The residual damage distributions created by MeV dopant implantation in semiconductors show that in-situ annealing is important for defect formation. Device-quality silicon can be obtained at the top few microns of the silicon substrate after a post-implant annealing cycle. A family of majority-carrier and minority-carrier devices have been fabricated by incorporating a MeV implantation step with conventional integrated-circuit processing.
The performance to date of the Model MV-H2O ion implantation system will be reviewed. The MV-H20 ion implanter is based on a single ended Pelletron accelerator with a 2 MV column rating. It is designed to provide 20 iA of singly charged boron, phosphorus, and arsenic over the energy range of 400 keV to 2000 keV, although it is also expected to perform well down to 200 keV. This system includes a serial cassette to cassette wafer handler capable of automatically processing up to 80 wafers/hr of 100 mm, 125 mm, and 150 mm wafers. Operational features such as implant currents, source lifetimes, wafer throughput, and time to service the source will be presented.
The emergence of gallium arsenide technology and the trend in silicon fabrication toward CMOS based systems and smaller device sizes suggest a number of important applications for implantation in the MeV range. These include: 1. The formation of deep wells with retrograde shape without the large thermal budget associated with "pre-dep and drive-in" procedures. 2. Post-fabrication customization of devices by implant through the sealing glass and other layers normally laid down as the final stages of a semiconductor process cycle. 3. Buried grid structures for device isolation against soft errors. 4. Gallium Arsenide device fabrication.
The use of MeV ion implantation for CMOS device fabrication is illustrated with a description of an n-type, retrograde well process which is being developed for 1 micron-scale devices. Process design and integration issues are described along with selected results for B and P ion ranges, process modeling and device characteristics.
MeV implantation has a number of potential device and IC applications in both Si and the III-V's. Some of these have already begun to be realized in the Si technology. The development of III-V applications is, however, in a more embryonic stage. The primary thrust in the III-V's is toward increasing the range of devices which can be made by direct ion im-plantation to increase the level of device integration which can be attained. This paper will contrast the III-V technology with the Si, outline some of the potential applications for MeV implantation in the III-V's and discuss the basic research areas in which progress must be made in order to increase the use of MeV implantation in the III-V device and IC technology.
Implantation into GaAs of n-type dopants having energies of 1 MeV or greater has been investigated. The resulting carrier profiles have been studied with an electrolytic CV technique and with Hall measurements. It was found that MeV Si implantation maybe used to form well defined n-layers in GaAs. Anneal temperatures of 800°C to 850°C are adequate for activation of fluences of 5x1013/cm2 or less for 1 MeV Si. For a fluence of lx1014 Si/cm2 an anneal temperature of 900°C must be used to prevent the formation of a surface p-type layer. MeV S implantation was found to form diffuse 2-layers. When 1 MeV Si and 1.25 MeV S were co-implanted, both with a fluence of lx10/cm2, an n-layer was formed wila anneal temperatures as low as 800°C. At 900°C, a peak carrier concentration of 2x101°/cm3 was achieved. 6 MeV Si produces a buried n-layer with a carrier concentration maximum at 3.0 μm. Uniform deep implants have been activated having carrier concentrations as low as 2x10 16/cm3 with a mobility of 5500 cm2/V's.
The critical requirements of CMOS device fabrication have placed new demands on "front end" equipment manufacturers. In particular, ion implant manufacturers face the challenge of producing higher Arsenic beam currents in the 50-100keV range, copious Boron beam currents in the 20-40keV range, reduced particulates, and greater equipment reliability. The impact of high packing density, smaller line widths, and low device power consumption, recognized with the newest CMOS designs, will continue to broaden the requirements for implanter equipment flexibility. Compared to NMOS technology, the added high dose requirement of the P+ Channel Source-Drain substantially increases the ion implant time necessary to meet production needs. To meet these needs, many high volume CMOS device fabricators have dedicated processing equipment to operate at specific energies, doses, and ion species. Varian has developed a series of High Current Implantation Systems to help CMOS manufacturers meet these challenges.
