Robert J. Hoekstra

Two‐dimensional modeling of high plasma density inductively coupled sources for materials processing

Inductively coupled plasma sources are being developed to address the need for high plasma density (1011–1012 cm−3), low pressure (a few to 10–20 mTorr) etching of semiconductor materials. One such device uses a flat spiral coil of rectangular cross section to generate radio‐frequency (rf) electric fields in a cylindrical plasma chamber, and capacitive rf biasing on the substrate to independently control ion energies incident on the wafer. To investigate these devices we have developed a two‐dimensional hybrid model consisting of electromagnetic, electron Monte Carlo, and hydrodynamic modules; and an off line plasma chemistry Monte Carlo simulation. The results from the model for plasma densities, plasma potentials, and ion fluxes for Ar, O2, Ar/CF4/O2 gas mixtures will be presented.

Two‐dimensional hybrid model of inductively coupled plasma sources for etching

Inductively coupled plasmas (ICPs) are currently being investigated as high density (≳1011–1012 cm−3), low pressure (<1–20 mTorr) sources for semiconductor etching and deposition. We have developed a two‐dimensional (r,z) hybrid model for ICP sources and have used the model to investigate Ar/CF4/O2 mixtures for etching applications. The simulation consists of electromagnetic, electron Monte Carlo, and hydrodynamic modules with an ‘‘off‐line’’ plasma chemistry Monte Carlo simulation. The model produces the temporally and spatially dependent magnetic and electric fields (both inductively and capacitively coupled), plasma densities, and the energy resolved flux of ions and radicals to the substrate. We discuss results for densities, power deposition, and ion energies to the substrate as a function of position.

Microtrenching resulting from specular reflection during chlorine etching of silicon

In an effort to increase throughput, the microelectronics fabrication industry has transitioned to high plasma density etching reactors using large source (>800 W) and moderate substrate bias (>100 W) powers in which the ion to neutral radical flux is large compared to reactive-ion-etching systems. These conditions can lead to microtrenching where etch rates are largest at the base of the sidewalls. Microtrenching has been attributed to specular reflection of high energy particles, usually ions, at grazing angles on the sidewalls of the mask and trench. These reflections produce a “focusing” of flux to the corners of the trench which results in locally enhanced etching. In this letter, integrated plasma equipment and Monte Carlo feature profile models have been used to examine the processes and conditions which produce focused fluxes and microtrenching, including the degree of specular reflection and sidewall slope of the mask. Quantitative comparisons are made to experimental measurements of etch profiles.

Integrated plasma equipment model for polysilicon etch profiles in an inductively coupled plasma reactor with subwafer and superwafer topography

Above wafer topography of the substrate, such as wafer clamps, is known to impact adjacent feature profiles during plasma etching of microelectronic devices. The consequences of subwafer topography, such as electrostatic chucks and cooling channels, on feature profiles is less well characterized. To investigate these issues we have developed and integrated a plasma equipment model and a Monte Carlo feature profile model, and applied the integrated model to investigate polysilicon etching in an inductively coupled plasma reactor. We find that, when using low conductivity wafers, subwafer topography reduces the sheath potentials above the wafer which results in lower ion energies incident on the wafer. Etch rates sensitive to ion power are therefore also reduced. Due to the perturbation of the presheath and sheath, subwafer topography can also affect the angular distribution of the ion flux incident on the wafer which then results in asymmetric etch profiles. Superwafer structures perturb both the magnitude...

Predictions of ion energy distributions and radical fluxes in radio frequency biased inductively coupled plasma etching reactors

Inductively coupled plasma (ICP) reactors are being developed for low gas pressure (<10s mTorr) and high plasma density ([e]≳1011 cm−3) microelectronics fabrication. In these reactors, the plasma is generated by the inductively coupled electric field while an additional radio frequency (rf) bias is applied to the substrate. One of the goals of these systems is to independently control the magnitude of the ion flux by the inductively coupled power deposition, and the acceleration of ions into the substrate by the rf bias. In high plasma density reactors the width of the sheath above the wafer may be sufficiently thin that ions are able to traverse it in approximately 1 rf cycle, even at 13.56 MHz. As a consequence, the ion energy distribution (IED) may have a shape typically associated with lower frequency operation in conventional reactive ion etching tools. In this paper, we present results from a computer model for the IED incident on the wafer in ICP etching reactors. We find that in the parameter spac...

