Industrial Applications of Plasma

Transcription

1 Industrial Applications of Plasma Features that make plasma useful for industrial applications: Ø It is characterized by a wide range of energy And/or power densities (thermal plasmas in DC arcs, high frequencies and RF inductive plasma torch with power density W/cm 3, corona and luminescence discharge with 10-4 to 1 W/cm 3 ). Ø It produces large numbers of active species of several types, with higher energy than those produced in conventional chemical reactors. 1 Industrial Applications of Plasma Technological Processes: High power density processes: Fusion or evaporation of solid materials Welding Arc furnaces Processing at high temperature of materials Processes with a large number of active species of multiple types and at high energies: Surface treatment of materials Plasma chemistry 2 1

2 Plasma Reactors for Surface Treatments Typical Parameters: Power density: ü Corona discharges: P ~ 10-3 W/cm 3 ü Glow discharges : P ~ W/cm 3 ü Arc discharge: P ~ W/cm 3 Frequency: cc or RF (~ 10 MHz) Voltages: 0.01 kv in corona d., 5-20 kv s. glow d. Gases: air at atmospheric pressure and other gases (production of ozone, NO x, N, A, He) Exposure time: ms - min. Temperature: within reaction ranges. 3 Active Species Photons ü Infrared: ε < 1.6 ev low effectiveness ü Visible: 1.6 < ε < 3.3 ev - break chemical bonds, excite; ü UV: 3.3 < ε < 95 ev - break long molecules of hydrocarbons, ionize and excite). Neutrals - Free radicals (high energy particles chemically active - atoms: O, H, F, Cl, etc., monomers and molecular fragments: CH 2, etc.. They can produce a considerable amount of chemical reactions and energy processes). Charge particles (electrons, positive ions, and negative ions that can be accelerated by EM fields): electrons and ions accelerated by he electric field (electrons transmit momentum and energy much less than ions. 4 2

3 Main plasma surface processes ü Cleaning, de-grease, and change of surface characteristics (hydro absorbance and hydro repellence, surface electrical conductivity, cohesion and adhesion, etc.) ü Treatment of solids with ion implantation (doping, enhancement of the mechanical strength) ü Deposition of thin layers by means of plasma (microelectronics and more) ü Plasma etching for micro-electronics 5 Change of surface energy The molecules of a liquid attract each other isotropic ally in all directions. Those on a surface are just attracted toward the directions occupied by liquid. The forces due to surface tension make the liquid contract which tends to the spherical shape typical of the drop. The surface tension γ is defined as a force per unit of length that keeps the surface of the liquid flat: γ [Newton/m; dine/cm] or [Joule/m 2 ] 6 3

4 Liquid on a solid surface γ LV : surface tension between liquid and steam γ SL : surface tension between solid and liquid γ SV : surface tension between solid and steam θ: contact angle γ SV - γ SL = γ LV cosθ Ø (γ SV - γ SL ) > 0: hydro absorbent surface (wettable); Ø (γ SV - γ SL ) < 0: hydro repellent surface (waterproof). θ = 0 : The adhesion and the cohesion forces are equal; θ > 90 : the material is hydro repellent. 7 Hydro Absorbance (Wettable) and Hydro Repellence (Waterproof) Waterproof material: - Hydrophobic - Large contact angles (θ> 90 ) - Low surface energ. (< N/m) Wettable material: - Hydrophilic - Small contact angles (θ < 20 ) - High surface energ. (> N/m) 4

5 Hydro Absorbance (Wettable material) Ø To increase the hydro absorbance of a material the surface tension must be remodulated (increase surface tension between solid and steam by increasing the surface energy). Ø In order to do this, dirt, coated films or monolayer must be removed. 9 Plasma Cleaning (Surface plasma cleaning) Ø De-grease: a few hundred mono-layers of gas, present on above surface, adhere to the surface. The layers closest to the surface are bounded by energies of about 4-5 ev. It is very difficult to remove such layers chemically or by heating, requiring temperatures above the melting temperatures of the material. Instead, small quantities of energy are required only directed to superficial layers. This is thus achieved by means of collisions with high energy particles. Ø Removal of thin layers of hydrocarbons and oils can be achieved by means of low pressure oxygen plasmas; 10 5