The effect of phosphorus contamination on an antimony collector implant has been studied by SRP, SIMS, and SUPREM modeling. The results indicate that this is one of the most serious cases of cross-contamination; where P levels in excess of 0.07% of the implant can modify the resulting junction depth and therefore all subsequent electrical parameters. The SRP curves resulting from phosphorus contamination may be recognized by a characteristic stepped curve which is reminiscent of P-push in As. SIMS data indicates that the antimony profile is marginally modified by the presence of phosphorus while the increase in junction depth is due not only to the larger diffusion constant of the phosphorus but also to an interaction that increases that constant. Methods of eliminating this contamination are discussed.
The effects of axial and planar channeling in Si (100) crystals on junction depth and sheet resistance are studied for shallow junction implants by modern ion implantation systems. Evaluation techniques such as SIMS, spreading resistance and sheet resistance contour mapping are used to characterize planar channeling. Variations in crystal to ion beam alignment due to wafer to crystal orientation, wafer flat orientation, wafer tilt, wafer flex and beam scan angle variations are tested and results given. A model for channeling is shown and calculations for the critical channeling angles for axial and planar channeling for common dopants is derived. The ability to avoid planar channeling by predictions from the model are tested for B+ and As+ using common doses and energies. Other techniques for avoidance of planar channeling such as amorphization layers of SiO2 and Si are tested and results given.
The solid phase epitaxy of amorphous silicon deposited by LPCVD on (100) Si windows was achieved by implanting the deposited Si with a high dose of either silicon or phosphorus ions and subsequently annealing it in a furnace. Recrystallization proceeded by columnar growth followed by a lateral enlargement of grains. The regrowth rate of LPCVD amorphous silicon was found to be slower than that of ion implanted damaged region. Values of annealing time (tr) required to recrystallize the LPCVD amorphous silicon for the P-implanted samples were consistently a factor four lower than values of tr for the Si implanted samples. For both P and Si implanted specimens, microtwins were observed in the recrystallized region with a higher density for the Si implanted samples. The random nucleation of polysilicon on Si02 depressed by high dose P-implantation gave enough time for the lateral growth of 2-3 μm to occur in the adjacent window regions.
Ionized cluster Beams (ICB) are widely used to deposit metal, semiconductor and insulating films. The paper describes the current state of this technology in both fundamental and applications areas. One important ambiguity in the theory of metal cluster formation is addressed. RBS is used to measure the extent of any displacement of surface atoms caused by ICB. The minimal amount of such surface damage is confirmed by the close-to-ideal values measured for Schottky barrier height on an Al/Si interface. Several examples of ICB application are described.
SiO2 films have been deposited on Si wafers at a substrate temperature of 300°C and a pressure of 10-2 Pa using the Ionized Nozzle-Beam Deposition (INBD) technique. SiO grains were used as the deposition material. The refractive index and infrared absorption spec-trum of the deposited SiO 0 films resembled to those of the SiO2 films deposited using SiO2 as the source material. We found that the film quality was improved significantly after a short time furnace annealing (800°C, 10 minutes).
A new one-dimensional, process estimator for the design of IC technologies (PREDICT) has been developed which rigorously solves coupled equations describing dopant behavior under modern processing conditions. All of the models in PREDICT have been verified with extensive experimental measurements. Such models include a new ion implantation algorithm with empirical parameters to describe the exponential tail formed through ion channeling, rapid thermal diffusion of B, As and P, accurate oxidation calculations including the effects of pressure, HC1 and doping concentrations, effects of stress and dopant precipitation and clustering, ion pairing, implantation through deposited or grown films (oxide, polysilicon, nitride), concentration effects, etc. PREDICT has been used to do accurate simulations of high dose B and BF2 implants/diffusions in both <100> and <111> Si. Considerations in these calculations include channeling during implantation, the effects of pre-amorphization, damage-induced dislocation networks and the enhanced diffusion of B outside of these networks, and precipitation of B using a 12 atom cluster model.
Unimplanted boron and phosphorous doped bulk silicon wafers were studied for fixed exposures at constant anneal conditions using an infrared graphite source. Effects of different chamber conditions, silicon wafer properties and the pyrometer system were determined. Wafer resistivity changes were investigated. Dopant concentration, as well as wafer type, was found to affect the wafer temperature rise; and, hence, the wafer end point temperatures in significantly different ways. Results of chamber memory effects showed least memory for an infrared graphite source annealing system. Wafer peak temperature data and bulk resistivity changes in multiple annealed wafers indicates that the mechanism of thermal donor annihilation and neutral trap concentration reduction play an active role in both n and p-type silicon. The elimination of wafer to wafer endpoint temperature and chamber memory is demonstrated by wafer temperature feedback control.
