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Gold Reduction Strategies

 Gold Graph

The price of gold has risen steadily over the last 10 years.  Based on the market price of 7-Oct-11, the price of Engelhard fab gold increased 16% since 3-Jan-11 and 47% since 3-Jan-10.   The price of gold is presently influenced by investors who buy gold as a hedge against inflation and recent debasement of fiat currencies like the US Dollar.  Output by the world’s gold mines and from the recycling industry has not tracked linearly with increased demand.  While most of the gold consumed world-wide is used either as an investment vehicle or for jewelry, Materion’s customers use gold and gold alloys for a number of electronic applications.  In the jewelry and investment markets, the price of gold-containing products is adjusted frequently (sometimes daily) in response to the market price.   However, electronics and photonics companies who consume gold typically cannot pass along fluctuating market prices of gold to their customers on a daily basis.  For example, manufacturers of LEDs that contain thin film gold layers do not charge their customers as gold prices rise or fall.  Any cost as a result of rising gold prices charged by their suppliers is absorbed by the device manufacturer until the next opportunity arises to negotiate the price of their devices.

Since the price of gold is beyond the control of Materion or its customers, many electronics and photonics manufacturers have accelerated development efforts to reduce or “design-out” gold and gold-containing alloys.  Design-out of gold is difficult because of the many unique and technical advantages of gold including: corrosion resistance, compatibility with other semiconductor materials, high ductility and good thermal and electrical conductivity.  Substitution or reduction of gold inevitably requires an investment in added processing steps and reliability studies, but as long as the market price of gold is high, these added efforts are cost justified.

Materion’s lifecycle approach to reduce precious metal cost…

In addition to designing products to use less gold, Materion works with some of its industrial gold consumers to minimize the cost by managing the entire gold lifecycle.  Customers can buy gold to put into an account, which makes it easy to send spent PVD targets or other reclaimed gold back for gold credit.  The cycle begins by Materion selling a gold sputtering target or evaporation slug to the customer.  In the course of using these input materials, gold will be deposited onto surfaces inside the vacuum chamber other than the product wafers.  Materion’s shield services business offers cleaning of PVD shield kits, removal of the precious metals from the kit surfaces, refining the reclaimed metals, and crediting the value of the metals back to the customer’s account.  When PVD targets are spent, Materion will take back and recycle the target, again crediting the customer’s account for the value of the reclaimed gold.  For very large customers of gold, Materion has placed an employee at the customer’s location to manage the entire life cycle of precious metals for the customer.

Below is a list of gold products and the steps currently being taken by gold consumers to reduce or eliminate gold.

Product:   Gold Bonding Wire

Semiconductor manufacturers worldwide have, or are in the process of, substituting aluminum alloys and copper alloys for gold bonding wire.   Aluminum alloys, typically Ai:Si alloys have been used by the RF transistor industry for over 5 years.   Today, Si memory and ASIC industries are rapidly substituting copper for gold wire.   In both cases, the switch away from gold wire was enabled by new wire bonding machines optimized for Al and Cu wire, as well as changes in the metal contact metallization atop the chip.  Materion offers a broad range of PVD sputtering targets and evaporation slugs to deposit metal pads compatible with Al or Cu wire.

Product:   Packages for RF Power Transistors

At RF frequencies, electrons penetrate only a shallow skin of the electrical conductor.  Therefore, the outermost surface of a material conducting RF electrons must have a high electrical conductivity, and this conductivity must not drift over time.   Since gold is easy to plate, has a high electrical conductivity, and will not oxidize, gold plated RF packages have been standard since the invention of the RF power transistors in the 1960’s.  Additionally, the gold plating on the package is dissolved with Si from the backside of the Si chip to create AuSi eutectic solder in-situ for die attach.  Materion and its customers are working on a variety of approaches to reduce or eliminate gold on the package:

Product:  AuSn Eutectic Solder

80:20 AuSn eutectic is a lead-free solder with a liquidus temperature of 280C.  AuSn is used for die attach and hermetic lid sealing.  It has the advantage of not requiring any flux as long as it is used in an ambient free of oxygen or water vapor.  However, the high cost of gold is driving customers to either (1) reduce the amount of AuSn solder they require, or (2) qualify alternatives to AuSn solder.

