Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/32—Gas-filled discharge tubes
  • H01J37/32917—Plasma diagnostics
  • H01J37/32935—Monitoring and controlling tubes by information coming from the object and/or discharge
  • H01J37/32972—Spectral analysis
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/32—Gas-filled discharge tubes
  • H01J37/32917—Plasma diagnostics
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/32—Gas-filled discharge tubes
  • H01J37/32917—Plasma diagnostics
  • H01J37/32935—Monitoring and controlling tubes by information coming from the object and/or discharge
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/32—Gas-filled discharge tubes
  • H01J37/32917—Plasma diagnostics
  • H01J37/32935—Monitoring and controlling tubes by information coming from the object and/or discharge
  • H01J37/32963—End-point detection
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/32—Gas-filled discharge tubes
  • H01J37/32917—Plasma diagnostics
  • H01J37/3299—Feedback systems

Definitions

• Invasive plasma diagnostic techniques are undesirable in most industrial circumstances given the often harsh processing conditions which prove detrimental to many probes, and the fact that the presence of the probes themselves can alter the characteristics of the plasma.
• the disruptive impact of fitting such immersive diagnostics to existing fabrication infrastructures must also be considered.
• Installation of invasive probes can potentially introduce additional complexities to a fabrication line which adversely impacts process replication, a key necessity for high volume production.
• Real time, non-invasive and installation-free plasma monitoring techniques are therefore particularly advantageous in many of these industrial scenarios, for example US patent publication number US2005183821 discloses a noninvasive radio frequency antenna placed in close proximity to an activated plasma and configured to monitor a plasma. Other applications are discussed in Seiji Samukawa, et al. The 2012 plasma roadmap.
• PCT Patent Publication number WO2007/041280A2 discloses a wafer based version of this strategy in which a dielectric covered coil is fixed on a wafer work-piece while another US patent publication US2006/169410, Maeda et al, installs a similar sensing element via a quartz rod inserted into the gas shower head of a chamber electrode.
• US patent publication US2015/364300A1 Galli, describes the submersion of such a coil probe within an inductively coupled plasma source.
• the prior art to date basically describes probes which require 'internal' placement and submersion of the sensor within the discharge vessel. Perturbation of the plasma due to probe immersion and in particular the presence of an electrostatic shielding in such scenarios is known to be significant.
• a system to remotely monitor a plasma comprising:
• a magnetic field antenna positioned in the near field electromagnetic field of a coupled plasma source wherein the magnetic field antenna is a magnetic loop antenna placed in the near electromagnetic field or the like.
• a novel approach to remotely monitor low pressure non-equilibrium plasmas is provided.
• a magnetic field antenna is positioned in the near field of a plasma source. Magnetic flux from plasma currents, present in the vicinity of the viewport, is intercepted via a calibrated loop antenna.
• a radio system spectrum analyser
• Plasma parameters such as series (or geometric) resonance plasma and electron-neutral collision frequencies are evaluated via a fitting of resonant features present on higher harmonics of the driving frequency.
• This approach advances remote, non-invasive and installation free plasma monitoring.
• the plasma is a low pressure non-equilibrium plasma.
• the magnetic flux radiated from a view-port (or equivalently any non-conducting external access port, is exploited in the present invention.
• This enables a 'remote' detection of plasma currents without any physical alteration of the discharge vessel during probe installation.
• This strategy removes any disturbance caused by internal probe installation and is therefore particularly advantageous for deployment on existing fabrication lines which are highly sensitive to alteration.
• the application of a partially shielded loop antenna, prevalent in Electro-magnetic compliance (EMC) testing in which the electro-static shielding via a gap in its coverage actually induces the probe current is distinct from previous approaches to intercept magnetic flux from a plasma discharge.
• EMC Electro-magnetic compliance
• a system to remotely monitor a plasma comprising:
• a magnetic field antenna positioned in the near electromagnetic field of a coupled plasma source wherein the magnetic field antenna is a magnetic loop antenna placed in the near electromagnetic field and adapted to measure near field signals emitted from the plasma source.
• the magnetic loop antenna is inductively coupled to the plasma source and the system is adapted to analyse near-field radio emissions of the plasma source using a radio emission spectroscopy (RES).
