Lee M. Loewenstein

Cost-effective cleaning and high-quality thin gate oxides

Some recent findings in the area of wafer cleaning and thin oxide properties are presented in this paper. Results are shown for a practical implementation of a simplified cleaning concept that combines excellent performance in terms of metal and particle removal with low chemical and DI-water consumption. The effect of organic contamination on ultrathin gate-oxide integrity is illustrated, and the feasibility of using ozonated DI water as an organic removal step is discussed. Metal outplating from HF and HF/HCI solutions is investigated. Also, the final rinsing step is critically evaluated. It is demonstrated that Si surface roughness without the presence of metal contaminants does not degrade gate-oxide integrity. Finally, some critical remarks on the reliability measurements for ultrathin gate oxides are given; it is shown that erroneous conclusions can be drawn from constant-current charge-to-breakdown measurements.

Method for semiconductor process optimization using functional representations of spatial variations and selectivity

The authors present a methodology for determining the optimal equipment settings for a processing step based on experiment designs and model-based optimization. The proposed method for semiconductor process optimization uses two-layered models. The first layer involves creating a spatial model-one for each film of interest-of the etch results. The second layer maps the coefficients of the spatial models to equipment settings. All this is done before any optimization scheme is employed. The process engineer then may optimize the etch process by maximizing coefficients which contribute to the desired maximum etch rate, while minimizing coefficients which contribute to nonuniformity. He also may minimize coefficients which represent undesired etches, and thus obtain etch selectivity. The results of a study of a plasma-assisted silicon nitride etch step are presented.<<ETX>>

Adsorption of Metal Ions onto Hydrophilic Silicon Surfaces from Aqueous Solution: Effect of pH

This work examines the pH and time dependence of metal deposition onto hydrophilic silicon surfaces. We find that the surface concentration of metal species is proportional to the pH. Time dependence is seen for only a few metals, notably Cr and Fe in this study, especially at high pH. Si surfaces with wet chemical oxides were exposed to a dilute solution of ten metals: Al, Ba, Ca, Cr, Cu, Fe, K, Ni, Sr, and Zn, in dilute nitric acid solution, from pH 3 to 5.6, for different periods of time, from about 1 to 1000 s. At equilibrium the surface concentration of metals is approximately proportional to the ratio of ionic charge to ionic radius of the metal ions for many of the metal ions which exist as hydrated cations at that particular pH. We observe this proportionality over the pH range under study, although the surface concentration varies over several orders of magnitude. Cations which form amphoteric species, such as Al 3+ which forms Al 2 O 3 and Al(OH) 4 at high pH, and several other cations that do not exist as simple cations at some pHs (e.g., Fe 3+ , Zn 2+ , and Ni 2+ at high pH), do not show this dependence on ionic charge and radius.

HIGH-SPEED SPECTRAL ELLIPSOMETRY FOR IN SITU DIAGNOSTICS AND PROCESS CONTROL

Real‐time sensors are key for flexible manufacturing environments where variable layer structures are processed concurrently. We have developed a high speed spectral ellipsometer capable of precisely measuring thicknesses and compositions of multilayer structures in situ and in real‐time (i.e., the time frame of process changes or about one second). This spectral ellipsometer has been integrated into several vacuum chambers of a flexible process flow. Utilizing phase modulation, multichannel detection, and digital signal processing techniques, less than 0.5 s is required for acquisition of 46 spectral points. Fast numerical algorithms and processor are used for reducing measured spectral ellipsometric data to physical parameters (i.e., thicknesses and compositions), in real time based on a ‘‘standard model’’ approach. Implementations of spectral ellipsometric sensing in rapid thermal oxidation and remote plasma etch are described. For thermally activated processes such as oxidation, temperature dependent ...

Temperature dependence of silicon nitride etching by atomic fluorine

The temperature dependence of the etch rate of silicon nitride by atomic fluorine has been measured using a discharge‐flow reactor. The activation energy of this process is 3.55±0.28 kcal/mol, quite similar to activation energies of Si and SiO2 etching, which were also measured (3.02±0.31 and 3.36±0.40 kcal/mol, respectively).

