Wafer Chucks Information

Wafer chucks handle or hold wafers or substrates during wafer processing applications. They are a critical component in wafer manufacturing and testing in cleanroom fab environments. Wafer and substrate processing steps can include thin polishing or chemical mechanical planarization, film deposition (PVD, CVD or electrodeposition), polishing, etching, patterning or lithography, photoresist spin coating, ion implantation, diffusion, and oxidation for dielectric film growth.

Wafer chucks are also a critical product for a variety of substrate testing, metrology, and analysis steps in the manufacturing process such as spectrometry or compositional analysis, flatness or bowing measurement, surface roughness and waviness metrology, automatic defection detection, thermal stress or thermal cycling testing, and failure analysis. Wafers and substrates are very thin, so clamping against an edge clamping as in a conventional chuck would cause severe bowing or warping and destruction of the wafer. Common wafer clamping technologies include vacuum, electrostatic, mechanical, and magnetic. Regardless of the clamping technology, all wafer chucks provide needed clamping uniformity without distortion or damage.

Specifications

Outer diameter/width – The outer diameter or width determines the size of the wafer or substrates that can be held or chucked.

Length – The length determines the size of the wafer or substrates that can be held or chucked.

Flatness – The flatness of the wafer chuck is an important specification and is usually specified in microns. For extreme UV photolithography a wafer chuck with high flatness is required for focus.

Temperature range – This indicates the range of temperature control the thermal chuck can provide. For ambient wafer chucks, the temperature range indicates the temperature extremes the chuck can operate in without damage. Temperature is usually specified in degrees C or F.

Thermal stability – Thermal stability indicates the level of temperature control of the thermal wafer chuck, which is usually specified in degrees C or F.

Thermal uniformity – The uniformity of temperature control across the wafer surface, which is usually specified in degrees C or F. A thermal chuck with high thermal uniformity would have no hot spots or cold spots.

Capacitance – The capacitance (microfarads) of the wafer chuck is an important parameter for chucks used in electrical testing or probing. Low capacitance is better for testing or probing.

Types

Ambient chucks – Ambient chucks or non-thermal chucks operate at ambient room temperature and have no heating or cooling capabilities.

Thermal chucks – Thermal chucks have integral heating or cooling capabilities, which keep the wafer or substrate at a specific temperature during processing or testing.

Actuation/Holding

  • Dipole
  • Electrostatic chucking (ESC)
  • Magnetic/electromagnetic
  • Mechanical (edge/ring clamp)
  • Multipole
  • Vacuum

Form or Shape

Round chucks – The wafer chuck has a round shape, which accommodates most semiconductor wafers.

Square/rectangular chucks – The chuck has a square or rectangular shape, which is useful for certain substrates for optical windows, flat panel displays, hard disk drives, or photonic applications.

Materials of Construction

Ceramic vacuum chucksAluminum – Chucks are made of aluminum, a relatively soft, lightweight, non-magnetic, corrosion-resistant material. Aluminum chucks are used to avoid marring workpieces. Aluminum and aluminum alloys can be very conductive thermally and electrically.

Brass/bronze – Chucks are made of or lined with brass or bronze. Brass chucks and bronze chucks avoid workpiece marring while still providing suitably rigid and precise holding and positioning. Copper and some copper alloys can be very conductive thermally and electrically. Brass is not suitable for wafer chucks in chamber environments with high vacuum and high temperatures because the zinc can evaporate easily. A copper or low-alloy bronze material would conduct heat rapidly in cooling or heating applications.

Ceramic – Ceramics are materials produced by the high-temperature fusion of minerals in a controlled environment designed to impart specific physical and electrical characteristics. Generally, ceramics are electrical insulators or semiconductors, and feature high resistance to thermal breakdown, erosion, and surface degradation (e.g., marring or pitting). Ceramic chucks are used in highly specialized applications such as semiconductor wafer-holding during processing.        

Dielectric – Chuck is made from a dielectric or electrically insulating material.

Electrically conductive – Chuck is made from an electrically conductive material.

Glass – Chucks are made from a glass material. Silicate glasses are typically electrical insulators at room temperature. Glass can be transparent, which facilitates sensing or temperature measurement of wafers through the chuck.              

Insulator on metal – Insulator on metal wafer chucks have a layer of quartz, sapphire, polymer, or glass bonded to metal base.

Magnetic – Chucks are made from a magnetic or ferromagnetic alloy or material, which can be useful if the substrate or wafer is magnetic.

Nickel – Nickel chucks are made of nickel or nickel alloy grades such as Inconel® or Hastelloy® alloys, which are extremely corrosion resistant materials. Nickel is electrically conductive and magnetic.

Plastic – Plastic chucks are very soft. They are used to prevent marring or damage to the workpiece, and in applications which require materials with good electrical or thermal insulation.

Stainless steelStainless steel chucks are made of specific, corrosion-resistant grades of steel. Typically, these metals are either non-magnetic or less magnetic than standard alloy steels. Stainless steel is less suited to hardening and certain types of surface finishing. Stainless steel are electrically conductive.   

Steel – Steel is not typically the material of choice for wafer chucks because it lacks corrosion and oxidation resistance compared to other materials. Carbon steel or alloy steel chucks are typically nickel or gold plated to provide a corrosion resistant surface.

Titanium – Titanium chucks are made of titanium or titanium alloy grades such a commercial pure titanium or Ti6Al-4V, which are extremely corrosion resistant materials. Titanium is electrically conductive and non-magnetic.

Surface Finishes

  • Anodize
  • Hardcoat
  • Plated (gold or nickel)
  • Thermal spray coating
  • Thin film coating

Features

Wafer chucks may include a variety of special features, including backfill gas lines, various heating and cooling methods, independent vacuum channels, electrical connections, integral temperature controllers, rapid thermal cycling, and integral wafer lifts.

Applications

  • Display components
  • Extreme ultraviolet (EUV)
  • Ion implantation
  • Lithography
  • Memory
  • Microfabricated foils
  • Microelectromechanical (MEMS)
  • Microelectronics/semiconductors
  • Microfluidic devices
  • Nanostructured substrates/nanomaterials
  • Optical components
  • Optoelectronics/photonics
  • Reactive ion etching (RIE)
  • Solar/photovoltaic substrates
  • Thin film deposition (CVD, PVD, electrodeposition)
  • Testing/probing
  • Specialty applications

Image credit:

CoorsTek



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