Jeffrey Hopwood

Review of inductively coupled plasmas for plasma processing

The need for large-area, high-density plasma sources for plasma-aided manufacturing of integrated circuits has created a renewed interest in inductively coupled plasmas (ICPs). Several ICP reactor geometries are briefly reviewed. Typically, inductive coupling of RF power (0.5-28 MHz) can produce ion densities in excess of 1012 cm-3 even at submillitorr pressures. Existing electromagnetic field models of ICPs are examined and found to be in reasonable agreement with experimental results. Sputter deposition, anodic silicon oxidation and polymer etching using ICPs are also described. It is concluded that ICPs are promising candidates for meeting the future requirements of plasma processing, although considerable process development, plasma characterization and modelling are still needed.

Metal ion deposition from ionized mangetron sputtering discharge

A technique has been developed for highly efficient postionization of sputtered metal atoms from a magnetron cathode. The process is based on conventional magnetron sputtering with the addition of a high density, inductively coupled rf (RFI) plasma in the region between the sputtering cathode and the sample. Metal atoms sputtered from the cathode due to inert gas ion bombardment transit the rf plasma and can be ionized. The metal ions can then be accelerated to the sample by means of a low voltage dc bias, such that the metal ions arrive at the sample at normal incidence and at a specified energy. The ionization fraction, measured with a gridded mass‐sensitive energy analyzer is low at 5 mTorr and can reach 85% at 30 mTorr. Optical emission measurements show scaling of the relative ionization to higher discharge powers. The addition of large fluxes of metal atoms tends to cool the Ar RFI plasma, although this effect depends on the chamber pressure and probably the pressure response of the electron tempera...

Magnetron sputter deposition with high levels of metal ionization

A new deposition technique has been developed which combines conventional magnetron sputter deposition with a rf inductively coupled plasma (RFI). The RFI plasma is located in the region between the magnetron cathode and the sample position, and is set up by a metal coil immersed in the plasma. A large fraction of the metal atoms sputtered from the magnetron cathode are ionized in the RFI plasma. By placing a negative bias on the sample, metal ions are then accelerated across the sample sheath and deposited at normal incidence. Results from a gridded energy analyzer configured with a microbalance collector and located at the sample position indicate the level of ionization is low at a few mTorr and rises to ≳80% at pressures in the 25–35 mTorr range. Optical measurements of metal ion and neutral emission lines show scaling of the relative ionization to higher discharge powers. Significant cooling of the plasma electron temperature is observed when high concentrations of metal atoms were sputtered into the...

Langmuir probe measurements of a radio frequency induction plasma

In this work a planar, radio frequency induction plasma source is characterized in terms of ion density, electron temperature, and plasma potential using a single Langmuir probe in oxygen and noble gases. Probe measurements of density were also verified using microwave interferometry. Measured argon ion densities increase nearly linearly with power from 1×1011 cm−3 at 300 W rf power to 6×1011 cm−3 at 1.2 kW at 1×10−3 Torr. Krypton ion densities are also linear with power but saturate above 1 kW at a density of 2×1012 cm−3 at 1×10−3 Torr. Electron temperatures increase with decreasing pressure from 3 eV at 26×10−3 Torr to 7 eV at 0.3×10−3 Torr. Plasma potentials are typically 15–30 V and increase with decreasing pressure. Ion saturation current in oxygen at 5×10−3 Torr is 2.5% uniform over diagonals of 20 cm when a magnetic multipole bucket is used to confine the plasma. Ion generation energy cost in argon is 100–250 W/A.

Mechanisms for highly ionized magnetron sputtering

A simple model for ionization of sputtered metals by a high‐density plasma is presented. Experimentally, ion flux fractions of greater than 80% can be obtained by sputtering aluminum into a region of dense plasma (ne∼1012 cm−3). Such a process has important applications in the filling of high‐aspect‐ratio features encountered in microelectronics fabrication. Both electron‐impact and Penning ionization mechanisms are considered in this model. Under conditions of low electron density (ne≪1011 cm−3), Penning ionization is found to be the dominant ionization path. This is consistent with the accepted ionization mechanism for conventional diode sputtering. When high electron densities are generated, however, electron‐impact ionization plays a significant ionization role. Langmuir probe measurements of the inductively coupled plasma indicate that the electron density lies between 2×1011 and 2×1012 cm−3. The model, in combination with measured plasma density, is used to calculate ion fractions. Modeled and exper...

