinders the formation of ultra-shallow, low resistivity junctions. Previous studies have shown that fluorine reduces boron transient enhanced diffusion, however it is unclear whether this observed phenomenon is due to the fluorine interacting with the boron atoms or silicon self-interstitials. Amorphization of a n-type Czochralski wafer was achieved with a 70 keV Si+ implantation at a dose of 1×1015/cm2. The Si+ implant produced a 1500Å deep amorphous layer, which was then implanted with 1.12 keV 1×1015/cm2 B+. The samples were then implanted with a dose of 2×1015/cm2F+ at various energies ranging from 2 keV to 36 keV. Ellipsometry measurements showed no increase in the amorphous layer thickness from either the boron or fluorine implants. The experimental conditions allowed the chemical species effect to be studied independent of the implant damage caused by the fluorine implant. Post-implantation anneals were performed in a tube furnace at 750° C. Secondary ion mass spectrometry was used to monitor the dopant diffusion after annealing. Transmission electron microscopy (TEM) was used to study the end-of-range defect evolution. The addition of fluorine reduces the boron transient enhanced diffusion for all fluorine energies. It was observed that both the magnitude of the boron diffusivity and the concentration gradient of the boron profile vary as a function of fluorine energy.

The Effect of Impurities on Diffusion and Activation of ion Implanted Boron in Silicon

AbstractThe interaction between boron and silicon interstitials caused by ion implant damage is a physical process which hinders the formation of ultra-shallow, low resistivity junctions. The possibility of mitigating the effective interstitial point defect population via introduction of nonmetallic impurities in ion implanted silicon has been investigated. Amorphization of a n-type Czochralski wafer was achieved using a series of Si+ implants of 40 keV and 150 keV, each at a dose of 1×1015/cm2. The Si+ implants produced a 2800Å deep amorphous layer, which was then implanted with 8 keV 1×1014/cm2 B+. The samples were then implanted with high doses of either carbon, oxygen, sulfur, chlorine, selenium, or bromine. The implant energies of the impurities were chosen such that the damage and ion profiles of the impurity were contained within the amorphous layer. This allowed for the chemical species effect to be studied independent of the implant damage caused by the impurity implant. Post-implantation anneals were performed in a tube furnace at 750° C. Secondary ion mass spectrometry was used to monitor the dopant diffusion after annealing. Hall effect measurements were used to study the dopant activation. Transmission electron microscopy (TEM) was used to study the end-of-range defect evolution. The addition of carbon and chlorine appear to reduce the boron diffusion enhancement compared to the boron control. Carbon and chlorine also appear to prevent boron out-diffusion during annealing compared to the control, which exhibited 20% dose loss following annealing.

Correlation of end-of-range damage evolution and transient enhanced diffusion of boron in regrown silicon

Amorphization of a n-type Czochralski wafer was achieved using a series of Si+ implants of 30 and 120 keV, each at a dose of 1×1015 cm2. The Si+ implants produced a 2400 Å deep amorphous layer, which was then implanted with 4 keV 1×1014/cm2 B+. Postimplantation anneals were performed in a tube furnace at 750 °C, for times ranging from 15 min to 6 h. Secondary ion mass spectrometry was used to monitor the dopant diffusion after annealing. Transmission electron microscopy (TEM) was used to study the EOR defect evolution. Upon annealing, the boron peak showed no clustering, and TED was observed in the entire boron profile. TEM results show that both {311} defects and dislocation loops were present in the EOR damage region. The majority of the {311} defects dissolved in the interval between 15 min and 2 h. Results indicate that {311} defects release interstitials during the time that boron exhibits TED. These results show that there is a strong correlation between {311} dissolution in the EOR and TED in the regrown silicon layer. Quantitative TEM of dislocation loop growth and {311} dissolution indicates that in addition to {311} defects, submicroscopic sources of interstitials may also exist in the EOR which may contribute to TED.

Characterisation of pyramid formation arising from the TMAH etching of silicon

An investigation on the influence of etchant concentration, ambient temperature and wafer thermal history on the formation of pyramids arising from the TMAH etching of silicon has been carried out. The number and size of the pyramids were used as parameters for the investigation. From the results obtained from this study, we are able to explain the influence of the TMAH concentration and ambient temperature on the changes occurring at the silicon surface satisfactorily using the pH theory. However, the results from the rapid thermal annealed, the as-received Czochralski wafers, and the float zone wafers seem to favour the defects theory. We suggest that in order to explain the etching mechanism of TMAH of silicon satisfactorily, a combination of the pH and the defects theories is necessary.