• RES radio emission spectroscopy
• the near field signals comprises near H field signals.
• the near H field signal intensity drops with distance as a function of distance from the magnetic field antenna and the plasma source.
• the signal intensity drops with distance as a function of 1/r 3 where r is the distance from the magnetic field antenna and the plasma source.
• a module to perform a frequency analysis of higher harmonics present in the near field signal to provide a resonance behaviour dependent to plasma parameters of the plasma source and outputting a condition of the plasma source based on said analysis.
• the frequency analysis is performed for different operating pressures.
• the magnetic loop antenna is inductively or capacitively coupled to the plasma source.
• the magnetic loop antenna is calibrated to enable a calculation of a frequency dependent coupling factor between a current associated with the plasma and an induced antenna signal.
• a radio system is configured to analyse low power signal levels across a wide frequency band.
• the signal is localised to a proximity of a viewport to enable signal isolation to the plasma source.
• a viewport can be interpreted as a typically sealed glass, quartz, or dielectric window in a chamber, these being very typical for most plasma tools.
• the system and method of the invention does not have to be able to visually observe the plasma through these viewports, and said viewports are still usable even if the system cannot directly "see" the plasma by line of sight.
• a measured resonance plasma frequency and an electron- neutral collision frequency are correlated via a fitting of resonant features present on higher harmonics of a driving frequency.
• the correlation provides a factor to identify one or more of the following: arcing, pump or matching failure events, associated with the plasma source.
• the near field signals comprises near E field signals.
• a first antenna to measure near H field signals and a second antenna to measure near E field signals In one embodiment there is provided a first antenna to measure near H field signals and a second antenna to measure near E field signals.
• a module to perform a frequency analysis of higher harmonics present in the near H field signals and E field signals to provide a resonance behaviour dependent to plasma parameters of the plasma source.
• a system to remotely monitor a plasma comprising: a magnetic field antenna positioned in the near electromagnetic field of a plasma source wherein the magnetic field antenna is a magnetic loop antenna placed in the near electromagnetic field.
• a method to remotely monitor a low pressure non-equilibrium plasma comprising the steps of:
• the magnetic field antenna is a magnetic loop antenna placed in the near electromagnetic field and adapted and adapted to measure near field signals emitted from the plasma source.
• the steps of measuring a resonance plasma frequency and an electron-neutral collision frequency; and correlating said frequency measurements via a fitting of resonant features present on higher harmonics of a driving frequency are performed by the steps of measuring a resonance plasma frequency and an electron-neutral collision frequency; and correlating said frequency measurements via a fitting of resonant features present on higher harmonics of a driving frequency.
• Figure 2 illustrates (a) Distance dependence (r in Fig. 1 ) of induced radio signal IRES, (b) Comparison of IRES with the bulk plasma current ⁇ buik. (c) Electron density values in centre of discharge using Langmuir probe.
• Figure 3 illustrates a correlation of antenna signal with an in-line current measurement of the plasma using an l-V probe
• Figure 4 illustrates the radio emission intercepted (see figure 1 ) for the first three harmonics of driving frequency (13.56 MHz) for an 02 plasma;
• Figure 5 illustrates near field signal frequency analysis: resonance behaviour found in higher harmonics;
• Figure 6 illustrates extracted values of the series resonance (co and electron-neutral collision frequencies (v) from harmonic resonance peaks shown in Figure 5.
• Figure 1 illustrates a system to remotely monitor a low pressure non-equilibrium plasma according to one embodiment of the invention indicated by the reference numeral 1 .
• At least one magnetic field antenna 2 is positioned in the near field of a capacitively coupled plasma source 3. It will be appreciated that the system is applicable to any plasma source, be it capacitively or inductively coupled, or a plasma induced by laser heating or other means. Magnetic flux from plasma currents, present in the vicinity of a viewport 4, is coupled via a calibrated loop antenna 2.
• the system 1 can be embodied as a radio system (spectrum analyser) and utilised to analyse the low power signal levels across a wide frequency band. The signal is localised to the proximity of a viewport, enabling signal isolation to the nearby plasma source.
