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					         ME 463
Micro – nanotechnology

        Lecture 3
         Etching

     IOANA VOICULESCU
        Etching
   Subtractive Process - removing materials

   Wet etching - liquid etchants:
       Acids
       Hydroxides


   Dry etching - etching gases / plasma:
       Physical etching (impact of atoms / ions)
       Reactive chemicals / ions
       Enhanced by RF energy (instead of temperature)
         Bulk Micromachining
   Types of bulk
    etching
       Anisotropic and
        Isotropic etching
Bulk Micromachining examples
    Isotropic Vs. Anisotropic Etching
Isotropic etching
  – Same etch rate in all directions
  – Lateral etch rate is about the same as vertical etch rate
  – Etch rate does not depend upon the orientation of the mask edge

Anisotropic etching
  Etch rate depends upon orientation to crystalline planes
  – Lateral etch rate can be much larger or smaller than vertical etch
  rate, depending upon orientation of mask edge to crystalline axes
  – Orientation of mask edge and the details of the mask pattern
determine the final etched shape
  • Can be very useful for making complex shapes
  • Can be very surprising if not carefully thought out
  • Only certain “standard” shapes are routinely used
        Isotropic Vs. Anisotropic Etching


   Isotropic – same etch rate in all directions
       Etch depth / surface uniformity problems
       Diffusion dependent
      Etch silicon wafer and deposited films
   Anisotropic – crystal plane dependent
       Etch ratios more than 100:1
       silicon wafers only                                <111>
       Crystal planes:                            54.7º   <100> Si
           Wet Vs. Dry Etching
                                       Dry etch
                                           Reactive ion etching (RIE)
                                           High – cost
   Wet etch                               Very “directional”
       Chemical reaction                  “Unlimited” 2D geometries
       Low – cost                         0.1 – 10 microns/min

       Hard to control direction
       Limited 2D geometries
       1 – 10s of microns/min
Wet etching set-up




   Experimental set-up for anisotropic wet etching.
        Anisotropic Etching
   Typically, etch rates are: (110) > (100) > (111).

   The (111) family of crystallographic planes are normally
    the “stop” planes for anisotropic etching.

   There are 8 (111) planes along the ± x ± y ± z unit
    vectors.

   Intersections of these planes with planar bottoms produce
    the standard anisotropic etching structures for (100) Si
    wafers:
       V-grooves
       pyramidal pits
       pyramidal cavities
Anisotropic Etching
                       <100>
          <111>

   54.7

           Silicon Substrate
Anisotropic Etch Examples




   Typical etch pit on (100) wafer
      Etching Topologies
 Anisotropic wet etching: (100)    Isotropic wet etching: Agitation




Anisotropic wet etching: (110)    Isotropic wet etching: No Agitation
        Concave & Convex Corners
   Anisotropic wet
    etching
       Convex corners are
        undercut
       Concave corners stop
        at (111) intersections.
      Anisotropic Etchants
   Alkali hydroxides (KOH, NaOH, etc.): very smooth
    walls, can use isopropyl alcohol to increase selectivity
    (111) vs. (100), attacks aluminium, oxide etches
    somewhat, nitride good mask.

   Ethylene Diamine Pyrochatechol (EDP): similar to
    KOH but much more toxic, does not attack metals
    (even Al in some cases) nor oxide.

   Tetramethyl Ammonium Hydroxide (TMAH): similar
    to EDP but safer, in some cases will not attack Al,
    can be masked with oxide.
Under cutting
    Isotropic Etching of Silicon

    HNA=Hydrofluoric acid (HF)+ Nitric acid
    (HNO3)+ Acetic acid (CH3COOH) or
    water (used for isotropic etching)
       HNO3 oxidizes Si to SiO2
       HF converts SiO2 to soluble H2SiF6
       Acetic acid prevents dissolution of HNO3
   Etch masks
       SiO2 etched at 30-80 nm/min
       Not etching Au or Si3N4
      Etch Stop
   Factors that determines
    etch rate:
    -temperature,
    -concentration of etchant
    -stirring,
    -light,
    -wafer irregularity,
    -surface preparation.
          Dopant Etch Stop Layer

   Controlling the absolute depth of an etch is often difficult,
    particularly if the etch is going most of the way through a
    wafer.

