Etching

Etching Etching - Selective removal of unwanted materials. Goal Primary goal : a exact transfer of the image from the mask into the wafer surface Removing surface damage Cleaning the surface to remove contamination Fabricating 3-dimensional structures. Oxide etching Al etching

Definition of Terms Etching process are characterized by three parameters: Etch rate : film thickness of removed per unit time. Etch selectivity : the ratio of etch rate of the material to be removed to the etch rate of another material. It is as large as possible : 25-50 are usually considered reasonable. Etch uniformity

- Isotropic etching : no spatial orientation dependence Anisotropy - Isotropic etching : no spatial orientation dependence - Anisotropic etching : directional etching The degree of anisotropy Lateral etch ratio : (a) Isotropic etching : RL=1 (b) Anisotropic etching : 0<RL<1 (c) Completely anisotropic (directional etching) : RL=0 More directional etching → (a) (b) (c)

Undercut and Over etch Lateral etching at the top and bottom interfaces of the etched layer when a mask is used to pattern the film.

Wet etching and Dry etching Wet etching - wet chemical etching Plasma-based dry etching - Isotropic plasma etching - Reactive ion etching (RIE) - Deep reactive ion etching (DRIE) Non-plasma based dry etching - Isotropic gas etching Gas XeF2, BrF3 Radical F Ion Ga, Ar, Cl, Fluoro-carbon Atom Ar

Wet Chemical Etching

Wet Chemical Etching Basic Mechanisms of Etching Etching proceeds by three essential steps: The reactants diffuse through boundary layer to surface of wafer. Chemical reactions take place at the surface. Reaction products are transported away from the surface by diffusion.

Common wet chemical etchants for various thin films used in IC fabrication.

Wet Etching of SiO2

Temperature dependence of oxide etch rate in buffered hydrofluoric acid.

Wet Etching of Aluminum Al etches in water, phosphoric, nitride and acetic acid mixtures. Converts Al to Al2O3 with nitric acid (evolves H2). Dissolve Al2O3 in phosphoric acid. Gas evolution leading to bubbles. Local etch rate goes down where bubble is formed - Non-uniformity

Mask Erosion

The Limits of Wet Etching - Wet etching is limited to pattern sizes of 3 um. - Wet etching is isotropic, resulting in sloped sidewalls. - A wet process requires rinse and dry steps. - The wet chemicals are hazardous and/or toxic. - Wet processes represents a contamination potential. Failure of the resist-wafer bond causes undercutting. Wet etching was used exclusively till 1970’s. Etch bias : bad for small scale features.

Dry Etching

Dry Etching Dry Etching Dry etching is a generic term that refers to the etching techniques in which gases are the primary etch medium, and the wafers are etched without wet chemicals or rinsing. Type of Dry Etching Technology Plasma based dry etching RF power is used to drive chemical reactions. - There are three dry etching techniques: >> Plasma etching : chemical etching >> Ion beam milling (etching) : physical etching >> Reactive ion etching (RIE) : chemical etching + physical etching Non-plasma based dry etching (Vapor Phase Etching) Isotropic etching of Si Typically fluorine-containing gases (fluorides or interhalogens) that readily etch Si High selectivity to masking layers No need for plasma processing equipment Highly controllable via temperature and partial pressure of reactants

Plasma-Based Dry Etching Working Principle in Detail The 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. Maintained at 1 Pa (75 mtorr) to 750 Pa (56 torr) with gas density of 27 x 1014 to 2 x 1017 molecules/cm3 Positive ions are accelerated towards the cathode due to the electrical field. High energetic ions hit the substrate and transfers the momentum to the solid particles which are sputtered. VT : voltage across target sheath Vs : voltage across substrate sheath AT : area of target electrode As : area of substrate electrode

Dissociation by Electron Impact Plasma-Based Dry Etching Dissociation by electron impact Dissociation by Electron Impact Ionization by electron impact Excitation by electron impact

Plasma-Based Dry Etching Radicals They are generated at a higher rate than ions > Lower threshold energy > Ionization is often dissociative Radicals have longer lifetime than ions Nature of radicals depends on residence time in plasma (they can be altered in plasma environment)

Plasma-Based Dry Etching Plasma etching : Chemical etching Plasma etching, like wet etching, is chemical process but uses gases and plasma energy to cause the chemical reaction. A glow discharge plasma (usually RF) is used to generate reactive species, e.g. atoms, radicals and ions, from parent gases. Barrel plasma etch - Ashing Planar plasma etch

Plasma-Based Dry Etching These reactive species diffuse to the surface to be etched, form volatile products which then desorb from the surface and are pumped away. Processes involved in chemical etching during plasma etch process.

