J. Middelhoek

Modeling and Optimization of the Step Coverage of Tungsten LPCVD in Trenches and Contact Holes

A model is presented to calculate the step coverage of blanket tungsten low pressure chemical vapor deposition(W-LPCVD) from tungsten hexafluoride (WF6). The model can calculate tungsten growth in trenches and circular contactholes, in the case of the WF6 reduction by H2, SiH4, or both. The step coverage model predictions have been verified experimentallyby scanning electron microscopy (SEM). We found that the predictions of the step coverage model for the H2 reductionof WF6 are very accurate, if the partial pressures of the reactants at the inlet of the trench or contact hole areknown. To get these reactant inlet partial pressures, we used a reactor model which calculates the surface partial pressuresof all the reactants. These calculated surface partial pressures are used as input for our step coverage model. In this studywe showed that thermodiffusion plays a very important role in the actual surface partial pressure. In the case where SiH4was present in the gas mixture trends are predicted very well but the absolute values predicted by the step coveragemodel are too high. The partial pressure of HF, which is a by-product of the H2 reduction reaction, may be very high insidetrenches or contact holes, especially just before closing of the trench or contact hole. We found no influence of the calculatedHF partial pressure on the step coverage. Differences between step coverage in trenches and contact holes, as predictedby the step coverage model, were found to agree with the experiments. It is shown that the combination of the stepcoverage and reactor model is very useful in the optimization towards high step coverage, high throughput, and low WF6flow. We found a perfect step coverage (no void formation) in a 2 µm wide and 10 µm deep (2 × 10 µm) trench using anaverage WF6 flow of only 35 sccm, at a growth rate of 150 nm/min. In general, it is shown that the reduction of WF6 by SiH4offers no advantages over the reduction by H2 as far as step coverage is concerned.

VIPMOS-a novel buried injector structure for EPROM applications

A buried injector is proposed as a source of electrons for substrate hot electrons injection. To enhance the compatibility with VLSI processing, the buried injector is formed by the local overlap of the n-well and p-well of a retrograde twin-well CMOS process. The injector is activated by means of punchthrough. This mechanism allows the realization of a selective injector without increasing the latchup susceptibility. The p-well profile controls the punchthrough voltage. The high injection probability and efficient electron supply mechanism lead to oxide current densities up to 1.0 AA.*cm/sup -2/. Programming times of 10 mu s have been measured on nonoptimized cells. The realization of a structure for 5-V-only digital and analog applications is viable. A model of the structure for implementation in a circuit simulator, such as SPICE, is presented. >

Determination of 2D implanted ion distributions using inverse radon transform methods

Two methods are presented for the experimental determination of 2D implanted ion distribution resulting from implantations with a line source into amorphous targets. It is shown that the relation between the 2D distribution and the depth profiles resulting from tilted angle implantations is described by the Radon transformation. The inverse transformation has been applied to accurately measured depth profiles. The first method uses a digitization of the 2D distribution and the second method uses a parameterized function for the 2D distribution. The methods are tested for a 400 keV boron implantation in an amorphous layer of silicon. The experimental obtained 2D distributions are compared with a TRIM Monte Carlo simulation. A good agreement between experiment and simulation is observed.

The Relationship between Intrinsic Stress of Silicon Nitride Films and Ion Generation in A 50 KHz RF Discharge

Silicon nitride films prepared by the Plasma Enhanced Chemical Vapor Deposition technique (PECVD) are widely used in microelectronics. The intrinsic stress value of the silicon nitride films is a key factor which determines their reliability. A low frequency (50 kHz) RF discharge and a N 2 /NH 3 /SiH 4 gas mixture were used to deposit silicon nitride films on 4-inch silicon wafers in a horizontal hot-wall reactor at a temperature of 350 °C. The compressive stresses in the deposited films were found to increase linearly with the nitrogen ionization created in the glow discharge. The nitrogen ion bombardment at the film surface induces ion implantation into the deposited film which results in film expansion. The stresses were deduced by the interference fringes technique and from surface profiler measurements. Optical Emission Spectroscopy of the N 2 /NH 3 /SiH 4 discharge was used for measuring the N + 2 peak intensity variation with the partial pressure of the nitrogen gas. When the nitrogen ionization was increased, a shift of the main infrared transmission peak corresponding to the Si-N bond in the silicon nitride films was observed using Fourier Transform Infrared Spectroscopy (FTIR). The Si-H and N-H infrared peaks in the films were not affected by the ion bombardment. A linear relationship was found between the wavenumber of the main infrared Si-N peak ν M (835–875 cm −1 ) and the intrinsic stress σ i of the deposited nitride films with the slope dν M /dσ i −4.85×10 −8 cm −1 /Pa.