• the present invention provides a system and a method for the measurement and analysis of near-field radio emissions near a low pressure non-equilibrium plasma using a concept hereinafter referred to as radio emission spectroscopy (RES).
• RES radio emission spectroscopy
• a near-field loop antenna 2 typically diameter ⁇ 5-25 mm intercepts the magnetic flux resulting from currents in proximity to a viewport 4 in the plasma chamber.
• the loop plane is orientated perpendicular to the viewport 4 to intercept currents transiting between the electrodes.
• a shielded loop antenna design often used for magnetic field sensing in electromagnetic interference testing, can be employed here.
• the spatial behaviour of the magnetic field (strictly magnetostatic field) surrounding plasma currents flowing in proximity to the viewport is given by application of the Biot-Savart law:
• dV is the volume element for current density J
• r represents the separation between the current volume and antenna position r.
• An important aspect of the invention is the fact that measured currents or voltages in the near field fall off is approximately 1/r 3 , indicating near field operation. If the signals fall off as 1/r 2 , or similar, then they are not listening to the near field. This is illustrated in Figure 2 where there is shown in (a) Distance dependence (r in Fig. 1 ) of induced radio signal IRES, (b) Comparison of IRES with the bulk plasma current ⁇ buik. (c) Electron density values in centre of discharge using Langmuir probe, (d) Resistive component (IR) of the total current measured by an in-line l-V probe.
• the plasma chamber 5 acts as a radiation source emitting electromagnetic radiation into its environs.
• the antenna system 2 is positioned in the 'near' portion of this radiation field. This is evidenced by observation of the signal intensity drop with distance as 1/r 3 .
• multi-pole characteristics of the emitting element i.e. plasma
• Extension of the Bio-Savart formulation to include dipole contributions of the emitting current leads to such a 1/r 3 dependence.
• This behaviour is typical of antenna operation within the near field of a radiation zone sufficiently remote from an emitter and in contrast to probing within a source where spatial variations deviate from such 1/r 3 behaviour. Indeed, this fact is widely exploited in near field communication systems to enable a remote ('contact-less') but localised coupling to a radiation source.
• the ability to localize an emitted radio signal from a plasma 3 by a near-field (magnetic loop) antenna 2 is essential for operation of the invention. This resolution ensures that emissions from nearby sources are effectively minimized as the antenna is sensitive to the H-field near the plasma viewport (or alternatively any other non-conductive surface bounding the plasma).
• the localized nature of the radio emission spectroscopy (RES) signal corroborates the source of the emission as the plasma 3 over any far-field radiation sources that likely includes emissions from nearby plasma chambers, the power unit, the match box, other sections of the transmission line, and ambient radio signals. Noise can also be easily removed from the signal in this scenario by an on-line background subtraction at a suitable point away from the near field, as illustrated in Figure 2a.
• the invention provides a practical method for signal extraction and is particularly advantageous for implementation in "radio-noisy" fabrication environments.
• the system 1 can provide a module to perform a frequency analysis of higher harmonics present in the near field signal to provide a resonance behaviour dependent to plasma parameters of the plasma source, as referenced below with respect to Figures 3 to 6.
• the system can output a condition of the plasma source based on said analysis to inform a user that the plasma is operating correctly.
• the near field loop antenna 2 intercepts the magnetic flux resulting from (displacement) currents transiting between the electrodes in proximity to the viewport.
• the loop antenna 2 can consists of a flat metal inner conductor coated in a non-conductive shielding blocking electric fields.
• the antenna can be calibrated using a 50 Ohm micro-strip transmission line and a vector network analyser using a technique commonly employed for electromagnetic compatibility testing in adherence to International Electrotechnical Commission (IEC) directives.
• IEC International Electrotechnical Commission
• numerous calibration techniques can be used for example using calibration files provided by RF Explorer (http://i3.rf- explorer.com/downloads) or using a calibration using Helmholtz coils as provided by Beehive Electronics (see for example - h ttps://www. beehive- electronics. com/datasheets/1 OOSeriesDatasheetCurrent.pdf ).
• Calibration enables the calculation of a frequency dependent coupling factor between the (circuit/plasma) current and the induced antenna signal.