Plasma-Based Dry Etching Ex. CF4(g) gives F(g) So CF4(g) can etch Si Adding O2 enhances Si etch : O2 combines with CF3, CF2 reducing their recombination with F. But too much O2 oxidizes Si

Plasma-Based Dry Etching Even though free radicals are highly reactive, multiple steps required result in low effective sticking coefficients, S~0.01. Isotropic arrival angle distribution and low sticking coefficient : These two properties of the reactive neutral species in plasma system result in the fact that purely chemical etching acts in as isotropic or near-isotropic manner. Benefit remains selectivity.

Plasma-Based Dry Etching Advantages - high etch rate, - good selectivity, - low ion- bombardment- induced damage. Disadvantages - tendency for isotropic etching - poor pattern transfer. Nowadays plasma etching is mainly used for removing of the photoresist using O2-based plasmas (a process called ashing).

Plasma-Based Dry Etching Physical etching Process of etching through physical interactions (momentum transfer) between accelerated chemically inert ions (e.g. Ar+) and etched solid. >> Plasma sputtering >> Ion milling Chemical Physical

Plasma-Based Dry Etching Reactive ion etching (RIE) (Chemical reaction + ion bombardment) RIE systems combine the purely physical character of ion beam etching with the chemical nature of plasma etching. Ion-enhanced chemical etching Physical and chemical processes not just independent of each other. Ion beam can enhance chemical etching : Further, the profile is highly aniostropic. - Ex.

Plasma-Based Dry Etching RIE is now one of the most commonly used methods for etching semiconductor materials. When the wafers are placed directly on the RF powered electrode, they are in direct contact with the plasma, and therefore exposed not only to reactants, but also to energetic ions. Asymmetric RIE system: Wafers are mounted on powered electrode.

Plasma-Based Dry Etching Reactive ions (e.g. CF3+, CCl3+) produced in a discharge are accelerated onto the wafer surface at high energies. In this case the wafer is placed onto the powered electrode, and high KEs are maintained by the use of low pressures (tens of mTorr). Formation of the plasma sheath and resulting ion bombardment.

Plasma-Based Dry Etching Anisotropic Etch Mechanisms (a) Perpendicular ion bombardment creates a damaged surface that is then more reactive toward neutral etchants. Ion breaks bonds, render XeF2 more reactive. (b) Sidewall passivation (Inhibitor-driven anisotropy) : Ions helps to desorb etch- inhibiting species, such as etch products, from the surface. Ions increases formation of volatile byproducts. Ion beam may sputter away byproducts.

Plasma-Based Dry Etching Fluorine-containing plasma - CF4 is inert - By electron impact (dissociation ionization) (impact dissociation)

Plasma-Based Dry Etching Chlorine-containing plasma (Ex 1) Poly-Si etch - Ions, radicals, and electron generations : - Etchant formation : - Adsorption of etchant on poly-Si : Ion-bombardment-assisted reaction to form product : Product desorption :

Plasma-Based Dry Etching (Ex 2) Al etch Chlorine(Cl)-containing reactants are used (e.g. BCl3), since the etch product, AlCl3, is volatile under the low pressure process conditions. Fluorinated gases cannot be used for etching Al since AlF3 is a hard, crystalline material nonvolatile. - Higher vapor pressure → higher volatility Cl2 as etching gas.

Plasma-Based Dry Etching Plasma Etching Selectivity

Non-Plasma based Dry Etching Non-plasma based dry etching (Vapor Phase Etching) Isotropic silicon etchants utilizes a gas (XeF2, BrF3) that has high selectivity. That is, it reacts well with the desired species, but does not react with mask material. XeF2 Etching Simple setup (No external energy input such as plasma nor heating are required.) Basic reaction Xenon difluoride (XeF2)

Non-Plasma based Dry Etching XeF2 shows a very high selectivity of silicon versus photoresist, SiO2, silicon nitride, Al, Cr, and TiN. > XeF2 etch selectivity to silicon nitride > 100:1 > XeF2 etch selectivity to silicon dioxide > 10,000:1. Can be the very last step Fully CMOS compatible Post processing for standard CMOS

Non-Plasma based Dry Etching Fold-up structures with conducting Al hinges Suspended and 3D structures

Comparison