A Reflectometric Study of the Reaction between Si and WF6 during W-LPCVD on Si and of the Renucleation during the H2 Reduction of WF6

The formation of W through the reduction of WF6 by Si is monitored in situ using a wavelength adjustable reflectometer.The reflectance-time relation can be understood and modeled by assuming island growth and a statistical distributionof the island thickness. The model is supported by SEM and Auger observations. The effect of surface layers like nativeoxides or a plasma treatment on the inhomogeneous Si consumption by the reaction between Si and WF6 (gouging) and itseffect on the reflectance-time relation are understood. The model is also applicable in the case of renucleation during theH2 reduction of WF6. A renucleation step consists of the deposition of Si from SiH4 followed by the Si consumption byWF6. A renucleation step reduces the surface roughing which occurs during the H2 reduction process.

In Situ Growth Rate Measurement of Selective LPCVD of Tungsten

The reflectance measurement during the selective deposition of W on Si covered with an insulator grating is proven tobe a convenient method to monitor the W deposition. The reflectance change during deposition allows the in situ measurementof the deposition rate. The influence of surface roughening due to either the W growth or an etching pretreatmentof the wafer is modeled, as well as the effect of selectivity loss and lateral overgrowth.

High-energy low-dose implanted silicon annealed by transient RTA

Transient RTA performed in the sub-second time range has been applied for low-thermal-budget post-implantation anneal of low-dose, high-energy P and B implanted wafers. This very rapid thermal step not only accomplished full activation and minimum profile motion but was also effective in reducing the resident damage-associated lifetime problems compared to conventional annealing. The generation lifetime and velocity depth-profiling used for process characterization shed light on the role of metallic impurities in the high-energy B and P implanted samples during the fast-quenching operations in determining the electrical behavior.

Low Temperature RF Plasma Annealing Using A NH 3 -N 2 Gas Mixture

Techniques such as plasma etching, electron-beam lithography, X-ray lithography, ion etching, sputtering and ion implantation are used to Increase the density of Integration of semiconductor devices. All these techniques introduce undesirable radiation damage into the processed device. Annealing techniques are used to reduce or remove completely the radiation induced defects. The conventional postmetallization low temperature (400–450 °C) annealing using Forming gas (10% H 2 -90% N 2 ) does not anneal all the charge pentres in many device structures. We have employed a novel low temperature (350°C) rf plasma technique using a NH 3 -N 2 gas mixture to anneal bipolar structures. Vertical pnp transistors made with high energy ion implantation and having poor electrical characteristics have been dramatically improved after 30 min annealing with this new technique. The value of the ideality factor of the base current which was about 1.4 before annealing approached the ideal value of 1.0 after 30 min annealing. Optical emission spectroscopy of the NH 3 -N 2 glow discharge shows the presence of NH radicals, atomic hydrogen and nitrogen ion molecules during the plasma annealing. The atomic hydrogen passivates electrically active defects in the oxide and at the Si/SiO 2 interface. A nitridation process occurs at the surface of the top BPSG layer, where a thin silicon nitride film is formed and plays the role of a capping layer which inhibits saturation phenomena. Angle-resolved XPS and ellipsometry have been used to analyze the surface of a silicon wafer exposed to this plasma annealing process. A qualitative model is also proposed to explain the mechanisms involved in this novel NH 3 -N 2 rf plasma annealing process.

An 1 GHz All-implanted Vertical pnp Transistor

Vertical all implanted PNP transistors have been fabricated using high energy ion-implantation. The PNP transistor process can be implemented in standard NPN buried collector processes, with epitaxial layers larger than 2.5 μm, to achieve a high performance complementary bipolar process. The collector is formed by implantation of doubly charged boron ions with an energy of 500 keV. Base and emitter regions are also implanted. Independent change of the base concentration is possible. The base and collector currents are ideal over 5 decades. The current gain is ≈ 35 and constant over 4 decades. Cut off frequencies of the PNP transistors of over 1 GHz have been measured.

In Situ Reflectometry During LPCVD Tungsten Growth.

The formation of LPCVD tungsten by means of the reduction of WF6 with Si, H2 and SiH4 is monitored in situ using a wavelength adjustable reflectometer. The initial self stopping growth of W by Si reduction is strongly dependant on surface status [1]. SEM observations together with Auger depth profiling and weight measurements support a growth model of islands that grow laterally and vertically until islands touch. After the self stopping Si reduction the W layer was increased in thickness by either the h or SiH4 reduction. The surface roughness calculated from the reflectance appears to increase linearly with thickness in the case of H2 reduction. Typical rms roughness was found to be 7% of layer thickness in the H2 reduction case. The reflectance of H2 reduced W layers could be improved by interrupting the growth process with a renucleation step using SiH4. Selective deposition and in situ growth rate measurements can be monitored when the deposition is carried out on a grating of SiO2 as a mask. Precleaning of the reactor with an NF3 plasma results in a strong retardation of the H2 reduction reaction.