Disadvantages: 1. Some gases are quite toxic and corrosive 2. Re-deposition of non-volatile compounds 3. Need for specialized (expensive) equipment

Transcription

1 Dry Etching Advantages: 1. Eliminates handling of dangerous acids and solvents 2. Uses small amounts of chemicals 3. Isotropic or anisotropic etch profiles 4. Less undercutting 5. No unintentional prolongation of etching 6. Better process control 7. Ease of automation (e.g., cassette loading) Disadvantages: 1. Some gases are quite toxic and corrosive 2. Re-deposition of non-volatile compounds 3. Need for specialized (expensive) equipment Types: 1. Non-plasma based = uses spontaneous reaction of appropriate reactive gas mixture 2. Plasma based = uses radio frequency (RF) power to drive chemical reaction

2 Non-plasma Based Dry Etching Isotropic etching of Si Typically fluorine-containing gases (fluorides or interhalogens) that readily etch Si High selectivity to masking layers Non need for plasma processing equipment Highly controllable via temperature and partial pressure of reactants I. Xenon Difluoride (XeF 2 ) Etching: Isotropic etching of Si High selectivity for Al, SiO 2, Si 3 N 4, PR, PSG 2XeF 2 + Si 2Xe + SiF 4 Typical etch rates of 1 to 3 µm/min Heat is generated during exothermic reaction XeF 2 reacts with water (or vapor) to form HF II. Interhalogen (BrF 3 and ClF 3 ) Etching: Nearly isotropic profile Gases react with Si to form SiF 4 Surface roughness: ~40 to 150 nm Masks: SiO 2, Si 3 N 4, PR, Al, Cu, Au, and Ni

3 Plasma Based Dry Etching RF power is used to drive chemical reactions Plasma takes place of elevated temperatures or very reactive chemicals Types: 1. physical etching 2. chemical etching 3. reactive ion etching (RIE) 4. deep reactive ion etching (DRIE) Plasma = partially ionized gas consisting of equal numbers of + (ions) and - (electrons) charges and a different number of neutral (un-ionized) molecules. An ion-electron pair is continuously created by ionization and destroyed by recombination

4 Typical kinetic energy (KE) of an electron in plasma is 2-8 ev KE = ½ mv 2 = 3 / 2 kt m = particle mass V = particle mean velocity k = Boltzmann constant T = temperature (K) 2 ev electron has T 15,000 K and V 6 x 10 7 cm/s = 1,342, mph

5 Plasma Formation: 1. chamber is evacuated 2. chamber is filled with gas(es) 3. RF energy is applied to a pair of electrodes 4. Applied energy accelerates electrons increasing kinetic energy 5. Electrons collide with neutral gas molecules, forming ions and more electrons 6. Steady state is reached (plasma); ionization = recombination Plasma discharge is characterized by central glow or bulk region and dark or sheath regions near electrodes Bulk region = semi-neutral (nearly equal number of electrons and ions) Sheath regions = nearly all of the potential drop; accelerates + ions from bulk region which bombard the substrate Maintained at 1 Pa (7.5 mtorr) to 750 Pa (5.6 torr) with gas density of 2.7 x to 2 x molecules/cm 3

6 I. Physical Etching (Sputter Etching): Based on physical bombardment with ions or atoms Plasma is used to energize a chemically inert projectile so that it moves at high velocity when it strikes the substrate Momentum is transferred during the collision Substrate atoms are dislodged if projectile energy exceeds bonding energy Very similar to ion implantation, but lowenergy ions are used to avoid implantation damage Highly anisotropic Etch rates for most materials are comparable (i.e., no masking) Argon is the most commonly used ion source

7 II. Chemical (Plasma) Etching: Plasma is used to produce chemically reactive species (atoms, radicals, and ions) from inert molecular gas Six major steps: 1. generation of reactive species (e.g., free radicals) 2. diffusion to surface 3. adsorption on surface 4. chemical reaction 5. desorption of by-products 6. diffusion into bulk gas production of gaseous by-products is extremely important

8 III. Reactive Ion Etching (RIE) RIE = process in which chemical etching is accompanied by ionic bombardment (i.e. ionassisted etching) Ionic bombardment: No undercutting since side-walls are not exposed Greatly increased etch rate Structural degradation Lower selectivity IV. Deep Reactive Ion Etching (DRIE) Uses electron cyclotron resonance (ECR) source to supplement RIE system Microwave power at 2.45 GHz is coupled into ECR Magnetic field is used to enhance transfer of microwave energy to resonating electrons DRIE uses lower energy ions less damage and higher selectivity Plasma maintained at 0.5 to 3 mtorr

9 Etch Chemistries: A. Organic Films Oxygen plasma is required By-products: CO, CO 2, H 2 O Masks: Si, SiO 2, Al, or Ti Addition of fluorine containing gases significantly increases etch rate but decreases selectivity (due to HF formation) B. Oxide and Nitride Films Fluorine plasma is required (e.g., CF 4 ) Mask: PR Addition of O 2 1) increases etch rate 2) adjusts PR : oxide and PR : nitride selectivity C. Silicon 1. Fluorine plasma (CF 4 or SF 6 ) 2. Chlorine plasma (Cl 2 ) 3. Mixed (fluorine and chlorine) plasma (Cl 2 + SF 6 )