   Etch stop layers can be used to drastically slow the etch rate,
    providing a stopping point of high absolute accuracy.

   Boron doping is most commonly used for silicon etching.
    Etch Stop layer




   Boron etch stop (p++) used for wet -etching
        Dry Etching Overview
There are 3 types of etching:
 Physical etching
       Sputter etch
   Chemical etching
       Plasma etch
   Combination of physical and chemical etching
       Reactive ion etching (RIE)
   Examples of applications
            Integration of CMOS and MEMS
       Deep reactive ion etching (DRIE)
            Bosch Process - etches deep tranches
            Plasma Basics
   What is plasma?
     Plasma is an ionized gas, in which a certain
      proportion of electrons are free rather than being
      bound to an atom or molecule. The ability of the
      positive and negative charges to move somewhat
      independently makes the plasma electrically
      conductive so that it responds strongly to
      electromagnetic fields.
         Most of the gas is made up of neutral particles but plasma is
        quasi-neutral
        Number of negatively charged particles = number of positively
          charged
          Physical Etching (Sputter)
The etching is produced by ions
   accelerated under the effect of a
   magnetic field towards substrate wafer
In the sputtering process the substrate is
   bombarded by ions
        Substrate atoms are dislodged during
         collisions
             Ion energy > Bonding energy
   Highly anisotropic

   Low selectivity
        Etches all surfaces equally

   Back-scatter can form particulates, that
    will forms impurities on the MEMS
    structures
         Chemical Etching
   Plasma creates chemically reactive
    species
        Diffuse to wafer surface
        Adsorb onto surface
        Chemically react with the wafer surface
        Reaction byproducts desorb and diffuse
         away

   Isotropic

   High selectivity

   Note: Gaseous byproducts can be harmful
      Chemical Vs. Physical Etching




   Wide arrival angle            Narrow, vertical arrival
   High selectivity              Low selectivity
   Low sticking coefficient      Anisotropic etch profile
   Isotropic etch profile
            Reactive Ion Etching (RIE)
   RIE= Chemical etching combined with
    physical ion bombardment
        Reactive ions damage wafer surface that is
         transformed in chemical reaction site
        Increases etch rate

   Anisotropic

   High selectivity

   Etch parameters (gas pressure, concentration,
    RF power, ...) control etch rate, anisotropy,
    selectivity
        RIE Advantages & Issues
   Advantages
       Control of selectivity/anisotropy, etc. by adjusting etch
        process parameters
       Minimizes shortcomings of pure sputter and chemical dry
        etch
       Etchant chemicals and byproducts are collected in
        confined systems


   Issues
       High ion energies can cause device damage
       Chemicals/byproducts environmentally harmful
         Dry Etching of Silicon
   Dry etching
       Plasma phase
       Vapor phase
   Parameters
       Gas and species generated ~
        ions, radical, photons
       RF frequency 13.56 MHz
       RF power 10’s – 1000’s W
       Pressure m Torr - >100 Torr
Wet Vs. Dry Etch Profiles in Si
           Vapor phase etching
 Vapor phase etching is another dry
    etching method, which can be done
    with simpler equipment than what
    RIE requires. In this process the
    wafer to be etched is placed inside a
    chamber, in which one or more gases
    are introduced. The material to be
    etched is dissolved at the surface in a
    chemical reaction with the gas
    molecules.
   XeF2
   High pressure, chemical only
   No plasma (just pump)
   10 µm/min
   No damage
   Isotropic
   Very selective (Si over Al, photoresist,
    oxide and nitride)
   CMOS compatible
        Bosch Process Steps
   Si Etch Step
       SF6 / Ar
       Anisotropic



   Polymer Deposition
       CHF3 / Ar
       Thin on horizontal
       Thick on vertical
        Bosch Process Steps
   Etch again
       Polymer protects sidewalls
       Trench deepens


   Continue alternate etch
    and deposition to desired
    trench depth
         Bosch Process Summary
   Alternate deposition/ etch
    processes

   HDP systems required

   Very high aspect ratio
       Up to 30:1 claimed

   Highly anisotropic
       Scalloped sidewalls

   Etch rate fast
       ~30x RIE etch rate