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Metrology and Characterization Tools in Micralyne
MEMS (MicroElectroMechanical Systems) are more difficult to characterize than integrated circuits due to the need to gauge sensing and/or actuation performance, therefore characterization of the constituent structures is a critical part of MEMS development and manufacturing. Integrated circuits are created using stacks of patterned thin films which constitute the semiconducting, conducting and dielectric layer. This requires considerable control of a number of parameters with respect to material properties and structural size.
MEMS (MicroElectroMechanical Systems) use films of these types as well as an esoteric suite of films to make the suspended and freed parts which enable movement of the device. This adds a level of complexity to the characteristics which require measurement and control to realize functional devices.
Micralyne uses a number of metrology and characterization tools to develop and maintain processes as well as to control in line manufacturing protocols.
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Fig.1: A series of Zygo interferometer images of the SLV deforming at 1 volt intervals
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Film thickness control is essential, and one of the most basic measurement tools is a stylus profilometer. Using a scanned tip over a surface of a film segment, the thickness can be determined. It is most useful for metal films, as well as measuring the depths of channels or openings in substrates as large as tens to hundreds of microns. If the film is transparent, then optical methods can be used to characterize the thickness and index. Micralyne has dual pass spectroscopic measurement systems and for greater index of refraction characterization, we have a spectroscopic ellipsometer to characterize multiple layers of thin films.
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Fig. 2: Stress characteristics of a triple film layer system over 200 degrees Celsius range
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Multiple thin films deposited in situ can make measurement of thickness difficult, but using XRF (X-Ray Fluorescence), the individual film thickness values can be determined individually.
The films have to be patterned lithographically and delineated by various etching techniques. As such, the feature width needs to be measured and controlled, usually centered on what is referred to as a CD (Critical Dim
ension). The CD measurement system can accurately determine these features to a submicron resolution.
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Fig. 3: SLV Chip with 1000 switches
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Since many thin films are part of a suspended structure, the contribution of stress from each film is important, as the released portion can deform due to mismatched stress values. The film stress is characterized by measuring the distortion of a circular silicon wafer and can cycle through a temperature range from ambient to 650 Celsius.
In order to measure smaller membrane deflections, or the actuation angles of devices while being stimulated with voltage from a probe station, white light interferometers can measure deflection to the nanometer scale.
Electrical probing can also be done to gather electrical characteristic information of devices, or just to determine contact or sheet resistance of films.
Particulate control is essential in the clean room environment which these devices are made in. Micralyne uses a particle measurement tool to scan flat wafer surfaces to map and characterize the effectiveness of process and environmentally induced particulate.
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Fig. 4: Cut away view of a single actuating SLV element
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A powerful tool for looking at many facets of the above parameters in detail is our scanning electron microscope), which also has EDX (Energy Dispersive X-ray analysis), useful for development, control and troubleshooting.
Surface characterization that provides lateral and depth spatial distributions is also another important analysis that is routinely performed. There are different techniques available to meet these needs, and the most common ones are AES, XPS and SIMS. The justification for using one technique and not other depends on the capabilities, limitations and the comparative strengths and weaknesses of one particular analytical method.
AES, XPS, and SIMS are true surface analytical techniques, since the detected signals are emitted from top 2-20 surface monolayers. Additionally, they are broadly applicable to detecting any element in the periodic table. However, the detection limit for AES and XPS is in the range about 0.1 to 1.0 atomic percent while SIMS is much more sensitive in the parts per million range. The lateral spatial resolution for AES method is extremely high, several hundred angstroms, while for SIMS is several microns and XPS is the poorest, a few hundred microns.
- By Glen Fitzpatrick and Siamak Akhlaghi -
email: glen@micralyne.com
email: siamak@micralyne.com
web: www.micralyne.com
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Contact Information: Micralyne Inc.
1911-94 Street, Edmonton, Alberta, Canada, T6N 1E6
Phone: 1.780.431.4400 Fax: 1.780.431.4422
Email: info@micralyne.com
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