6 Plasma Sterilization Sanification Survival curve for 50,000 micro-organisms exposed to a discharge in air at 1 atm. 11 Plasma Etching for micro-electronics In 1947 J. Bardeen and W.H. Brattain (Bell Lab.) invented the transistor (A). A In 1958, the first chip, micro-electronic solid state device that contains several circuit elements,, was realized by J. St. Clair Kilby (B, B '). In 1961 the first completely monolithic chip (C) was produced. The realization of a multiple component monolithic circuit (integrated circuit), has allowed to reduce size of electronic element. This has led to the rapid progress of Electrical Technology. B B 12 C 6

7 Characteristic Dimensions in Micro- Electronics Dimensioni Elemento Microns, µ Amstrongs, Å Polvere Batterio Strato per circuito microelettronico Dimensione di un circuito con grande configurazione (regole di progetto) Dimensione di un circuito con piccola configurazione (regole di progetto) Lunghezza d onda del visibile Monostrato <111> di Silicio Diametro dell atomo dell Argon Diametro dell atomo dell Ossigeno The design dimension or design rule (δ) is the typical size of the smallest active electronic element (in the case of figure: δ ~ 0.5 µm). 13 Development of Technology Chip production has been firstly based on etching by meas of chemical baths (wet chemical etching). In the early 1970s in the industry the etching of silicon made by means of plasmas of RF low pressure discharges (dry plasma etching.) When in the early 80s the size of the component reached 2 µm the plasma etching almost completely replaced the chemical etching. 14 7

8 Active Species Plasma Etching Etching on a SiO 2 surface using a beam of argon ions at 450 ev and a current of 2.5 µa on a surface of 0.1 cm 2 in a bi-fluoride of Xenon (XeF 2 ). 15 Chemical Etching and Plasma Etching Etching in Chemical Baths : The etching is obtained isotropically: the dimension of the layer of material removed horizontally beneath the mask is the same of that as vertically removed. Therefore, the minimum horizontal dimensions of the mask must be at least twice the size of the layer to be removed. Dry Plasma Engraving: Anisotropic etching obtained by plasma techniques (RF glow discharge within CF 4 inert gas at p < 1 torr). CF 4 is decomposed into CF 3 and F that attack the silicon producing SiF 4 volatile. This attack is anisotropic and selective (only the layer that is intended to be engraved is chemically attached and not the mask and the substrate below the mask. 16 8

9 Plasma Etching Technology Plasma etching uses relatively inert molecular gas (carbon tetrafluoride CF 4 ). This gas interacts with the plasma of an RF glow discharge to produce active species able to react chemically with the layer to be etched. The CF 4 molecule is dissociated in F and CF 3, both chemically highly reactive with silicon. Reaction products (SiF 4 ) must be volatile so they can leave the etched layer and be pumped away from the vacuum system. CF 4 neutral gas does not chemically react with silicon and does not attack the side walls of the etched channel. Interaction is facilitated by the collisions with energy carrying particles (ions, electrons and photons) and promoters of surface reactions. This mechanism guarantees directional etching. 17 Advantages and Disadvantages of Plasma Etching Chemical baths based etching has produced fluid deposits from the highly toxic chemical reactions required. Only in Silicon Valley there are more than 150 toxic waste sites from chemical etching. The remediation of an IBM site (San Jose's underground deposit) is estimated to cost more than $ 100,000,000. Ø Advantages of plasma etching: It is highly directional with large precisions. It is a clean process with very low waste production. It requires very low chemical consumption. It has as limiting factor for the technology the mask realization and not the etching technology. Ø Disadvantages of plasma etching: Requires vacuum systems made using chemically resistant materials. For dimensions of the order of 1 µ, pressures of 0.1 to 1 torr are required. For dimensions below 1 µ, the pressure must be below 10 mtorr. Therefore, a sophisticated vacuum technology is required. 18 9

10 Anisotropic Etching Etching catalysed by plasmas (plasma etching): Plasma is used to produce ions or other active particles (electrons, photons). No ion bundle of an external source is used. Plasma ions are accelerated by the sheath voltage of 10 to 100 volts at energies of ev. The ion energy must not exceed 50 ev to avoid damaging the surface. Etching catalysed by electronic beams: An electron beam at 1500 ev for I = 45 µa catalyses reactions between XeF 2 and SiO 2 gas, etching the silicon oxide (etching time rate ~ 200 Å/min.). There is no etching without XeF 2 within the electronic beam. Etching catalysed by a bundle of photons: A bundle of photons produced by a laser can produce etching. There is not much difference between etching catalysed by ions, electrons or a laser beam. 19 Surface bombardment Interactions with active energy particles involve more states of matter and give rise to the following processes: Secondary Electronic Emission: The bombardment of a surface with active plasma species can lead to electron emission; Sputtering: ions or neutrals hit atoms or surface molecules with detaching of them from the solid surface; Erosion: a massive sputtering removes a consistent layer of the surface; Plasma-cleaning due to plasma-surface interaction: layers to the surface are removed for bombardment with active species