Comparison of two-dimensional and three-dimensional models for profile simulation of poly-Si etching of finite length trenches

The development of two-dimensional (2D) profile simulators for fabrication of microelectronics features has significantly progressed during the past few years and now enables modeling of etch profile evolution for many different plasma processing conditions. Increasingly complex devices which have three-dimensional (3D) (that is, asymmetric) structures are now being designed. These structures require improved dimensionality in profile simulators to capture their most important features. Under many conditions, such as circular via etching, two-dimensional profile simulators can be used to address 3D structures. A legitimate issue is to what degree these 2D approaches can indeed be applied to truly 3D structures. In this article, we present results from a 3D profile simulator for the purpose of comparing profiles for innately 3D features to results obtained from a 2D profile simulation. It has been found that profiles obtained from the 3D simulators exhibit greater sidewall sloping in three-plane corners th...

The effect of subwafer dielectrics on plasma properties in plasma etching reactors

Nonplanar electrode topographies in plasma etching reactors are known to perturb plasma properties. In this article results from a computational study of plasma etching reactors having nonuniform dielectric structures below the wafer are presented. The system is an inductively coupled plasma reactor having a 13.56 MHz bias applied to the substrate. The model we have used is a hybrid simulation consisting of electromagnetics, electron Monte Carlo and fluid kinetics modules, and an off‐line plasma chemistry Monte Carlo simulation. We found that the subwafer dielectric adds a series capacitance to the sheath and wafer resulting in voltage division of the applied potential between the sheath, wafer, and dielectric. This produces a smaller sheath potential and smaller sheath thickness above the dielectric. The ion energy distribution is therefore depressed in the vicinity of the dielectric. The effect is more severe at high plasma densities where the capacitance of the sheath is larger compared to the subwafer...

A time dependent propagator method for long mean free path transport of neutral particles in plasma processing reactors

Plasma etching reactors for microelectronics fabrication are moving towards operating at lower gas pressures (<10 mTorr). These pressures are sufficiently low that simulations using continuum modeling techniques may not be strictly applicable. A time dependent kinetic method based on the use of a transition matrix (propagator) has been developed and applied to the calculation of long mean free path transport of neutral species in an inductively coupled plasma (ICP) etching reactor. The propagator P(r,r′) provides the probability that particles originating at location r′ will have their next collision at location r. The species densities obtained from this model are compared with results from fluid and Monte Carlo simulations for various mean free paths. We find that the propagator model is valid when the mean free path of the particles is larger than the numerical cell dimension and that fluid methods for long mean free path transport can be corrected to obtain the Monte Carlo or propagator results by emp...

Simulation tools for the design and analysis of plasma processing equipment

Summary form only given. Plasma processing equipment has become one of the most important tools used for microelectronics fabrication. The increasing complexity of microelectronics devices is making it imperative to develop a thorough understanding of this equipment. In order to accomplish this goal, a hierarchy of simulation tools have been developed at the University of Illinois in the recent years. These tool are centered around the Hybrid Plasma Equipment Model (HPEM), detailed plasma model. This paper gives an overview of the HPEM, its extensions and the variety of problems these tools can address.

Modeling of 2-dimensional and 3-dimensional etch profiles in high density plasma reactors

Summary form only given. In order to model the plasma etching process from plasma generation to etch profile evolution, processes from the macroscopic reactor scale to the microscopic feature scale must be simulated. An integrated Monte Carlo feature profile model (MCFPM) has been developed to examine the time evolution of etch profiles in high density plasma systems. By integrating the MCFPM with the hybrid plasma equipment model (HPEM), we are able to self-consistently determine the etch profiles for specific regions on the wafer in specific reactor geometry with specified parameters for power, chemistry, gas flow, etc. The latest improvements of the model include the effects of incoming particle angle and energy on reaction and reflection based on the results of molecular dynamics simulations.