Diversification: that’s the central pillar of RF Micro Devices’ growth strategy. To continue to execute on that front it is opening up its MBE facility and starting to offer various services that include shipments of arsenic- and phosphorous-based epiwafers. Richard Stevenson investigates.

RF Micro Devices is renowned for its manufacture of power amplifiers. In the late 1990s it convinced handset makers to turn away from MESFET-based amplifiers and switch to its HBT-based variant. Since then it has gone from strength to strength to become one of the world’s largest manufacturers of power amplifiers, which it churns out in-house on a 6-inch line equipped with multi-wafer MBE tools.

While there is no denying that RFMD’s fame derives from its manufacture of billions of HBTs at its Greensboro, NC, headquarters, it is wrong to think of this company as just a captive GaAs chipmaker. Since 2003 it has been outsourcing growth of InGaP HBTs – it now has several established suppliers of MOCVD-grown epiwafers – and during the last few years it has added another string to its bow, GaN. The company continues to expand its portfolio of wide bandgap RF products, and last year it started to offer foundry services for this technology, including fabrication of GaN-on-SiC wafers to customer-specific circuit designs.

The company’s diversification strategy has taken another significant stride this year: The introduction of MBErelated epitaxial products and services, including ultrahigh vacuum cleaning services. Profitable new revenue streams could result from this move, leading to a good return on the company’s substantial investment in capital equipment.

“Our offerings include working with customers to develop epitaxial structures and MBE growth conditions; the delivery of epiwafers grown to exact customer specifications; and designing epitaxial DOEs [design of experiments],” explains Robert Van Buskirk, president of RFMD’s multi-market products group.

RFMD’s MBE services are available to all. There is no minimum order, so start-ups and small companies will not be put off working with this RF giant. And RFMD’s proven pedigree in high volume manufacturing makes it an appealing option for far bigger players looking to off-load epiwafer growth, or qualify an external material supplier that can supplement internal manufacture during peak periods.

 



An RFMD technician loads substrates in preparation for epitaxial growth

 

The battle ahead

If RFMD is to have significant success in this new venture, it will have to tender and win MBE-based epiwafer supply contracts. The strongest competition will surely come from IQE, an epiwafer supplier with large MBE facilities in Singapore and Bethlehem, PA, that has experience in producing material for RF applications in high volume, thanks in part to its contract with Anadigics.

The Greensboro outfit is certainly up for the challenge of competing in the epiwafer supply market. It is no stranger to high-volume manufacture, and Van Buskirk points out that the company can draw on its experiences associated with developing several generations of epitaxial structures that have been instrumental in the creation of one of the most successful RF companies in the world.

What is certain is that RFMD’s new venture will get off the ground. Over the years the company has had several inquiries and engagements for MBE products and services, including the ones that they are offering, and this interest will not diminish now.

A significant proportion of RFMD’s revenue from its new venture is likely to come from the sale of arsenic and phosphorous-based epiwafers. These structures can be grown on 4-inch or 6-inch GaAs substrates, which can either be supplied by the customer, or purchased from RFMD. “We do not have the capability for nitride growth, but beyond that limitation, we are open to any epi structure,” says Chris Santana, director of the company’s MBE operations. According to him, RFMD has experience in growing and developing many different types of structure, including metamorphic designs, BiHEMTs, MOSFETs and even optoelectronic material such as VCSELs.

RFMD’s introduction of a GaN-based foundry service in 2009 has provided a great stepping-stone for this year’s foray into MBE services. “We now have all the aspects of a full-service, commercial turn-key foundry in place – including purchasing and IP agreements, work-flow procedures, and web-based customer support processes – and we can quickly tailor those commercial business processes and systems to our MBE based service,” explains Van Buskirk.

Customer services

Although GaN and MBE foundry customers can just instruct RFMD to supply wafers to their specifications, there is more help on hand if they want it. For example, in RFMD’s GaN foundry, customers can tap into the company’s design kits that are supported by industry standard design software and device models. What’s more, it is normal for RFMD’s engineers to interact with customers in the final stages of their circuit design efforts to make sure that customer designs do not violate any inhouse design rules or layout services, as documented in RFMD’s design kits. “However, while we offer a wide range of GaN-based proprietary products, we do not offer circuit design services for our foundry customers,” explains Van Buskirk.

