X-Fab Interview: SOI Solutions for analog/mixed-signal, high-voltage, high-temperature and MEMS Applications


X-FAB_logo_print_loresWith five manufacturing sites around the world and 72,000 wafer starts/month, X-Fab is a leading pure-play analog/mixed-signal and specialty foundry for automotive, industrial and medical applications. ASN recently had the opportunity to talk to Tilman Metzger, Product Marketing Manager for the X-Fab Group, about when customers choose an SOI-based offering.

Advanced Substrate News (ASN): Can you give us an overview of the SOI offering at X-Fab?

Tilman Metzger (TM): X-FAB offers a range of SOI solutions from 1µm to 0.18µm. We support high voltage (HV) requirements from 20V to 650V. X-FAB also targets very high temperature applications of up to 225˚C.

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Tilman Metzger, Product Marketing Manager for the X-Fab Group. (Courtesy: X-FAB)

Our latest addition to the SOI family is XT018, our first 0.18µm SOI solution. The modular XT018 platform combines a state-of-the-art 180nm mixed-signal process with benefits of a robust SOI HV technology. XT018 supports voltages up to 200V and targets next generation automotive and industrial applications.

ASN: When did X-Fab first start offering SOI and why?

TM: We started more than 15 years ago with a 2µm HV SOI process. Our first SOI development was driven by specific customer requirements for an HV motor driver application.

ASN: What sorts of chips are currently being manufactured by X-Fab using SOI?

TM: X-FAB solely focuses on analog and high-voltage SOI applications. We do not target RF-SOI or high density SOCs like CPUs etc.

Typical products include high-side gate pre-driver ICs, motor driver ICs, ultrasound driver ICs, solid state relays, optocoupler and analog switch arrays.

ASN: For X-Fab, what are the traditional SOI markets (both in terms of end-markets and geography)? How do you see it evolving?

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X-FAB headquarters in Erfurt, Germany (Courtesy: X-FAB)

TM: Historically, we have seen demand for SOI-based technologies mainly from the industrial sector. That said, we expect to see more automotive customers adopt our SOI solutions in the future.

Geographically, our SOI customer base mostly originates from North America, Europe and Japan. Customers from Greater China and South Korea are generally slower in adoption but gaining momentum.

ASN: When and why do your customers choose an SOI-based process?

TM: Typically, we see two types of SOI customers:

  1. Those that tried and failed a particular design in a BCD/Bulk technology and hence turned to a SOI solution; and
  2. Those that focus on SOI technology right from the start due to IC or system requirements (or past experience). Some of the challenges of such designs may include:
  • Very high temperature of 175-225°C
  • Resistance to EMI* or stringent EMC and ESD requirements
  • Latch-up concerns
  • Negative voltage swings / inductive loads
  • Stringent noise immunity / cross-talk requirements
  • Low leakage at high temperature
  • Aggressive time-to-market requirements

ASN: Can you expand on the time-to-market (TTM) issue a bit?

TM: Since SOI substrates are more expensive than normal bulk wafers, the average wafer price is also higher. Typically customers look at a straight cost-per-die calculation when evaluating the business case for their product. But there’s also the aspect related to ease of design – with SOI, design is easier, so the design cycle might be faster and less costly in terms of engineering time. As a result, if customers can launch their product faster, they can grab more market share and increase their profits.

ASN: What kind of support do you offer designers for SOI-based chips? Is it different from the sort of support for bulk processes?

TM: Generally, for our SOI technologies we offer the same comprehensive support as for our bulk solutions. In addition, we provide SOI application notes that discuss SOI related design considerations. With the exception of XI10, the SOI material we are using is “thick film” SOI, where the device layer is up to 55µm thick, so the behavior of active devices is similar to those on non-SOI substrate. Let’s consider the designers doing high-voltage analog: in bulk, they do standard junction isolation, but in SOI they use deep trench isolation, which actually comes with fewer parasitics, so it’s easier to simulate and design.

ASN: Would you say the SOI ecosystem is well established in the markets X-Fab serves?

TM: There are no special SOI ecosystem requirements for X-FAB’s SOI solution. We use established SOI wafer suppliers and support all major EDA platforms (Cadence, Mentor, Synopsys, Tanner). with complete design kits. Analog and high voltage is all about customization. In the analog world, there are some generic IPs, but most of it is specialized. We offer basic IPs for SOI solutions including I/O and standard cell libraries and memories such as OTP, SRAM etc. which is similar to our offering for non-SOI processes..

ASN: Can you tell us more about X-Fab’s SOI offerings?

