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Research

1. Pupil Function Engineering in Solid Immersion Microscopy for IC Fault Isolation

Aplanatic solid immersion lens (SIL) microscopy is required to achieve the highest possible resolution for next generation silicon IC backside inspection and failure analysis. However, aplanatic SILs are susceptible to spherical aberration introduced by substrate thickness mismatch. We have developed a wavefront precompensation technique using a MEMS deformable mirror and demonstrate an increase in substrate thickness tolerance in aplanatic SIL imaging. Spot intensity increases by at least a factor of two to three are demonstrated for thicknesses deviating several percent from ideal (Patent pending).

Besides aberration control, we also consider polarization control and beam apodization to reach highest resolution.These approaches can be used for next generation silicon IC fault analysis where higher NA aSIL is required

 

   

setup

Closer to the theoretical limit: spherical correction to aplanatic solid immersion imaging with adaptive optics

 

2. Implementation of Adaptive Optics into Different Imaging Systems

a). Imaging through scattering medium:
A reflective microelectromechanical mirror array was used to control the intensity distribution of a coherent beam that was propagated through a strongly scattering medium. The controller modulated phase spatially in a plane upstream of the scattering medium and monitored intensity spatially in a plane downstream of the medium. Optimization techniques
were used to maximize the intensity at a single point in the downstream plane. Intensity enhancement by factors of several hundred were achieved within a few thousand iterations using a MEMS segmented deformable mirror (e.g. a spatial light modulator) with 1020 independent segments.
Increased temporal bandwidth is needed to accommodate scattering media with increased dynamics, including greater rates of bulk translation and eventually scatterers that evolve on a microscopic scale such as fog, smoke, or human tissue. In future work we plan to increase the temporal bandwidth of our adaptive system by using high-speed cameras and improved optimization algorithms.

b). MEMS deformable mirror zoom:
DMs at fixed locations in an optical microscope imaging system was used to provide 2.5x zoom capability, dynamic focus control, and aberration compensation. Zoom is achieved by simultaneously adjusting focal lengths of the two DMs, which are inserted between an infinity-corrected microscope objective and a tube lens.

 

   

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MEMS spatial light modulators for controlled optical transmission through nealy opaque material
zoom

Variable zoom system with aberration correction capability

 

3. Design and Modeling New Types of Wavefront Control Devices

a). Deformable mirror open loop control:
We characterize the errors associated with open-loop control of a microelectromechanical system deformable mirror using an approach that combines sparse calibration of the electrostatic actuator state space with an elastic plate model of the mirror facesheet.We quantify sources of measurement error and modeling error and demonstrate that the DM can be shaped in a single step to a tolerance of ∼8 nm of that achievable with iterative feedback-based closed-loop control. Zernike polynomials with up to 2.5 μm amplitude were made with this approach and yielded a shape error of <25 nm rms in most cases.
Residual errors were shown to be due primarily to spatial resolution limits inherent in the DM (e.g., uncontrollable errors).

b). Manufacturing technology development for MEMS optical components:
One example showing a tip-tilt-piston MEMS DM which has a 1µm stroke @ 50kHz. Potentially good for sweep laser cavity.

 

   
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