• Low level signals are analysed using a spectrum analyser with a high dynamic range and resolution bandwidth set ⁇ 100 Hz.
• a background scan can be performed at a distance sufficiently remote from the plasma (i.e. where the near field signal is lost), as illustrated in Figure 1 and subtracted from the near field signal to isolate frequencies intercepted from the plasma.
• the plasma source used here is a Plasma Lab 100 etching system from Oxford Instruments, as provided by Oxford Instruments Ltd. http://www.oxfordplasma.de/svstems/100ll.htm . Oxford Instruments Plasma Lab 100.
• the voltage induced in the loop antenna from magnetic fields resulting from plasma currents (I) is based on Faraday's induction principle and can be given by the following: dl
• the signal is dominated by frequencies near the driving frequency (13.56 MHz).
• Figure 3 shows this signal as an effective proxy for monitoring current variations in the plasma.
• the monitored current variations have utility as a contact-less sensor for monitoring arcing, pump or matching failure events in fabrication equipment and in endpoint detection where optical diagnostic are limited (e.g. due to window coating).
• Figure 4 illustrates the radio emission intercepted (see figure 1 ) for the first three harmonics of driving frequency (13.56 MHz) for an 02 plasma ashing of a photo-resist coated wafer. Measurements of the oxygen 777 nm line intensity are carried out using an optical spectrometer. Endpoint (total removal of photoresist) occurs as the line intensity plateaus (500 seconds in figure 2). Monitoring near field radio emission 'Radio Emission Spectroscopy' or RES, according to the invention, is found to be an effective indicator for endpoint here.
• the sheath width is normalised to the gap width (0 ⁇ s ⁇ 1 ) and co ⁇ is the plasma frequency.
• Electric field (which is proportional to the induced antenna voltage) coincides with the occurrence of s min and s mgx enhancing the two primary resonances shown in Figure 5.
• Damping of the series resonance is due to the electron- neutral collisions given by the frequency v and is constrained in the upward direction by the driving frequency ⁇ (i.e. v>13.56 MHz).
• a Gaussian distribution is fitted to the upper resonance peaks shown in Figure 4.
• the maximum sheath width is estimated here as 0.23 or 23 % of the discharge gap and is found to be approximately static across the power range of interest. This estimation is achieved by employment of the Child sheath law coupled with a combination of Langmuir probe measurements for electron density and temperature and l-V measurements for estimation of the sheath voltage.
• the plasma density can now be calculated using the relation ⁇
• Figure 6 illustrates extracted values of the series resonance ( ⁇ ) and electron- neutral collision frequencies (v) from harmonic resonance peaks shown in Figure 5. Investigations of the radiated (far field) radio signal were also carried out. Results showed a more limited diagnostic potential and insensitivity to the plasma parameters. Frequency analysis revealed harmonic peaking at wavelengths of -0.5-1 meters across a range of applied powers. This insensitivity likely reflects some physical aspect of the transmission line (e.g. chamber diameter) rather than correlation to any plasma parameter. A mentioned previously the ability to localise the emitted radio signal from the plasma via a near field (magnetic loop) antenna is important for the operation of the invention.
• the system can comprise a first antenna to measure near H field signals and a second antenna to measure near E field signals.
• the module can perform a frequency analysis of higher harmonics present in the near H field signals and E field signals to provide a resonance behaviour dependent to plasma parameters of the plasma source.
• This provides a practical method for signal extraction and is particularly advantageous for implementation in 'radio-noisy' fabrication environments.
• the embodiments in the invention described with reference to the drawings comprise a computer apparatus and/or processes performed in a computer apparatus.
• the invention also extends to computer programs, particularly computer programs stored on or in a carrier adapted to bring the invention into practice.
• SDR software defined radio
• the program may be in the form of source code, object code, or a code intermediate source and object code, such as in partially compiled form or in any other form suitable for use in the implementation of the method according to the invention.
• the carrier may comprise a storage medium such as ROM, e.g. CD ROM, or magnetic recording medium, e.g. a memory stick or hard disk.
• the carrier may be an electrical or optical signal which may be transmitted via an electrical or an optical cable or by radio or other means.

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• 2017
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