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X-FAB cleanroom in Kuching, Malaysia, where the company runs their new XT018 SOI process. (Courtesy: X-FAB)

TM: X-Fab has two one-micron SOI ultra-high-voltage process offerings for 650 Volt and 350 Volt which are used by customers for applications that plug directly into the grid. There is also a big market for 600V IGBT and MOSFET driver ICs. Some customers select these processes for their inherent robustness in applications like avionics and aerospace. (We do not offer specific radiation-hardened solutions, but our customers use these when they have particular reliability requirements.)

Our one-micron process XI10 targets very high-temperature applications: it offers different metallization schemes, and can support up to 225°C.

XT06 is a 0.6µm SOI technology that supports voltages up 60V and is popular across a range of industrial applications.

XT018 is our latest SOI solution. As mentioned earlier it not only targets industrial and medical applications, but also next generation automotive products. An example is the new CAN FD** standard which is more complex and challenging to implement. XT018 offers the right process options to address these requirements. X-FAB has a long successful track record of serving the automotive market. This is also reflected by the fact that the automotive segment accounts for more that 50 percent of our total revenue.

ASN: For MEMS, when and why do your customers opt for an SOI-based solution? Do you see any growth in interest in putting MEMS on SOI?

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X-FAB MEMS Foundry received the “MEMS Foundry of the Year” award at the Best in MEMS & Sensors Innovation Awards ceremony, as part of the MEMS Industry Group’s 10th annual MEMS Executive Congress® held in Scottsdale, Arizona in November 2014. (Courtesy: X-FAB)

TM: For MEMS, we definitely see the opportunity to take advantage of SOI material. In general, SOI wafers are interesting for the formation of highly uniform silicon membranes or other mechanical structures, especially if we prefer to use SOI’s mono-crystalline properties rather than depositing poly silicon. The top device layer is ideal for defining silicon features with thicknesses from a few microns to several tens of microns, without the effort of very long silicon deposition times. The buried oxide (BOX) layer acts as a natural etch-stop layer during silicon etching, at the etching either from the front or from the back of the wafer. Stopping at the BOX layer mitigates any non-uniformity for the deep silicon etch and allows for great process control.  

For instance, at X-FAB, we use SOI wafers to manufacture our open-platform gyro sensor / accelerometer process. We use the SOI wafer’s device layer to make single-crystal masses with uniform thickness for predictable and robust performance. In this case the buried oxide layer not only acts as an etch stop when etching the silicon but is also a sacrificial material to remove from underneath silicon structures such as inertial masses and comb-drives.

We also have our newer three-axis gyro / accelerometer process where X-FAB is making its own SOI substrate with buried cavities. In other cases, we etch a pattern all the way through the back side of the wafer to leave thin membranes on the front side of the wafer. Again, the etch is well-controlled, stopping on the buried oxide and the remaining oxide / device layer silicon membrane could be used on its own or with further layers and structuring to form a variety of device types such as pressure sensors, force sensors, thermopile structures or microphones.

ASN: Do you see SOI becoming a more important part of X-Fab’s offering? If so, why?

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Chips manufactured by X-FAB go into key automotive systems. (Courtesy: X-FAB)

TM: Yes. One of the factors that we foresee to drive SOI based designs is the increasing challenges of automotive systems and ICs. This is largely driven by newer standards like CAN FD. While SOI is is still a relatively small part of our business, we see opportunities, especially with our XTO18 offering, which may open new high-volume markets.

We have customers that require a stable supply of their product over a long period in time, often for a decade or more. In the automotive industry, those customers are using a 10-year old process. We need to be able to guarantee that those processes will be available for ten to fifteen years.

We have customers in consumer markets using SOI – either because they’ve tried and failed on bulk, or they’re looking for long-term solutions. They see the benefits in the ease and speed of design, which helps them ensure that they don’t miss windows of opportunity. But they need to crunch the numbers themselves. SOI will give them a smaller chip size, but there is not a “one fits all” approach – it depends on the design topology.

ASN: Will the SOI-based processes offered by X-Fab evolve? If so, how and why?

TM: Remember, analog and mixed-signal is not a linear shrink like for digital. The node at 0.18 microns is the leading edge for high-voltage. We can add more functionality and more voltage classes. We’ll continue to add features and modules where we see opportunities for increased performance or new markets. That said, for the five platforms in our current SOI offering, the mature ones won’t change too much except for increasing performance. The markets are evolving, but they’re also very conservative.

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X-Fab has organized a series of design webinars, including a number that cover SOI-related topics. Click here to access the list.

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* EMI = electromagnetic interference; EMC = electromagnetic compatibility; ESD = electromagnetic discharge

**CAN stands for controller area network, a protocol that allow microcontrollers and other devices to communicate without a CPU. It is used extensively in automotives for connecting electronic control units (ECUs) and in industry for factory automation. CAN FD is CAN with Flexible Data rates.

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