Yukinobu Hikosaka

Diagnostics and control of radicals in an inductively coupled etching reactor

Comprehensive measurements of charged particles and neutral radicals in an inductively coupled plasma (ICP) are performed to understand and control the etching process using a CF4/H2 gas. The electron density in the ICP reactor decreases exponentially in a downstream region while the most abundant ionic species CF+ increases in proportion to the rf power with the CF+3 density almost constant. The neutral radical diagnostics by appearance mass spectrometry indicate 10 times more F atoms and somewhat fewer CFx radicals (x=1–3) in ICP, compared with a high‐pressure capacitively coupled plasma diode. Such a small ratio of the CFx density to the F density is possibly a cause of the low etch selectivity of SiO2 to Si in ICP etching. Two innovative methods to achieve the high selectivity in ICPs are demonstrated. One is wall heating (100–200 °C), which leads to a drastic increase in CFx densities with the F density almost constant. The other is a pulse modulation of rf power at 30–50 μs durations where the time‐...

Free Radicals in an Inductively Coupled Etching Plasma

A high-density (>1011 cm-3) CF4/H2 plasma was produced in an inductively coupled plasma (ICP) reactor where an external helical coil is wound around a quartz tube. Capacitive coupling from the coil to the plasma caused the release of a large number of impurities ( SiF4 and CO) from the warm quartz wall close to the coil. These impurities significantly deteriorate the etch selectivity of SiO2 to Si in the ICP reactor. Water cooling and a Faraday shield are effective to suppress the release of impurities. Neutral radicals CF3, CF2, CF and F were measured in addition to ionic species. The high-density high electron-temperature ICP causes the formation of a large number of F atoms and CF+ ions with fewer CFx radicals, in comparison to a low-density capacitively coupled plasma (CCP). H2 addition to the CF4 discharge drastically modifies the CF3 and CF2 densities in the ICP as well as in the CCP. The high etch rates and the low selectivity of SiO2 to Si obtained in the ICP were discussed taking account of the residence time and the dissociation time of reactive species in the etching reactor.

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.

Radical Kinetics in a Fluorocarbon Etching Plasma

Neutral free radicals, CF3 and CF2, in a radio-frequency CF4/H2 discharge were detected using appearance mass spectrometry. The spatial density distributions of these radicals were obtained with the mixing ratio of H2/CF4 as a key parameter. Adding a 10% H2 gas to CF4 enhanced the CF2 density by a factor of 50 and flattened the spatial profile. By the injection of the H2 gas abruptly into the CF4 discharge, the temporal transition to the CF4/H2 discharge was investigated. When the percentage of H2≥5%, a strange time variation is found for the CF3 radical: its density sharply rises by a factor of 10 and slowly falls to a value close to the initial one. The slow time response was attributed to the H2-induced polymer deposition. The surface loss probability of CF2 and CF3 was measured in various conditions which suggested the importance of surface chemistry of radicals.

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...

Drastic Change in CF2 and CF3 Kinetics Induced by Hydrogen Addition into CF4 Etching Plasma

CF2 and CF3 radical in a CF4/H2 etching system were directly measured by means of threshold-ionization mass spectrometry. Addition of 10% H2 to CF4 increased the CF2 density by two orders of magnitude, flattened the spatial profile, while it doubled the CF3 density. After the abrupt addition of H2, the temporal transition to steady CF4/H2 discharge was investigated on reactive species such as CFx, F, H, HF and CHF3. In particular, an anomalous time variation was found on the CF3 radical: its density sharply rises by a factor of 20 and slowly falls to a value close to the initial one. The slow time response was attributed to fluorocarbon film deposition induced by H2 addition. A good correlation was obtained between the CFx density and the surface loss probability measured.

Realistic Etch Yield of Fluorocarbon Ions in SiO2 Etch Process

The energy distribution and flux of ions striking an rf-biased electrode were measured by using an rf floating ion energy analyzer. Energies of CF1+, which was the dominant species, were distributed over a voltage range of about half the peak-to-peak bias voltage. Energetic ions with neutral radicals, forming the reactive fluorocarbon polymer layer on a SiO2 film, affected etching characteristics such as rate and selectivity. To investigate the chemical activity of the reactive layer, we estimated the etch yield of SiO2 from the given energy distribution of the ions and the etch rate of SiO2. We found that the energy dependence of the etch yield should be controlled by precisely regulating the flux and composition of neutral radicals under a given ion flux, in order to obtain a high etch rate under the actual SiO2 etching conditions.

Reduction of Electron Shading Damage Using Synchronous Bias in Pulsed Plasma

A novel method is proposed for reducing charging damage due to the “electron shading” effect. The concept is to selectively utilize the coolest electrons in a pulsed plasma at the end of its off period by synchronizing rf bias; thereby one should be able to reduce the negative charge build-up responsible for the damage. This concept has been examined using an inductively coupled plasma (ICP) apparatus. Exposure to a cw Ar ICP damaged most MOS capacitors with a 6-nm-thick gate oxide and connected to 105 shaded antennas. This damage was reduced only slightly even with 5-µ s- on/10-µ s- off pulse modulation when the rf bias was asynchronous (60 kHz). When the rf bias (66.7 kHz) synchronized at the expected optimal phase, the most significant reduction of the damage was observed. This effect is discussed based on the results of time-resolved probe and optical emission measurements.

Investigation of ion transportation in high-aspect-ratio holes from fluorocarbon plasma for SiO2 etching

Abstract We investigated the behavior of ions in deep contact holes under actual dry etching conditions by comparing results between experimental and numerical simulations. In the experiments, we directly measured the ions that penetrated a real-contact-hole-sized (0.2 μm o ) micro-capillary plate, which was a membrane with many throughholes fabricated on a Si wafer with a structure customized to simulate the actual electric field profiles in the contact holes. As expected, both results suggested that the ion flux and the ion energy were depressed at the bottom surface and strongly depended on the hole aspect ratio. However, we found some differences between the experiments and the calculations. We believe these were caused by variations in the experimental results and by unknown parameters in the calculation model of the ion kinetics in the deep contact holes. Quantitative analysis will be possible in the future after the results conform to each other and the unknown parameters are clarified.