EE-527: MicroFabrication Plasma Processing Systems R. B. Darling / EE-527 / Winter 2013 Outline • Fundamentals of plasmas • Plasma sheaths and biasing • Glow discharges • Conservation equations • DC plasmas • RF plasmas • Common plasma sources and configurations • Excitation and matching • Applications survey R. B. Darling / EE-527 / Winter 2013 What Are Plasmas? • A gaseous-like state which contains both positively and negatively charged species, in roughly equal numbers. – Even though the particles are charged, the plasma itself is overall neutral. • It does not have a definite size or shape; its extent is determined by its container. • Sometimes called the 4th state of matter. (science over-simplified…) • Most of the matter in the universe exists in a plasma state. • Because it contains charged particles, it responds strongly to EM fields. – It can form filaments, beams, vortices, eddies, curtains, and layers. • It usually involves the ionization of a neutral species into positive ions and negative electrons as part of its excitation process. – Recombination of the excited ions and electrons often produces luminous glows. The aurora or Northern lights are a prime example of a plasma. – The loss of energy through radiation requires a plasma to be continuously excited. R. B. Darling / EE-527 / Winter 2013 The Range of Plasmas -3 25 Plasma Density, n = n = n , cm 10 i e Center of the Sun Crystalline Silicon at room temperature 1020 Laser plasmas High Shock pressure tubes Theta arcs pinches 1015 Nuclear fusion RF Alkali processing Fusion 10 metal plasma 10 plasmas experiments DC glow Flames discharges Earth's 105 ionosphere Solar corona Solar Interplanetary 100 wind Electron Energy, Ee, eV Electron Temperature, Te, K Interstellar Gallactic 10-5 10-2 eV 10-1 eV 100 eV 101 eV 102 eV 103 eV 104 eV 105 eV 102 K 103 K 104 K 105 K 106 K 107 K 108 K 109 K R. B. Darling / EE-527 / Winter 2013 Why Use Plasmas for Microfabrication? • Reactions can be run faster, more localized, and with higher uniformity. – Plasmas can create high excitation energies without high substrate temperatures. – Ion bombardment can conform to the shape of the object. – Plasma processing allows efficient use of source materials. • Highly anisotropic rates can be obtained. – This allows the final feature size to be closer to the mask size. – Deep etches with high aspect ratios can be created. • High levels of process control can be achieved. – Plasmas are electrically excited, and their internal parameters can be directly controlled by the external excitation. • Plasma processing provides good contamination control. – The process is carried out in a vacuum chamber, so no airborne contaminants enter the process. – A cold wall system produces less cross-contamination than a hot wall system. – Yields tend to be very high. R. B. Darling / EE-527 / Winter 2013 Characteristics of a Plasma • Particles: – Neutral atoms or molecules (0) – Positively charged ions (+) – Negatively charged electrons (−) • Typical composition of a microfabrication etching plasma: 16 −3 – Etch gas: ng~10 cm , T~300 K (~25 meV) (P~1 Torr) – Etch products: n~1015 cm−3, T~300 K (~25 meV) Note the rather 14 −3 – Free radicals: n~10 cm , T~300 K (~25 meV) low level of 10 −3 −6 – Plasma ions: ni~10 cm , Ti~300 K (~25 meV) ionization: ~10 . 10 −3 – Plasma electrons: ne~10 cm , Te~10,000 K (~1 eV) – Bombarding ions: n~1010 cm−3, T~1,000,000 K (~100 eV) • Conversion between temperature and energy units: –q/kB = 11604 K/eV. • Conversion between gas pressure and density (at T = 300 K): −3 16 –ng (cm ) = 3.250 x 10 P (Torr) R. B. Darling / EE-527 / Winter 2013 Plasma Sheaths • The interface between a plasma and an electrical conductor will deplete the electron density within a thin layer known as the sheath. • The net positive space charge density within the sheath will cause the plasma to rise to a potential higher than that of the electrode. This potential difference is known as the sheath potential. • Nearly all plasma processing occurs within the sheath. • Ions, electrons, and neutrals leave the plasma at approximately their ideal gas impingement rates. Electrons are retarded by the sheath potential, while positive ions are accelerated by it. • The bombarding ion flux is roughly set by the impingement rate. • The bombarding ion energy is roughly set by the sheath potential. 1/ 2 dNi N kBT P P nkBT 1/ 2 Adt V 2 m 2 mkBT R. B. Darling / EE-527 / Winter 2013 Sheaths for an Unbiased Plasma sheathplasma sheath • Plasmas are attached to any conductive surface by a sheath. • The conductor depletes the sheath of electrons, density giving it a positive ni space charge (ni > ne). n • This space charge e 0 x biases the plasma 0Ls1 L - s2 positively, so that the potential plasma is the most V positive potential in P x the system (VP). 