Satoshi Morishita

ETCH RATE CONTROL IN A 27 MHZ REACTIVE ION ETCHING SYSTEM FOR ULTRALARGE SCALE INTEGRATED CIRCUIT PROCESSING

The etch rates of SiO2, photoresist, Si, and SiN in a 27 MHz reactive ion etching system at constant ion flux of 6×1016 cm−2 s−1 and ion energy of 1450 V were studied. Typical incident flux densities of CF2 and CF+ were on the order of 1017 and 1016 cm−2 s−1, respectively. The SiO2 etch rate was determined by the balance of the energy supplied by the total ion flux and the amount of the C–F reactive species supplied by radicals and ions. When we roughly assumed the surface reaction probabilities of F, CF, CF2 and CF3 to be 0.1, 0.1, 0.1, and 0.5, the SiO2 etch rate could be expressed well as a function of the total number of F in the net radical fluxes. To clarify the dominant flux including radicals and ions, however, further research on surface reaction probabilities on the actual etched surface must be conducted because the incident fluxes strongly depend on these constants of the surface reaction probability. Lowering the total ion flux or ion energy decreased the etch rate of SiO2. A higher ion flux ...

Mechanism of Radical Control in Capacitive RF Plasma for ULSI Processing

The radicals of capacitive plasmas actually used in mass production were analyzed using various measurement systems. The composition of radicals in bulk plasma depends on the gas chemistry, the dissociation process, and interaction with the wall. It is revealed that parent gas (C4F8) is dissociated by multiple collision with electrons according to τne , where τ is the residence time, ne is the electron density, σ is the dissociation collision cross section and v is the electron velocity. A high-performance etching process, which can realize 0.09 µm contact holes with aspect ratio of 11, was achieved using a short residence time to suppress the excess dissociation and the control of deposition species through the addition of O2 to C4F8/Ar plasma as well as the reduction of the density of F radicals through the reaction with the Si wall.

Mechanism of C4F8 dissociation in parallel-plate-type plasma

To investigate the mechanism of C4F8 dissociation in parallel-plate-type plasma, we used several of the latest diagnostic tools and made extensive measurements of electrons, radicals, and ions under conditions that greatly suppressed the effects of plasma-surface interaction. These measurements showed that the amount of light fluorocarbon radicals and ions increased with increasing electron density. The dissociation of C4F8 was analyzed by using rate equations, after confirming the stability and uniformity of the plasma. The total dissociation rate coefficient of C4F8 was 1×10−8 cm3/s, and CF2 radicals were mainly generated from products of C4F8 dissociation. F was mainly generated from CF2 by electron-impact dissociation and lost by pumping. We could estimate that the C2F4 density was roughly comparable to the densities of CF and CF3, and that the surface loss probability of C2F4 increased with increasing electron density. C2F4 might play an important role in the etching because of its rich polymerizatio...

Plasma-Wall Interactions in Dual Frequency Narrow-Gap Reactive Ion Etching System

The effects of plasma-wall interactions on the plasma chemistry in a dual frequency narrow-gap reactive ion etching (RIE) system is investigated as a function of electron density, residence time, and partial pressure of additive O2 gas. It is found that there is a critical point in the residence time, where the dissociation dominant region and the wall-interaction dominant region are separated. The net flux of chemically reactive species is estimated. The net flux of carbon-inclusive ions is of the order of 1016 cm-2s-1, and is larger than that of CFx radicals. Carbon-inclusive and silicon-inclusive ions dominate the total flux of chemically reactive species. The deposition rate of the fluorocarbon film is strongly dependent on the O2 partial pressure, and is controlled by the chemical etching with oxygen as well as the reduction of fluorocarbon radical density in the gas phase. Based on the estimation of the net flux of chemically reactive species and the effect of added oxygen, dominant chemical species that control the etching reaction in the RIE system are discussed.

Characterization of 100 MHz inductively coupled plasma (ICP) by comparison with 13.56 MHz ICP

The effect of the excitation frequency on the dissociative process of the C4F8 gas was investigated by comparing a 100 MHz [very high-frequency (VHF)] inductively coupled plasma (ICP) with a 13.56 MHz [radio frequency (rf)] ICP. The same apparatus except for the wave generator and matching network was used for both ICPs in order to investigate the frequency effect as precisely as possible. The electron density and electron temperature in an Ar plasma were measured by using a Langmuir probe. From the dependence of the radial distribution of the Ne on the ICP source power, it was found that the rf ICP was produced in a cylindrical space under the coil area, while the VHF ICP was generated throughout the reactor. In C4F8/Ar plasma, the CFx (x=1, 2, 3) radical densities near the reactor wall were measured by using appearance mass spectrometry, and the F radical density was measured by using actinometry through the optical emission spectroscopy of Ar (750.4 nm) and F (703.7 nm). The degree of dissociation of t...