For freestanding AuSn solder ribbon and performs, Materion has developed thinner versions of AuSn.   Although pure gold is very ductile and malleable, the 80:20 AuSn eutectic is brittle.  After years of development, Materion can now roll AuSn eutectic solder down to a thickness of 0.0007”, and 0.0005” (13 µm) in special circumstances.  Where AuSn is used for die attach, Materion provides PVD materials that can deposit AuSn solder onto the backside of wafers.  Thin film deposited layers of AuSn can be made much thinner than 0.0005” (13 µm), and are typically 2 – 8 µm thick .  Materion offers AuSn sputtering targets whose composition is tailored to compensate for the unequal sputter rates of Au and Sn.  The sputter-deposited film has approximately 80:20 ratio of Au to Sn in order to achieve the desired liquidus temperature.  Materion also offers high purity Au and Sn evaporation slugs that enable co-evaporation or alternative evaporation of Au and Sn.  PVD deposition of AuSn solder on the backside of wafers is currently employed by certain LED and RF device manufacturers.

For customers who wish to eliminate AuSn completely, Materion offers a complete range of Pb-containing solders with liquidus temperatures near 280C.  If a leadfree solder is required, Materion offers SnSb alloys with a liquidus temperature ranging from 240 - 232C.  Solders other than AuSn require the use of flux or vacuum ovens.  For hermetic lid sealing, alternatives to AuSn solder include:  low temperature, PbO-containing solder glass and lid welding.  For die attach, alternatives to AuSn solder include Pb-containing and Pb-free solders, as well as highly loaded silver-filled epoxies.  Materion offers a complete range of Pb-containing and Pb-free solders that are alternatives to AuSn.

Product:   Thin Film Au Layers

Thin film gold has been a standard metallization layer on GaAs, InP and GaN-based compound semiconductor devices for over 30 years.   Au is used to coat or fill vias, as a conductive trace material, and as a backside metallization.  Today, most GaAs devices used for RF wireless communication (like cell phone power amplifiers) and GaN-alloy devices used for LEDs, contain ohmic contacts with a layer of gold.   Researchers are working to develop ohmic contacts that are gold free.  For example, researchers at MIT have demonstrated ohmic contacts on GaN devices using a metal stack of Ti/Al/W.   However, extensive reliability testing will be required before this technology is embraced by industry.   Materion offers PVD consumables including Ti, Al, W, Au and many other metals that are widely used by the compound semiconductor industry.

Product:   Electrical Contacts

Materion Technical Materials (MTM) can reduce precious metal content by plating or cladding a thin layer of precious metal onto the surface of a less expensive metal alloy.   For most electrical contacts, it is important that the surface be kept free of corrosion (oxidation scale) in order to provide a low electrical resistance when plugged in.   Bulk contact alloys like beryllium copper and zirconium copper can be clad or plated with precious metals in order to protect the surface from oxidation.   Plated layers can be comprised of successive layers of Ni + Pd or Ni + Au, and can range in thickness from 10 to 100 microinches  ( 0.25 µm  to 2.5 µm).   However, cladding is a much more versatile surface protection strategy than plating because for cladding: (1) the thickness of the clad layer can cover a wide range, namely from 30 to 200 microinches (  0.76 µm  to 5.1 µm);  (2)  a wide variety of alloys can be clad, such as Au, AuAg, PdNi, PdAg or AuCu;     (3) cladding can be applied single sided, double sided, selective on a surface, or as an inlay.   MTM’s unique factory in Rhode Island has the ability to produce clad products on a very large scale at a competitive price.