Low-power microwave plasma source based on a microstrip split-ring resonator

Microplasma sources can be integrated into portable devices for applications such as bio-microelectromechanical system sterilization, small-scale materials processing, and microchemical analysis systems. Portable operation, however, limits the amount of power and vacuum levels that can be employed in the plasma source. This paper describes the design and initial characterization of a low-power microwave plasma source based on a microstrip split-ring resonator that is capable of operating at pressures from 0.05 torr (6.7 Pa) up to one atmosphere. The plasma sources microstrip resonator operates at 900 MHz and presents a quality factor of Q=335. Argon and air discharges can be self-started with less than 3 W in a relatively wide pressure range. An ion density of 1.3/spl times/10/sup 11/ cm/sup -3/ in argon at 400 mtorr (53.3 Pa) can be created using only 0.5 W. Atmospheric discharges can be sustained with 0.5 W in argon. This low power allows for portable air-cooled operation. Continuous operation at atmospheric pressure for 24 h in argon at 1 W shows no measurable damage to the source.

Electromagnetic fields in a radio‐frequency induction plasma

The electromagnetic fields which drive a radio‐frequency induction plasma are both modeled and measured. The plasma source consists of a planar, square coil separated from a low pressure plasma chamber by a 2.54‐cm‐thick quartz window. A small loop antenna, which is sealed in a pyrex tube, is immersed in the discharge to determine the magnitude and direction of the rf magnetic field. The measured B field is primarily radial and axial. Typical rf field strengths vary from 2 to 7 G for rf powers of 0.1–1 kW. The radial B field decays exponentially in the axial direction. The skin depth of the electromagnetic field is 1.6–3.6 cm which is consistent with Langmuir probe measured ion densities (typically 3×1011 cm−3) in argon. Invoking Maxwell’s equations to deduce the rf electric field from the measured B field, we find the E field to be primarily azimuthal. Peak field strengths increase from 100 V/m at 100 W to 200 V/m at 600 W where they saturate for higher powers. Finally, we present a 3D finite element sol...

Split-ring resonator microplasma: microwave model, plasma impedance and power efficiency

The microstrip split-ring resonator (MSRR) microplasma source is analysed and characterized using a microwave model of the device. Throughout the discussion, experimental data for three MSRR designs are also presented. The model identifies the key parameters that control the performance of the device and results in the formulation of closed-form expressions useful for designing, analysing and comparing MSRR designs. Matching the microstrip characteristic impedance to the microplasma impedance is found to be a key factor in the performance of these devices and it can be even more critical than the quality factor of the ring resonator. Based on the model, average rf electric fields of up to 4 MV m−1 at 1 W of input power are estimated to be generated in a 45 µm gap device. Furthermore, the model is used to determine the plasma impedance and thereby obtain information on physical properties of the microdischarge. Electron densities of the order of 1014 cm−3 are estimated in a 1 W argon discharge at atmospheric pressure. Based on the values of the plasma impedance, it is also determined that up to 70% of the power input to the MSRR is coupled to the electrons in the microdischarge.

Ion bombardment energy distributions in a radio frequency induction plasma

Ion bombardment energy distributions to a grounded substrate in a low pressure, rf induction plasma source are measured. The plasma source consists of a planar, spiral coil driven at 13.56 MHz which is separated from a low pressure discharge vessel by a quartz vacuum window. Ion bombardment spectra were determined using a differentially pumped retarding grid energy analyzer which samples the plasma through an 80 μm diam grounded, conducting orifice. The ion flux was found to be nearly monoenergetic for heavier ionic species such as Ar and oxygen. A double‐peaked distribution was observed in water vapor plasmas where the sheath transit time of light ions is much less than the rf period. The average ion energy follows the average plasma potential and the width of the ion energy distribution correlates with the rf component of the floating probe potential, which is typically 2–6 Vp–p.

A microfabricated inductively coupled plasma generator

The design, fabrication, and characterization of a surface micromachined plasma generator is described for the first time in this paper. The plasma is sustained without electrodes by inductively coupling a /spl sim/150-MHz current into a region of low-pressure gas, Both argon and air plasmas have been generated over a range of gas pressures from 0.1 to 10 torr (13.3-1333 Pa). Typically, the power used to sustain the plasma is 350 mW, although /spl sim/1.5 W is required to initiate the discharge. Network analysis of the plasma generator circuit shows that over 99% of the applied RF power can be absorbed by the device. Of this, /spl sim/50% is absorbed by the plasma and the remainder of the power is dissipated as ohmic heating. An argon ion current of up to 4.5 mA/cm/sup 2/ has been extracted from the plasma and the electron temperature is 52 000 K at 0.1 torr. This plasma source is intended for electronic excitation of gas samples so that the presence of impurities and toxins may be detected using optical emission spectroscopy.