Introduction to Plasma

Transcription

1 What is a plasma? The fourth state of matter A partially ionized gas How is a plasma created? Energy must be added to a gas in the form of: Heat: Temperatures must be in excess of 4000 O C Radiation Electric Field Magnetic Field Examples of Common Plasmas: Stars are almost entirely composed of plasma this makes plasma the dominant state of matter in the universe Aurora Borealis (Northern Lights) Lightning Plasma - 1

2 In practice, a plasma is a gaseous medium containing: neutral gas atoms or molecules ions (atoms with a charge) free radicals (highly reactive molecules) electrons photons (massless particles of light) The net charge is zero: # of positively charged particles = # of negatively charged particles The Relative number of charged species is very low: ~ 1,000,000 neutral particles for every charged particle (ion) Plasma - 2 Neutral Atoms or molecules Positive ions Negative electrons Photons

3 Glow Discharge: A plasma is identified by a visible glow. The color of the glow is dependent on the gasses present in the chamber. Plasma - 3

4 Why Plasma? Light Generation: Fluorescent light bulbs, Neon lights. Low Temperature Chemical Reactions Creation of Unique Materials that could not be accomplished via ordinary chemical means. Accelerated Chemical Reactions for greater throughput processing. Highly Directional Processes: anisotropic etching. More Efficient Energy Utilization Less Waste Product Generation Industrial Applications: Aerospace industry Communications industry Public utilities Plasma - 4 Wide variety of manufacturing industries

5 Plasma Formation: 1. begins with neutral gas particles (atoms or molecules) 2. there are also some free electrons present the presence of heat energy generates more free electrons 3. an electric field is introduced that accelerates the free electrons 4. the accelerated free electrons collide with neutral gas molecules 5. following the collision, one of three things can happen: a. dissociation b. ionization c. excitation Plasma - 5

6 Dissociation: (plasma chemistry) Gas molecules are broken down into smaller fragments called Free Radicals : M + e - => M 1 + M 2 + e - Free radicals are high-energy chemical species. Although they are electrically neutral, they are unstable. They readily react with other substances in order to achieve a more stable configuration. Plasma - 6

7 Dissociation: In the example shown, the free radical Cl is generated. The stable state of chlorine is Cl 2. Since Cl (by itself) is unstable, it readily reacts with aluminum as follows: Al (s) + 3Cl (g) AlCl 3 (g) This is an aluminum etch process. Plasma - 7

8 Dissociation: The reverse process of Dissociation is called Recombination. If they are not used up in other chemical reactions, the free radicals will spontaneously recombine to resume their more stable states: CCl 3 + Cl CCl 4 Free Radicals are the useful products of a plasma that are used in: Plasma Enhanced Chemical Vapor Deposition (PECVD) outcome of the chemical reaction is a solid Plasma Etching and Plasma Cleaning outcome of the chemical reaction is a vapor Plasma - 8

9 Ionization: Electron(s) are knocked loose from a neutral atom or molecule M + e - => M + + 2e - The resultant positively charged particles are called ions. Since they carry an electric charge, ions can be manipulated by an electric field. Plasma - 9

10 Ionization: As an example, in a typical sputtering operation, Argon (Ar) gas is introduced into a vacuum chamber. A plasma is then ignited. The ionization process that occurs is described by the equation: e - + Ar 2e - + Ar + The positive argon ions are attracted to the negatively charged target in a sputtering system. When they strike the target, a vapor is created that deposits on the wafer. Plasma - 10

11 Impact Ionization: In the equation on the previous slide, it should be noted that there are now TWO free electrons, following the collision. This doubles the available electrons for ionization. This ongoing doubling process is called "impact ionization. Plasma - 11

12 Ionization: The reverse process of Ionization is also called Recombination. If they are not absorbed by the wafer or target, the ions will spontaneously recombine to become neutral atoms again. e - + Ar + Ar Ions are the useful products of a plasma that are used in: Sputter Deposition Reactive Ion (highly directional) Etching Ion Implantation Plasma - 12

13 Excitation: Following electron impact, the molecules hold together, but they absorb energy and enter an excited state. Valence electrons are bumped up to a higher energy level (shell). After a few nanoseconds, these excited electrons relax back to the valence band. This is called Relaxation. The additional energy acquired is dumped, and a photon of light is emitted. This is what gives a plasma its glow. Plasma - 13