First‐wafer effect in remote plasma processing: The stripping of photoresist, silicon nitride, and polysilicon

We have identified a first‐wafer effect for photoresist ashing and silicon nitride‐polysilicon stripping in remote plasma reactors. The first‐wafer effect consists of the first wafer etching differently from the subsequent wafers in a lot. For photoresist ashing, the first wafer ashes faster than subsequent wafers. For silicon nitride and polysilicon stripping, first wafers show higher etch rates of silicon nitride and polysilicon, while silicon dioxide first wafers etch faster for the polysilicon strip process, and slower for the silicon nitride strip process. We have modeled the first‐wafer effect for photoresist ashing. We found an inverse relationship between the percentage change in the time to clear the photoresist from the wafer and the time delay between processing sequential wafers. We have included this first‐wafer effect in the on‐line statistical process control strategy for the photoresist asher in our laboratory. Examination of this first‐wafer effect suggests that it may be caused by the ge...

MMST manufacturing technology-hardware, sensors, and processes

This paper describes the equipment and processes utilized in the Microelectronics Manufacturing Science and Technology (MMST) program. The processes were carried out in a combination of testbeds (AVP, the TI designed and built Advanced Vacuum Processor) and commercial equipment, all in the single-wafer mode. All AVP processing was performed with the wafers in an inverted, face-down, configuration. All the processing equipment was connected to a Computer-Integrated Manufacturing (CIM) system, which both collected the designated data and communicated the process parameters from the CIM database to the particular processing unit. Where available, in situ sensors were utilized for monitoring the process parameters, with measurements made on a metrology die in the center of the wafer. Many of these processes were controlled by the model-based process control algorithms in the CIM system. Otherwise, the processes were controlled by standard statistical process control (SPC) methods. This paper emphasizes the processing methodology that was developed and followed in order to operate in this CIM environment and successfully execute an approximately 150 step 0.35 /spl mu/m CMOS process in less than 72 hours. >

Competitive Adsorption of Cations onto the Silicon Surface: The Role of the Ammonium Ion in Ammonia‐Peroxide Solution

Study of metal ion adsorption in aqueous solutions using a spin-contamination method has shown that NH 4 + can effectively prevent Ca 2+ adsorption onto the hydrophilic, oxidized silicon surface. In ammonia-hydrogen peroxide-water solution (APM) the relatively low Ca 2+ adsorption on the wet oxidized silicon surface can be explained by its competition for adsorption sites with NH 4 + , which is the dominant cation in APM. NH 4 + prevents Ca 2+ adsorption onto the oxidized silicon surface, by an ion-exchange mechanism analogous to that of H + in acidic solutions. The equilibrium constant governing the adsorption of NH 4 + is comparable to, or larger than, that of H + , meaning that NH 4 + is at least as effective as H + in removing metals from the oxidized silicon surface and in keeping metal ions off the surface.

Photoresist Stripping Using a Remote Plasma: Chemical and Transport Effects

Photoresist removal from silicon substrates has been achieved by a dry processing method using a plasma remote from the substrate to produce active species. Although the principal gas used is oxygen, the addition of small amounts of other gases (N 2 O, HCI, HBr, H 2 , Cl 2 , CF 4 , and CHF 3 ) has been shown to significantly enhance resist removal. The measured temperature dependences indicate that the effect of these additives may be a combination of oxygen atom production enhancement by neutral and ionic mechanisms as well as independent reaction with the resist.

The interaction of ion implantation with photoresist ashing: A statistical experimental design study

We applied statistical experimental design concepts to the study of the interaction of ion implantation with remote‐plasma photoresist ashing. A 28–4IV fractional‐factorial screening experiment revealed that implant species and resist hard bake conditions were not significant factors for resist ash rate and uniformity. We obtained quadratic polynomial response surface models using a G‐optimal experimental design as a function of the remaining four ash variables (temperature, pressure, oxygen, and hydrogen mass flow rates) and two implant factors (implant dose and energy). Ion implant dose had a significant effect on ash rate while implant energy was an important factor for ash rate uniformity.