In Situ Metrology for Real-Time Process Control

By Katherine Derbyshire

Chipmakers don't like metrology tools. The tools take up space and delay wafer production. They require test and monitor wafers, wasting equipment time on non-product silicon. Chipmakers can't do without metrology tools, though, because they provide critical information for process monitoring and control. The expense of metrology is more than justified by improvements in yield.

Still, in situ sensors offer important advantages over standalone tools. Some applications, like rapid thermal processing, require real time measurements. In others, such as endpoint detection for etch or CMP, in situ sensors are more accurate than process times derived from monitor wafers.

As Bob Goodman, president and CEO of Luxtron (Santa Clara, CA), told Semiconductor Online, in situ sensors also provide other information about the health of the equipment. For example, changes in CMP planarization time can indicate that the polishing pad needs conditioning or replacement.

Installing in situ sensors in a process chamber is difficult, though. Hostile environments—high temperatures, plasmas, and caustic chemicals—damage sensors and complicate data transmission. Space inside a process tool is limited, and the best location for a sensor may be impossible to reach with power and data cables. Luxtron specializes in solving these packaging and installation problems.

The company supplies fiberoptic temperature and optical sensors for semiconductor, medical, and electric utility applications. Minimally-invasive surgical techniques using lasers, microwaves, and RF energy require precise temperature control to avoid damage to healthy tissues. Fiberoptic temperature probes are small, flexible, and chemically inert. Being non-metallic, the sensors are not affected by strong magnetic fields or microwave and RF energy. Electric utilities use ruggedized versions of these probes to monitor transformer temperatures.

Rapid thermal processing systems, in contrast, need accuracy more than ruggedness. Conventional optical pyrometry calculates temperature from the light emitted by the wafer, but different backside coatings have different emissivities. An accurate temperature measurement must account for unpredictable emissivity variations. Luxtron's Ripple technology does this by measuring small changes in radiant output created by the alternating current in heating lamps. By analyzing light reflected from the wafer surface, the sensor determines its emissivity and thermal radiance.

Recently, STEAG AST Elektronik (Germany) signed a patent licensing agreement to commercialize the Ripple technology. According to Peter Augustin, president of STEAG AST, "Now we are offering our customers temperature control to within ±2°C, regardless of wafer backside variation."

While relatively few semiconductor applications require precise temperature control, many require endpoint control. Overetching and overplanarization waste process time and can damage device structures. CMP endpoint detection is especially difficult. In plasma etching, the wafer and plasma emit light that directly correlates with the chemistry of the wafer surface. In CMP, it is impossible for a single sensor to image the entire wafer. At the same time, non-uniformities make it difficult to extrapolate a single-point measurement.

Luxtron has developed two distinct sensing methods for CMP. The first relies on optical detection through the backside of the wafer. A low-power, battery-operated laser illuminates the detection spot, and infrared light transmits the signal. Placing wires inside the polishing head, a tricky engineering problem, is unnecessary with this design. A second approach monitors the polish head motor current, detecting changes in friction. Besides indicating planarization endpoint, friction also varies with pad and slurry characteristics.

Cybeq Nano Technologies (San Jose, CA) plans to incorporate both types of sensors in the company's CMP systems. Optical sensors will monitor shallow trench isolation, while motor control sensors will monitor tungsten and copper polishing. "It was imperative that Cybeq partner with a process control manufacturer in the early stages of product design and that we integrate the in situ endpoint control detection capability into our product," said Robert Shinagawa, director of engineering for Cybeq. "Luxtron has been very supportive by optimizing their products to meet our design and performance criteria."

As Goodman explained, such customization is a common thread in these diverse applications. Luxtron's application engineers work closely with customers to select the appropriate sensor and package it for the particular environment.

Wet etching presents yet another set of challenges for in situ metrology. Luxtron has agreed to collaborate with SEZ America (Phoenix, AZ) to integrate optical endpoint detection into SEZ's spin-etch equipment. The resulting Wet Master series tools are intended for applications such as backside film removal, cleaning, and polymer removal. "With enhancement from the Luxtron endpoint system, the Wet Master series products have higher selectivity. Unlike other technologies, this process makes it unnecessary to protect the opposite side of the wafer with photoresist or tape," according to Jim Mello, process/applications manager at SEZ.

Current market conditions have forced many chipmakers to delay major equipment purchases. These companies have been forced to enhance yield and extend the life of existing process tools. In situ metrology, customized for particular processes, offers improved productivity at substantially less cost than new equipment.

For more information: Luxtron Corporation, 2775 Northwestern Parkway, Santa Clara, CA 95051-0941. Tel.: 408 727 1600, fax: 408 727 1677.