Hocheol Lee

CMOS chip planarization by chemical mechanical polishing for a vertically stacked metal MEMS integration

In this paper we present the planarization process of a CMOS chip for the integration of a microelectromechanical systems (MEMS) metal mirror array. The CMOS chip, which comes from a commercial foundry, has a bumpy passivation layer due to an underlying aluminum interconnect pattern (1.8 µm high), which is used for addressing individual micromirror array elements. To overcome the tendency for tilt error in the CMOS chip planarization, the approach is to sputter a thick layer of silicon nitride at low temperature and to surround the CMOS chip with dummy silicon pieces that define a polishing plane. The dummy pieces are first lapped down to the height of the CMOS chip, and then all pieces are polished. This process produced a chip surface with a root-mean-square flatness error of less than 100 nm, including tilt and curvature errors.

Horizontally progressive mirror for blind spot detection in automobiles.

The blind spot of automobiles has been a critical issue in driving safety performance. Side mirrors that use an aspheric shape to achieve a wider angle rather than conventional spherical or flat mirrors have been recently permitted from European Union safety regulations. However, these mirrors also cause difficulty in perceiving the speed and distance of an approaching vehicle in the aspheric mirror zones with their decreasing radii of curvature. We demonstrated new side mirrors showing a stable vehicle image by inserting a horizontally progressive zone between the two outer spherical zones used for the far and near views.

Planarization of a CMOS die for an integrated metal MEMS

This paper describes a planarization procedure to achieve a flat CMOS die surface for the integration of a MEMS metal mirror array. The CMOS die for our device is 4 mm × 4 mm and comes from a commercial foundry. The initial surface topography has 0.9 μm bumps from the aluminum interconnect patterns that are used for addressing the individual micro mirror array elements. To overcome the tendency for tilt error in the planarization of the small CMOS die, our approach is to sputter a thick layer of silicon nitride (2.2 μm) at low temperature and to surround the CMOS die with dummy pieces to define the polishing plane. The dummy pieces are first lapped down to the height of the CMOS die, and then all pieces are polished. This process reduces the 0.9 μm height of the bumps to less than 25 nm.

Photorealistic ray tracing to visualize automobile side mirror reflective scenes

We describe an interactive visualization procedure for determining the optimal surface of a special automobile side mirror, thereby removing the blind spot, without the need for feedback from the error-prone manufacturing process. If the horizontally progressive curvature distributions are set to the semi-mathematical expression for a free-form surface, the surface point set can then be derived through numerical integration. This is then converted to a NURBS surface while retaining the surface curvature. Then, reflective scenes from the driving environment can be virtually realized using photorealistic ray tracing, in order to evaluate how these reflected images would appear to drivers.

Slumping Process of the Horizontally Progressive Type of Automobile Side Mirror

The horizontally progressive mirror has been suggested to reduce the blind spot in the automobiles by wide rear-view angle. And glass molding by slumping process and measurement technique are introduced to meet the curvature distribution.

An Optical Surfacing Technique of the Best-fitted Spherical Surface of the Large Optics Mirror with Ultra Precision Polishing Machine

This paper describes a novel method to surface large optics mirror with an extremely high hardness, which could replace the high cost of the repetitive off-line measurement steps and the large ultra- precision grinding machine with ultra-positioning control of 10 nm resolution. A lot of diamond pellet to be attached on the convex aluminum base consists of a grinding tool for the concave large mirror, and the tool was pressured down on the large mirror blank. The tool motion at an interval on the spiral path was controlled with each feed rate as the dwell time in the conventional computer-controlled polishing. The shape to be surfaced was measured directly by a touch probe on the machine without any separation of the mirror blank. Total 40 iterative steps of the surfacing and measurement could demonstrate the form error of RMS 7.8 µm, surface roughness of Ra 0.2 µm for the mirror blank with diameter of 1 m and spherical radius of curvature of 5400 mm.

A novel polishing head with a gimbals-like structure for the high-speed polishing process

In this paper, we suggest a polishing head with a mechanical gimbals-like structure for the optics fabrication. In the small tool polishing processes, several types of polishing mechanism have been tried to get more deterministic and high efficient optical fabrication. The conventional polishing processes to use the contacting pad material of polyurethane or pitch need the higher polishing rate to shorten the overall polishing time for large optics. Therefore, new polishing head mechanism is designed to use the air backpressure and gimbals-like hinge structure to increase polishing velocity. In the following experiment, mechanical adaptability was confirmed both on the flat glass and the convex aluminum surface.

Large-scale metal MEMS mirror arrays with integrated electronics

Design, microfabrication, and integration of a micromachined spatial light modulator ((mu) SLM) device are described. A large array of electrostatically actuated, piston-motion MEMS mirror segments make up the optical surface of the (mu) SLM. Each mirror segment is capable of altering the phase of reflected light by up to one wavelength for infrared illumination ((lambda) equals 1.5 micrometers ), with 4-bit resolution. The device is directly integrated with complementary metal- oxide semiconductor (CMOS) electronics, for control of spatial optical wavefront. Integration with electronics is achieved through direct fabrication of MEMS actuators and mirror structures on planarized foundry-type CMOS electronics. Technical approaches to two significant challenges associated with manufacturing the (mu) SLM is discussed: integration of the MEMS array with the electronic driver array and production of optical-quality mirror elements using a metal-polymer surface micromachining process.