Rapid annealing has been in use for several years but no simple modeling tools have been developed to help the process designer evaluate applications of rapid thermal processing. This paper gives a brief development of simple but comprehensive models for evaluating the redistribution of Gaussian implants when processed by a shuttered blackbody or similar sources. A detailed solution is obtained for the intrinsic diffusion case and an algorithm and a first order solution are obtained for the concentration dependent diffusion case. A method for comparing rapid thermal processing (RTP) machines is also described where two parameters are fit to the heat-up and cool-down portion of the cycle and an effective time at peak temperature, teff, is derived for the transient portion of the thermal cycle.
A brief review is presented of the basic properties of ion beam mixing of layers of material on Si and Si02, and current models for the observed phenomena. Following this, some potential applications of ion beam mixing to processing of Si devices are reviewed. These applications are: (1) dispersal of impurities which would otherwise block thermal reactions, (2) formation of uniform, well-aligned metal-silicide contacts to devices, and (3) adhesion of metal interconnects to Si02 layers. In each of these, the mixing is executed with a simple implantation step, conducted at room temperature. The alternative processes are mostly either more complex, or require annealing at very high temperatures.
Schottky barrier heights may be modified by means of shallow ion implantation. In this paper, a numerical method is presented to calculate effective barrier heights for metal-semiconductor contacts with arbitrary doping profiles. Thermionic current, image force lowering and tunneling current are considered. The transmission (tunneling) coefficient is computed by numerical solution of Schrodinger's equation and wave function matching at boundaries. A metal-germanium contact test case is investigated. This scheme may be applicable to practical implant conditions where dopant redistribution is significant.
Measurements of electrical resistivity are used to monitor changes in CrSi2 thin films induced by Xe irradiation over a fluence range of ≈ 1010 - 1014 cm-2. Behavior associated with defect generation and recombination are evident at high fluences. A temperature dependence at low fluences is reported and tentatively identified.
Ion mixing (IM) has been of considerable interest over the last several years.1 It has emerged as a convenient method to produce various amorphous and metastable crystalline phases.2 Several attempts have been made to predict the formation of amorphous phases by this technique. Liu and coworkers have formulated a rule which states that an amorphous binary alloy will be formed by IM of the multilayered sample when the two constituent metals are of different structures.3 It has also been suggested that IM is likely to produce a crystalline phase at a composition which corresponds to a compound of simple lattice struc-ture.4 Recently, the application of thermodynamic considerations to IM processes have proven fruitful.5,6 The present authors have provided some general criteria regarding amorphous and crystalline phases formation by IM6 of metal-metal systems based on considerations of thermodynamic free energy diagrams and the restricted growth kinetics of competing phases. In this paper we shall examine these ideas by studying the IM of metal-metal systems of Ru-Zr and Ru-Ti.
Rutherford backscattering, Auger electron spectroscopy and Cross-sectional Transmission electron microscopic techniques have been utilized to study the ion beam mixing of alternating Au-Ge film structure deposited by e-gun evaporation on GaAs. Ion beam mixing was carried out with 1 MeV Au+ and with 160, 140 and 125 keV Si+ ions at various doses. Alloying was observed between Au and Ge, and between Au-Ge and GaAs induced by ion beam mixing at room temperature. The mixing efficiency of Au+ ions is about a factor of five higher than that of Si+ ions. Depending on the mass, energy and dose of ions and thickness of Au-Ge film structure, ion beam mixing produced various amounts of damage in the interface region of GaAs. Electrical characterization of a selected ion beam mixed sample revealed the rectifying nature of the contact.
