Use of Precious Metals in Semiconductors
Gold (Au) and Platinum (Pt) are metals vital to the manufacture of compound semiconductor devices. These precious metals are chosen because they offer high electrical conductivity and resistance to corrosion necessary for superior performance. They also facilitate ohmic contacts to compound semiconductors. The downside is that Au and Pt represent a significant cost to the manufacturer and the majority of the metal is deposited onto the surfaces of the shield kit during the evaporation or sputtering process.
It is important to recover the maximum amount of Au and Pt from the shield kit to minimize total cost of ownership. Materion has developed proprietary chemical processes that offer distinct advantages over traditional shield kit cleaning. The comparative advantages and disadvantages of the cleaning methods will be discussed in this article.
Unrecovered Output Increases Costs
The output from evaporation or sputtering processes ends up on a variety of surfaces inside the vacuum chamber. Most of the evaporated Au and Pt is not deposited on the final production wafers. The majority of the precious metal can be found in a variety of places including: shield kits; shutters; Cu parts such as hearths/turrets, e-gun inserts, V plates; spent crucibles; chambers; vacuum bags; etch tools, viewing windows, etc. Only a small portion (3%-8%) of the evaporated Au and Pt ends up coated onto the wafer substrate.
A typical range of coatings on Evaporator Kits is 600-200,000 grams of gold, translating to approximately $25K-$8M per annum. Including Planned Maintenance on top of that figure means each kit could run $200K to $70M annually. Thus, there is the potential for financial loss.
A similar challenge exists for precious metal utilization during the sputtering process. Only around 10-15% of the Au and Pt ends up coated onto the wafer substrate. Precious metal will be found on spent targets, shield kits, chamber flakes, vacuum bags, scrap wafers, etch tools, etc. While the majority of Au and Pt in sputter tools is recovered as spent targets, the amount transferred onto the shield is significant.
Critical to minimizing costs for either of these processes is recovering the maximum amount of Au and Pt output from the PVD tools. This can be achieved through precision parts cleaning using traditional cleaning processes or Materion’s advanced chemical method. The following data is per Evaporation and Sputter tools.
Even a 1% improvement in Evaporator Tools cleaning return would equate to $1,000-$2,100 savings for a low end user and as much as $322,000-$697,000 for a high end user. (With Jan 2014 gold price at $1250/ozt). For sputter tools, a 1% return improvement would mean potential savings of $482-$1,045 for low usage and $4,019-$8,707 for high usage.
The typical range of coatings on Sputter Kits is 300-2500 grams of precious metal for a cost of approximately $12K-$100K per Kit. With Planned Maintenance added, this equates to a broad range of $48K to $850K annually per tool. Thus, even a 1% improvement in Shield Kit Cleaning, Precious Metal Reclaim and Refine would equate to an annual savings of $500-$8.5K per sputter tool.
Traditional Shield Kit Cleaning v.s. Chemical Cleaning – Pros and Cons
Scraping involves the physical removal of the metal deposition using a scraping tool and manual force. It usually must be followed by bead blasting, as not all deposition can be removed by scraping. It has the advantage of being easy to perform with no special training and relatively low capital investment. The problem is that it may not fully remove the deposition, therefore it does not provide an efficient return of precious metal. In addition, the physical impact of scraping can damage shields or run the risk of risk igniting the dust.
Bead Blasting involves the removal of the deposition using a media (usually alumina) and a carrier gas (compressed air). Like scraping, it is easy to operate with little training and at relatively low operational cost. The biggest disadvantage is that it produces lower refine yields. This is because Au and Pt dust can easily be lost and bead blast material refine is low/dilute in precious metals and more difficult to refine. In addition, erosion from bead blasting may damage shields.. Bead blasting can also result in deformation or warpage of the shield kit, rendering it unusable.
Materion has developed proprietary chemical processes that offer distinct advantages over traditional scraping and bead blasting technologies. We also offer a chemical process for cleaning copper turrets which is unique in the industry. Our processes:
• Provide the highest quality cleaning
• Avoid the problem of shield damage
• Lower the frequency at which shields need to be replaced
• Produce optimal Au and Pt reclaim and refine yields
In addition, the reclaim material from Materion’s chemical etch processes contain concentrated Au and Pt and thus are easier to refine.
The chemical etch process is slightly higher in cost, requiring capital and chemical expenditure. It also requires special training to conduct the work. However, it returns the highest precious metal yields that have often been found to more than off-set those disadvantages.
Maximizing Metal Return
In summary, to increase refine yield and minimize costs, chemical etch cleaning is Materion’s recommended choice. It produces the highest quality of cleaning and can be used to effectively remove a variety of depositions:
• Single stack Au and Pt depositions
• Stacks containing Au and Pt along with base metals
• Thick stacks containing Au and Pt along with base metals
The Materion chemical cleaning technology also delivers above industry returns for platinum along with faster settlement times.
Materion’s experts will be happy to visit your site and recommend a precious metal recovery plan designed to minimize your costs and maximize your uptime. To learn more about our removal technologies and other services such as in-house refurbishment, custom design and certification per customer specifications, please contact: Jim McMullen, Senior Project Manager, James.McMullen@Materion.com, +1-716-446-2273.