CF and CF2 Radical Densities in 13.56-MHz CHF3/Ar Inductively Coupled Plasma

Radial and axial distributions of CF and CF2 radical densities in CHF3/Ar inductively coupled plasma (ICP) generated by a radio frequency of 13.56 MHz were evaluated using laser-induced fluorescence (LIF). The radical densities in the bulk plasma were estimated by combining the density distributions measured by LIF with the absolute radical density measured by appearance mass spectrometry (AMS) near the reactor wall. Axial and radial distributions of CF and CF2 radical densities were extremely hollow. The radial distribution was strongly correlated with the electron density distribution and was significantly influenced by the dissociation process resulting from the electron excitation in the reactor space. The axial distribution, on the other hand, was mainly determined by the surface reaction of radicals on the wafer.

An integrated CMOS-MEMS probe having two-tips per cantilever for individual contact sensing and kelvin measurement with two cantilevers

The MEMS-made probe cards can drastically improve semiconductor wafer test quality as compared to traditional tungsten probe. To further take advantage of MEMS technology, the authors propose a CMOS-MEMS integrated probe card, to solve the tradeoff problem of measurement precision and excess pad damage by skating, by 4-terminal (Kelvin) measurement with two-tracks-per-cantilever needle. Putting two tips on each cantilever enables us to detect electrical contact and to decrease skating. And by this structure, electrical properties of a device under test are measured precisely with 4-terminal measurement which can eliminate track resistance and contact resistance. We measured the resistance of a gold thin film. With 2-terminal method, the resistance was measured to be about 74 ohms. However with Kelvin measurement, the resistance was 0.012-0.022 ohms. This result shows the successful implementation of 4-terminal measurement probe with MEMS technology.

A Balanced-SeeSaw MEMS swing probe for vertical profilometry of deep micro structures

An actuator-integrated MEMS needle probe is improved to measure vertical surface profile of narrow and deep test structures such as microholes and trenches. A newly developed surface scanning method, called ¿swing probing¿, can reduce the measurable feature size by factor of up to eight (i.e. from 40 ¿m down to 5 ¿m for 50¿m-deep trenches, and down to 25 ¿m for 1 mm-deep ones) as compared to traditional ¿slide probing¿. The improvement is due to the new ¿Balanced-SeeSaw¿ probe design that guarantees rotational movement without wobbling as well as sensitivity increase. Since the design is highly scalable, the probe can further reduce target feature size as well as measurement resolution thus may enlarge the application field of such surface quality assessment technology to MEMS process test structure monitoring.

Demonstration of hardware-accelerated formal verification

A semi-formal verification technique, which performs a brute-force compiled simulation with a sophisticated search space pruning, has been proposed and shown to be competitive with the state-of-the-art SAT-based verification techniques [1]. This paper presents a novel approach for accelerating the semi-formal verification by utilizing hardware/software co-execution. To maximize the gain from hardware acceleration, we propose two novel techniques such as hardwired conflict analysis for learning and speculative input pattern generation. We demonstrate that our FPGA-based prototype system achieves about 7x speedup compared against the software implementation of the semi-formal verifier.

Mechanism of fluorine reduction in C4F8/Ar parallel-plate-type electron-cyclotron-resonance plasma by a Si top plate

The density of F in C4F8/Ar plasma could be reduced by 34% when the Si top plate was bombarded by energetic ions in a parallel-plate-type 500 MHz electron-cyclotron-resonance plasma reactor, but that in CF4/Ar plasma was not reduced. We measured the densities of CFx (x=1–3), Si, and F in both plasmas as a function of ion-bombardment energy and found that F was generated from CF2 in C4F8/Ar plasma but not in CF4/Ar plasma, and that the CF2 density decreased to a similar extent with increasing ion-bombardment energy in both plasmas. We conclude that the reduction of the F density in C4F8/Ar plasma was caused by the decrease in CF2 density and not by a direct reaction of F with Si when the Si plate was irradiated by energetic ions.