14 Glow Discharge Color: Different gasses, when excited, will glow with different colors. Nitrogen glows purple, Helium glows blue, Sodium glows yellow, Boron glows green, Neon glows red. (neon lamps!) The color of the glow (the wavelength) is related to the energy lost during relaxation by: E = hc / E is the photon energy in Joules h is Planck's constant (6.6 x Joule-seconds) c is the speed of light (3 x 10 8 meters per second) is the wavelength in meters This property enables the use of spectral analysis to obtain detailed information about the nature of the gasses that make up the plasma. Example: Endpoint detection in a plasma etch process. Plasma - 14

15 Semiconductor Applications of Plasma Etching: W et Etch only features > 3 isotropic (sloped walls) more contamination issues greater resist lifting (undercut) environmental impact D ry (Plasma) Etch submicron features anisotropic (straight walls) less contamination issues less resist lifting (less undercut) lower environmental impact endpoint detection capability Plasma - 15

16 Semiconductor Applications of Plasma Chemical Vapor Deposition: Low Pressure CVD vs. Plasma Enhanced CVD high temperature reaction lower temperature reactions slower deposition rate faster deposition rate more hazardous gasses less hazardous gasses deposition on chamber walls less deposition on chamber walls Plasma - 16

17 Semiconductor Applications of Plasma Metal Deposition: Evaporation vs. Sputtering point-like source planar source (imp. step coverage) difficult to do alloys easy to do alloys small/many grain structures large/fewer grain structures electromigration issues fewer electromigration issues poor adhesion improved adhesion Plasma - 17

18 Semiconductor Applications of Plasma D oping: Thermal Diffusionvs. very high temperature difficult to control Ion Implantation low temperature very controllable only one basic doping profile custom (retrograde) doping profiles Plasma - 18

19 Plasma Ignition: Plasmas can be generated wherever the following conditions exist: Power: an electric field (AC or DC) of sufficient energy to accelerate electrons to begin impact ionization. Pressure: sufficient molecular density (pressure) to provide an adequate number of collisions, but... low enough molecular density to create a sufficient mean free path that allows particles to accelerate before colliding. Therefore, there is a pressure sweet spot at which point conditions are optimal for striking a plasma. The relationship between pressure and ignition power (voltage) is called the Paschen Curve. Plasma - 19

20 Plasma Ignition: Sweet Spot Plasma - 20

21 Plasma Ignition: Additional Factors that Affect Plasma Ignition: ionization potential of the gas (some gasses are harder to ionize than others) AC power frequency temperature chamber and electrode geometries Plasma - 21

22 Plasma Ignition: Ignition Process: (voltage vs. current curve) Region 1 - Initial Increase: Voltage is increased to the ignition voltage. Up to this point, there is little detectable current. Region 2 - Negative Impedance: Once ignition occurs, electrons avalanche (similar to a short circuit). Voltage decreases rapidly while current continues to increase. This sends energy back into the RF generator and RF match. RF components must be designed to handle this energy spike. Region 3 - Steady State: Plasma reaches steady state. Increase in voltage yields linear increase in ion current. Plasma grows brighter. Plasma - 22

23 Plasma Ignition: Voltage and Current Characteristics: Plasma - 23

24 DC Plasmas: A DC (constant) voltage is applied to two parallel plates (or electrodes: cathode (negative polarity) and anode (positive polarity) The glow discharge divides itself into the following regions between the cathode (-) and the anode (+): Plasma - 24

25 DC Plasmas Regions: Glow Regions: multiple ionizations/recombinations and excitations/relaxations cause the bright glow color of the glow is characteristic of the gas being used note: relatively little work (i.e. etching) is accomplished in the glow regions Dark Spaces: large voltage drops cause ions to travel rapidly little recombination occurs, hence no glow most of the work is accomplished in these areas Frequently the narrow glow regions known as the cathode glow and the anode glow are very faint, so the DC plasma appears to consist of only three primary regions (dark, glow, dark). Plasma - 25

26 Sheath Formation in a DC Plasmas: If an electrically insulated object (such as a wafer) is placed within a plasma, it will begin to build up a negative charge. This is due to the higher velocity of electrons vs. ions (~1000x). The negative charge will act as a repelling force to any additional incoming electrons. This will create a positively charged space around the object known as a dark space sheath. Plasma - 26

27 Arcing in a Plasma: An arc is NOT a glow discharge. An arc IS a low-resistance breakdown of the dielectric space between the two electrodes. It occurs at high pressure and high voltage. The electrode surface partially vaporizes, and the metal vapor becomes the main conductor. An arc will divert current from the glow discharge, nullifying any of the desired results of the plasma. Whenever an arc is observed in a plasma system, the system should be shut down immediately! Sodium vapor and mercury vapor lamps are examples of USEFUL arc discharges. Plasma - 27