Bilayers of Si/metal (metal on top) and metal/Si (Si on top) were annealed in vacuum to produce the Si/silicide/metal and metal/silicide/Si configurations, respectively. The sandwich structures were then irradiated with Xe ions of energies ranging from 125 to 300 keV such that the Xe ions traverse only one of the two interfaces (metal-silicide or silicide-Si). In the case of CrSi2, irradiation above 150°C induces further and laterally uniform growth of a stoichiometric layer of silicide, but only when the Cr/CrSi2 interface is traversed, not when the other interface is traversed. We conclude that the formation of CrSi2 is an interface-limited process, which is consistent with the linear time dependence of the growth of CrSi2 under thermal annealing, and that the limiting reaction occurs at the Cr/ CrSi2 interface. On the other hand, in TaSi2 for temperatures up to 500°C, and in Pt2Si at room temperature, Xe ions penetrating through only one of either of the two interfaces does not induce silicide growth in a layer-by-layer fashion. This observation is consistent with the fact that under thermal annealing, the growth of Pt2Si is a diffusion-controlled process. The absence of further growth of TaSi2 is attributed to unidentified processes which control the formation of the silicide.
When a film of CoSi2 on a SiO2 substrate is thermally annealed in an oxidizing ambient in the temperature range, 750 - 1000°C, a SiO2 layer grows on the surface by depleting the film of its Si. As a result, the chemical composition of the film passes through a sequence of silicides with increasing metal content. The film also breaks up into islands. This morphological instability is observed also for pure Co film on SiO2 when annealed in vacuum. We show that less than 10 atomic percent of Si, or higher, in Co suppresses this instability under vacuum annealing. The oxidation of a CoSi2 film on a Si <111> substrate establishes that the displacement of Si atoms against Co atoms in the silicide does not create a morphological instability. The irradiation of a film on SiO2 prior to oxidation by Kr ions whose range is commensurate with the thickness of the film can partly suppress this instability during oxidation under certain circumstances.
The diffusion of silicon from an underlying silicon substrate into a Ti film has been investigated as a function of the annealing temperature and time of a Rapid Thermal Annealer, using Auger spectroscopy. It was found that at 600°C, silicide formation is initiated witin 10 sec. and the thickness of the silicide formed increased approximately as the square root of time, at this temperature. The rate of silicide formation is higher than that ex-pected from the kinetics obtained for conventional diffusion furnace anneals. It was found that the interfacial native oxide between the Ti and Si greatly affected both the kinetics of silicide formation and the quality of the film. By using a high dose (1x1016 ions/cm2) heavy ion implantation through the Ti film (ion beam mixing) the interfacial oxide could be dispersed. As a consequence, for a given temperature and time of anneal, the ion-beam mixed films show more silicide formed than a non ion-beam mixed film. Also, the ion-mixed films retained a smooth surface and interface after annealing.
This paper will discuss the application of Rutherford backscattering technology and secondary ion mass spectrometry to advanced ion beam processes. Two particular processes, high current implantation and high energy implantation will be investigated.
Four-point probe sheet resistance measurements, which are used throughout the semiconductor industry to monitor ion implant uniformity and repeatability, can also be used to determine implanter-to-implanter agreement on dose. Factors which affect accuracy, precision and probe performance of an automated sheet resistance mapping system widely used in this application are discussed. It is also shown that the spatial resolution that can be obtained using the four-point probe is better than 0.5mm, making it possible to detect short-range variability in implant doping.
Three experimental techniques have been used to examine the material and optical properties of proton irradiated n-type GaAs implanted atiroom temperature and subsequently furnace annealed. The depth profiles of the implanted hydrogen atoms (H) have been determined as a function of anneal temperature for temperatures1up to 600°C using secondary ion mass spectrometry. These profiles display a major redistribution of the IH atoms with movement beginning at 200'C and terminating by 700°C. The optical changes produced by irradiation have been characterized by infrared reflectance measurements that indicate an optically uniform layer is initially formed in the as implanted specimen. Annealing causes this layer to first broaden and then return to almost its as implanted state. The optical changes qualitatively follow the behavior of the SIMS hydrogen profiles. The structural damage caused by irradiation has been studied using cross-sectional transmission electron microscopy. The micrographs show no evidence of damage in the as-implanted GaAs to a resolution of 5 nm. Only in specimens annealed at temperatures at or above 500°C are precipitates and dislocation loops noticeable.