11 Plasma Sputtering Sputtering consists in the release of atoms and surface molecules due to ion bombardment (the cumulative effect of sputtering, which leads to the removal of a thick surface layer, determines surface erosion). Electrons, due to their small mass, and neutrals, as they are not sufficiently energetic, do not produce sputtering for industrial applications. For sputtering, energetic ions are used, accelerated by high electric fields. Sputtering coefficient or sputtering yield is defined by γ = Emitted number of atoms and molecules Number of incident particles γ depends on E i, A i, Z i (energy, atomic weight and atomic number) of the incident particle, incidence angle, atomic weight of the surface particle, its crystalline nature. 21 Plasma Sputtering Typical dependence behaviour of the sputtering yield on the kinetic energy of the incident ion. Sputtering yield as a function of the kinetic energy of hydrogen, deuterium, helium, and nickel ions on nickel surface

12 Erosion The cumulative effect of sputtering, leading to the removal of a consistent surface layer, results in erosion. Industrial process erosion is made with ions. Sputtering flux density: Γ s = γ Γ i = γ J i /e [released atoms/(m 2 s)] Erosion speed: v ε = Γ s /n w = γ Γ i /n w = γ J i /(en w ) [m/s ] (n w wall particle density) Time needed for the erosion of a thickness L: T = L/(3600 v ε ) [ore] 23 Ion implantation Ion implantation consists of bombarding solid surfaces with sufficiently energetic ions ( kev) which penetrate the structure and stop several atomic layers beneath the surface. Some ions run along channelled pathways through the crystalline solid structure (usually channelled trajectories have to be avoided). Most ions are scattered immediately below with the surface and go inside the material along non-channel trajectories. Usually it is necessary to avoid the sputtering material or at least the released material to be less than the implanted one

13 Ion implantation Channelled pathways 25 Ion implantation Methods to avoid channelled pathways 26 13

14 Ion implantation Crystalline lattice The dimensions of the channel are indicated by the dotted line. The figure also shows the characteristic dimensions of some ions. 27 Ion implantation Applications Ø Ø Ø Ø Ø Doping of semiconductor in micro-electronics (first wide application); Increase in hardness of metals; Increase wear resistance (metals and ceramics in the aerospace industry and in the medical industry); Increased corrosion resistance; Changes in surface electrical and optical properties Dipartimento Department of Electrical, di Ingegneria Electronic, dell Energia and Information Elettrica Engineering e dell Informazione (DEI) - University of Bologna - DEI 28 14

15 Ion implantation Characteristics Dose: / cm 2 inhibits corrosion; / cm 2 increases the hardness and wearresistance. Ion Energy: kev Depth of implantation: µ inhibits corrosion; - 1 µ increases the hardness and the wear resistance. Fig Ø The carbon and nitrogen implantations increase corrosion resistance, hardness and wear. For the wear it would require 2 µ implantation depth that is obtained with energies of about 1 MeV (difficult to get). Indeed, ev are sufficient to provide implantation depths of about 0.1 µm as the ions migrate toward intimate layers during wear. Thin Film Deposition Applications Ø Glass reworking to vary the reflection. Ø For ornamental and adhesion characteristics in food plastic. Ø For the layered processes in multilayer microelectronic circuits

16 Thin Film Deposition Micro-electronics Compliant recoat: It is obtained by transport after deposition and the diffusion by means the surface tension forces. Unidirectional Coating: It is obtained by means of a ion beam. Isotropic coating: It is obtained with a gas deposition: tan θ = w/h 31 Thin Film Deposition Micro-electronics Electronic microscope pictures of a deposit executed by a plasma with SiO x (a) and ) SiN x (b), the first at 200 C, the second at 330 C. In (c) a theoretical result is shown

17 Integrated circuit schematic Layer Structure Fig CMS circuit schematic Fig Section diagram of a CMS circuit showing two layers connected by means of the "Via" junction

18 5 layer circuit schematic The design dimension (or project rule) d is the typical size of the smallest active electronic element. Current technology is characterized by a minimum dimension of 0.10 µm and integrated circuits with at least 5-7 layers. Fig Electron microscope scan (SEM) of a five-layer metal circuit (M1-M5) with tungsten junctions between the layers (W1-W3). 35 Multilayer circuit schematic 36 18