Santana claims that his team will be even more flexible when it comes to MBE structures and profiles. In this case, so long as they can support the customization required, the engineers will be willing to help to develop epiwafer designs needed to develop a product. “We will be happy to engage customers in this technical dialogue.”

Any company weighing up the pros and cons of working with an epiwafer supplier will demand the protection of their intellectual property. RFMD can assure customers of this, and show them the plans put in place that draw on its previous foray into foundry services. “We have established robust firewalls within RFMD for our GaNbased foundry service,” explains Van Buskirk. “Our foundry service teams are separated from our internal development teams, and RFMD employees at large cannot access the IP or data for foundry services.”

There are times when customer-sensitive information has to be transferred to RFMD employees, but this is minimized, with technical data and information disclosed on a ‘need-to-know’ basis. When a customer wants to access design kits and models, they can do this through an external, web-based portal that allows them to see the status of their wafer fabrication orders.

Aside from IP issues, the big question for many customers is how long it will take them to get their epiwafers. Van Buskirk has some reassuring news for them: “We have the industry’s fastest cycle times, and expect to use that as a key performance discriminator in our foundry service.” Cycle time commitments are already in place for customers using RFMD’s GaN services, and they have a clause in these contracts entitling the customer to a discount if shipments are late.

Van Buskirk can also assure customers that they will not lose out if RFMD has a substantial hike in orders for its own GaAs chips. “We are committed to growing our foundry services, and we have the installed capacity to meet our internal needs and the needs of our potential external foundry customers.” In fact, this installed capacity is so large that it makes RFMD one of the world’s largest MBE, GaAs and GaN wafer production facilities, claims Van Buskirk.

The MBE facility at RFMD runs ‘24-7’, but it is only staffed from 7 a.m. to 7 p.m. Automation allows the MBE tools run through the night, with growth aborted if in-situ monitoring tools determine that the processes havedeviated beyond acceptable windows for production.Customers can select the tool for epiwafer productionfrom a portfolio of MBE reactors: a Veeco Gen2K, whichhas a 7 x 6-inch capacity; a Riber R7000 with identicalcapacity; a Riber R6000, which can accommodate four 6-inch wafers; and a single wafer, 6-inch tool, the VG V100.RFMD equips these tools with state-of-the-art monitoringapparatus. “Our MBE systems are outfitted with what we believe to be the most effective tools at delivering quality, consistent products,” claims Santana.

 



RFMD’s portfolio of multi-wafer MBE tools includes reactors built by Veeco, Riber and VG

 

To ensure that the facility is run as efficiently as possible, the company’s process engineers interlace growths for the customers with those for internal production. “However, if the customer chooses, we can enter into an agreement that provides exclusive use of an MBE tool for a period of time,” reveals Santana. In fact, RFMD has already taken this type of arrangement with one of its MBE foundry customers.

The Greensboro outfit has an impressive toolkit for characterizing epiwafers. Alongside the more common methods for determining material characteristics, such as a Lehighton instrument for resistivity measurements and an X-ray diffraction tool, the company can analyze wafers with a multi-field Hall probe and a photoreflectance technique.

“Multi-field Hall is primarily used to give the mobility and carrier concentrations of the conductive layers in the epi,” explains Santana. It can, for example, be used to determine the mobility and carrier concentrations in both the channel and the highly conductive cap layer of pHEMT epiwafers. This is beneficial, because it eliminates the need to grow a ‘capless’ calibration structure. “In addition, it is also an excellent method for process control,” argues Santana.

Photoreflectance, another non-destructive technique, provides a qualitative measurement of HBT gain. Samples are probed with a broadband light source, and insights into the structure are gleaned by collecting and spectrally analyzing light that has been reflected off of the many interfaces of the transistor. The data from all of these measurements can accompany material shipments to customers. And if the customer wants the data collected by the in-situ monitoring tools, including values for growth rates and temperatures, this can be sent as well.

 



The throughput of MBE-grown material at RFMD has climbed over the last decade. The dip in 2008 resulted from a sharp decline in handset orders, plus a move from within that industry to slash inventory levels. The continuous line shows actual wafer output, and the dashed line is a linear fit of this data.

 

In short, it seems that RFMD is willing to stay as flexible and accommodating as possible to meet the customer’s needs. At present the only tasks that the company is not prepared to do are to process GaAs wafers into chips and package them. But even this may become an option one day – after all, it certainly ties in with the company’s goal of diversification. Will it do it? We’ll just have to wait and see.