We report a new method for the determination of ion-implanted dose and uniformity of n-type and p-type dopants in GaAs. The technique, employing thermal waves, is noncontact and nondestructive and therefore particularly attractive for monitoring production wafers for GaAs IC production. We demonstrate dose measurements over the 1012 -1015 ions/cm2 range, contour mapping of implanted dose on a GaAs wafer, and dose measurements on discrete circuit elements of micron spatial scale on a GaAs FET.
The rare earth (RE) doping of III-V compounds and Si is currently of interest due to the potential development of these materials for LED's and electrically injected lasers which operate in the near infrared (1-3 pm). Er and Yb have been studied extensively, and have been incorporated into semiconductor hosts either during growth or by subsequent implantation. Implantation is of interest because of the greater degree of flexibility it affords for device fabrication in an integrated format. The use of conventional implantation energies is hampered by the large mass of the rare earths which restricts the range and creates a high density of displacement damage. In this paper, we discuss the use of MeV implantation for the incorporation of Er in Si. Rutherford backscattering (RBS), photoluminescence (PL) and electrical measurements have been carried out on Si substrates implanted with Er at MeV energies. The RBS data show that 1E13cm-2 1 MeV implants do not produce a distinct damage peak and are well annealed by a 2 900°C 30 min anneal. They also show that MeV implants of 2.5E14cm-2 produce a thick amor-phous layer while 5E13cm-implants result in a damage peak which is 50% of the random. A characteristic 806 meV Er PL peak is present in all the samples annealed at 700°C or higher. The integrated Er PL intensity is found to decrease with increasing anneal temperature, and may be related to interstitial Er3+. All samples annealed at 650°C or higher also show an n-type layer associated with the implanted Er. The carrier concentration is a maximum for 700°C anneals and decreases monotonically for higher anneal temperatures.
We demonstrate the ability of thermal-wave techniques to nondestructively measure ion-implanted dose and dose uniformity on the same spatial scale as the implanted features of micro-electronic devices. We present results on the variation of the thermal-wave signal with implanted dose, ion energy, surface oxide thickness, and silicon substrate resistiv-ity. We demonstrate ion dose measurements over the 1011 -1015 ions/cm2 range, automatic wafer mapping of ion dose uniformity, and measurement of dose variations on actual integrated circuit features. The attributes of this measurement technique make it particularly suited for production monitoring of ion implantation, especially in the critical threshold-voltage-adjust implant regime.
Recent work has demonstrated that the infrared properties (refractive index and absorption) of amorphous silicon and germanium prepared by ion implantation depend upon the low temperature thermal annealing history (1500C<T<6000C). This thermal relaxation phenomenon is the subject of this review. The data suggest the change in refractive index is caused by a structural reorganization of a continuous random network but that changes in absorption and spin density are chiefly caused by the annealing of defects within the amorphous structure.
The synthesis of buried dielectric layers by the implantation of reactive ions (0+, N+) to form silicon on insulator substrates suitable for very large scale integration (VLSI) circuits is described. Silicon (100) wafers have been implanted with ions of energy 100-300 key and doses in the range 0.25 x 1018 to 2.6 x 1018 cm2. These structures have been annealed at temperatures of up to 1200oC. The composition, microstructure and electrical properties are reported and a comparison is made between substrates formed by O+ and N+ implantations.
Silicon on Insulators (SOI) formed by ion implantation of oxygen has been examined by several researchers including the present authors. This paper gives a brief review of this subject. The advantages of SOI versus Silicon on Sapphire (SOS) and bulk Si are discussed. The materials properties and the effects of ion implantation and anneal conditions are reviewed. Device modeling as it applies to SOI has been presented. Characteristics of devices built in SOI formed by ion implantation of oxygen are examined. Finally some circuit results are discussed.
High power oxygen ion beams have been used to produce a buried insulating layer in silicon wafers. Standard materials analysis techniques show that a thermally stable insulating layer is formed beneath the single crystal surface layer of silicon, serving to isolate the surface from the substrate.
The formation of a buried dielectric in single crystals of silicon by high dose implantation of oxygen ions is investigated. The dependence of the microstructure on implantation conditions is determined. It will be shown that the microstructure can be tailored by changing the implant conditions to optimize its suitability for the silicon-on-insulator technology. Mechanisms responsible for the formation of the microstructure and the influence of implantation conditions will be discussed.