Persistent photoconductivity in silicon wafers

Persistent photoconductivity effect in silicon wafers was observed using specifically designed test structures. Persistent changes in conductivity can reach 0.01% in wafers with a doping level of . The recovery time constant is in the range 10-30 min at room temperature. Oxygen-rich-Czochralski-grown wafers were compared with oxygen-poor float-zone-grown ones. The persistent photoconductivity is much higher in the Czochralski wafers. This indicates that this effect is most probably related to oxygen in silicon. To the best of our knowledge, such an effect has never been observed before in silicon crystals.

Noncontact Measurement of Doping Profile for Bare Silicon

In this study, we evaluate the doping concentrations of bare silicon wafers by noncontact capacitance–voltage (C–V) measurements. The metal-air-insulator-semiconductor (MAIS) method enables the measurement of C–V characteristics of silicon wafers without oxidation and electrode preparation. This method has the advantage that a doping profile close to the wafer surface can be obtained. In our experiment, epitaxial silicon wafers were used to compare the MAIS method with the conventional MIS method. The experimental results obtained from the two methods showed good agreement. Then, doping profiles of boron-doped Czochralski (CZ) wafers were measured by the MAIS method. The result indicated a significant reduction of the doping concentration near the wafer surface. This observation is attributed to the well-known deactivation of boron with atomic hydrogen which permeated the silicon bulk during the polishing process. This deactivation was recovered by annealing in air at 180°C for 120 min.

Multicrystalline silicon material: Effects of classical and rapid thermal processes

For photovoltaic applications silicon is still the predominant material. Besides monocrystalline Czochralski wafers (Cz-Si), multicrystalline sheets (mc-Si) play an important role in terrestrial power applications (almost 50%). Large mc-Si ingots (up to 250 kg) are now produced in large scale by the industry using various directional solidification methods in appropriate crucibles (or molds). However, if the crystallographic properties are now quite satisfactory (columnar structure with large grains of more than 1 cm2, dislocations and intragrains defects), multicrystalline silicon contains larger quantities of impurities than single crystalline silicon which can have detrimental effects on the bulk minority carrier diffusion length (Ln,p). These impurities, including metals as well as high concentrations of carbon and/or oxygen, can degrade the photovoltaic properties of solar cells. Thermal treatments such as gettering, performed in a classical or rapid thermal furnace, studied separately or in conjunction with the doping steps can limit or avoid the degradation of the bulk diffusion length, but its efficiency is strongly dependent on the presence of these impurities in Si.

Effect of Oxide Precipitate Size on Slip Generation in Large Diameter Epitaxial Wafers

The effect of oxide precipitate size on slip generation was investigated in Czochralski (CZ), p/p-, p/p+ and p/p++ epitaxial wafers during device manufacturing. In high-temperature processes where maximum temperature reaches 1100°C, the mechanical strength of epitaxial wafers was far superior to that of CZ wafers. In contrast, the mechanical strength of all wafers was sufficiently maintained in low-temperature processes where maximum temperature reaches 1050°C. The slip dislocation generation can be explained according to the size of oxide precipitate formed in each wafer; precipitates larger than approximately 200 nm can generate slip dislocations.

Accurate extraction of the diffusion current in silicon p-n junction diodes

An accurate method for the extraction of the reverse diffusion current component in a silicon p-n junction diode is proposed. It combines capacitance–voltage and current–voltage measurements on an array of diodes with different geometry in order to separate the peripheral and the volume leakage current components. The corrected volume capacitance is then used to calculate the depletion width as a function of the reverse bias. Extrapolation of the reverse current to zero depletion width results in the diffusion current part, both for the volume and for the peripheral component. From the temperature dependence, a thermal activation energy of 1.12 eV is obtained. The volume diffusion current density of the p-type Czochralski wafers studied, shows a pronounced substrate dependence, while the peripheral diffusion current density is constant. Finally, the implications for the extraction of the effective bulk recombination lifetime are discussed.

Proton irradiation effects in silicon junction diodes and charge-coupled-devices

Proton irradiation effects in silicon devices are studied, for components fabricated in various substrates in order to reveal possible hardening effects. The degradation of p-n junction diodes increases in first order proportionally with the fluence, when submitted to 10 MeV proton irradiations in the range 5 × 10 9–5 × 10 11 cm −2. The damage coefficients for both p- and n-type Czochralski, Float-Zone and epitaxial wafers are reported. Charge-Coupled-Devices fabricated in a 1.2 μm CCD-CMOS technology are shown to be quite resistant to 59 MeV H + irradiations, irrespective of the substrate type.