0 0Ls1 L - s2 R. B. Darling / EE-527 / Winter 2013 Sheaths for a DC Biased Plasma sheathplasma sheath • Application of an external bias VB alters VB the potential distribution of the system. • The applied bias is density absorbed entirely by one sheath, while the plasma ni remains at its potential ne 0 x of VP. 0Ls1 L - s2 • Ions bombarding the potential electrodes are V accelerated by the P potential difference 0 x s L - s across the corresponding 0 1 2 L sheath. V B R. B. Darling / EE-527 / Winter 2013 Plasma Oscillations sheathplasma sheath • Unforced oscillations in an unmagnetized plasma: • Also known as Langmuir oscillations. electron cloud displacement = s • The displacement of the electron cloud is s (in the −x direction as particle density shown). no ni • This electron displacement ne creates a surface charge on 0 x opposite sides of the plasma. 0Ls1 L - s2 • The surface charge produces an charge density electric field which acts to qno restore the electron 0 x displacement. 0 s1 L - s2 L • Plasma oscillations result. -qno Ex = -qnos/o R. B. Darling / EE-527 / Winter 2013 Plasma Oscillations • Unforced oscillation in a non-magnetized plasma: • s = displacement of the electrons away from equilibrium where they compensate the positive ions, ni = ne = no. • A surface charge density of ρs = ±qnos is created on opposite sides of the plasma, leading to a restoring electric field Ex = −qnos/εo. • The force equation for the electrons is then 2 2 d s q nos 2 me 2 qEx me pes dt o • The plasma oscillation frequency is: 2 2 2 2 2 2 q no 2 q no p pe pi pe pi ome o M i •fp is typically 1-10 GHz. • Most RF excitation is below fp, so the electrons move instantaneously with the applied field. An exception are ECR systems. R. B. Darling / EE-527 / Winter 2013 AC Excitation of a Plasma • AC excitation allows the use of insulating targets. – This is important for sputtering and etching of dielectric materials. • AC excitation allows more flexible control of the potential distribution by means of capacitive voltage division and the creation of self-biasing electrode potentials. – Higher sheath potentials can be created, leading to higher ion energies incident upon the driven electrode. – Typically, obtain 100s of Volts instead of 10s of Volts. – Most importantly, these can be easily adjusted. • AC excitation frequencies are chosen so that ωpi < ω < ωpe. – The ions are too heavy to respond to this frequency and are effectively stationary. – The electrons are light and respond instantaneously to this frequency. – The electron cloud moves back and forth synchronously with the excitation. – Drive frequencies are usually in the mid-MHz. R. B. Darling / EE-527 / Winter 2013 Electrical Equivalent Circuit for a Plasma Electrode 'a' L sa d sb Da Ca sa Sheath 'a' Ra Plasma Admittance: Lp 1 L d Cp Plasma YP jCP RP jLP Rp 0 A CP Rb d sb Sheath 'b' 1 LP Db Cb LP 2 RP peCP m Electrode 'b' R. B. Darling / EE-527 / Winter 2013 Electrode Asymmetry Effects • The driven electrode is usually smaller than the grounded electrode, since the chamber itself is grounded. • If driven through a blocking capacitor (capacitively coupled), the electrode asymmetry will affect the voltage distribution across the two sheaths, self-biasing the driven electrode to a negative voltage. • With no blocking capacitor, Vbias = Vb − Va = 0. • With a large CB, voltage divides by: blocking V C capacitor a b Vbias Vb Ca CB •C> C , so V < 0. Ca Va sheath 'a' b a bias RF A source C for each sheath C V sheath 'b' s b b 2 1/ 2 Va Ab If s V then Vb Aa R. B. Darling / EE-527 / Winter 2013 DC Glow Discharges • Low pressure gas discharges involve ionization of the gas. • Atomic, ionic, and molecular collisions are an essential part of these. • Glow discharges are a sustainable ionization state, usually characterized by high voltage, low current, and a luminous output caused by recombination and radiative de-excitation of the gas ions. cathode dark space Faraday (Crookes or Hittorf) dark space CATHODE ANODE cathode negative positive anode glow glow glow glow Colors are approximately those of air at 10 millitorr (N2 + O2). R. B. Darling / EE-527 / Winter 2013 I-V Characteristic of a Low Pressure Gas Discharge • Distinguish between glow discharge and arc discharge regions: Current, I ARC DISCHARGE IA GLOW DISCHARGE Voltage, V IT 0 0 BVS BVI BVA R. B. Darling / EE-527 / Winter 2013 Electrical Excitation of a Gas Discharge • Capacitively coupled (primarily electric field) – Use plates for coupling. • Inductively coupled (primarily magnetic field) – Use coils for coupling. • Electromagnetically coupled (TEM wave-heated) – Use antennas for coupling.