Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2000-05-31
2002-12-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S237500, C356S237600, C250S372000, C118S7230AN, C156S345420
Reexamination Certificate
active
06490032
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an apparatus and method for detecting workpiece defects and, more particularly, to a dark field inspection apparatus and method for detecting defects on a workpiece in a clean room environment.
BACKGROUND OF THE INVENTION
Workpieces such as, for example, wafers, are typically in the form of a flat, substantially planar disk. Workpieces may include semiconductor wafers, magnetic disks and optical disks. For many applications, particularly in the area of integrated circuits, the wafer serves as a high-tech building block. In order to produce quality microelectronic devices, it is desirable that the wafer surface be uniform, planar and devoid of any imperfections.
Chemical mechanical planarization (CMP) is an abrasive process used for polishing the surface of the wafer. The CMP process involves the use of chemical slurries and a circular (sanding) action to polish the surface of the wafer. Slurry is a chemical polishing agent deposed upon the wafer while on a polishing pad. After the polishing step, the wafer is cleaned and rinsed to remove the slurry. A wafer may undergo several steps of cleaning, rinsing, polishing and drying in the CMP machine to remove any debris from the wafer surface. For example, the manufacture of a semiconductor wafer generally includes “layering” dielectrics or metals on the wafer surface. After each layer is formed, the wafer surface is typically cleaned, polished, rinsed and dried. The smooth surface is now ready for further processing steps or for the next layer to be applied to the surface.
The CMP process to some degree smooths out minor defects in the wafer surface which result from, for example, slicing the wafer from a silicon ingot. However, the CMP process itself can introduce or reveal several types of yield-limiting defects, including residual slurry, surface voids and surface particles. Residual slurry remaining on the wafer as a result of inadequate cleaning can unevenly raise the surface level.
Wafer surface defects are considered “events” that lead to or can lead to electrical faults in microelectronic circuitry. The events are first identified and then reviewed to determine if they represent defects that can adversely affect subsequent device performance. Defects are found on both patterned and unpatterned wafers. An unpatterned wafer is a bare silicon wafer or a wafer with various blanket films coated thereon. A patterned wafer has undergone a photolithography process wherein geometric shapes have been transferred to the surface of the wafer.
A surface void is a divot on the wafer surface caused by an embedded particle (e.g., dust) or a weak point in the top layer that is ripped out or dislodged during processing. For example, a small foreign particle may be “coated” on the surface during the layering step and subsequently dislodged from the surface. Thus, a void or dimple remains where the particle once was. Alternatively, surface particles may adhere to the surface after the layering step resulting in a small rise (mound) in the surface. Similarly, the dielectric or metal used in the layering step may become contaminated with a trapped particle prior to or during the layering step. Once applied, the layer will exhibit “rough” areas where contaminates are present.
Microscratches on the wafer surface are caused by small particles, debris or similar foreign objects caught between the polishing pad and the wafer surface. During the polishing step, the particle causes a “scratch” on the wafer surface. In some cases, the microscratch may be inadvertently filled with a deposited material (e.g., tungsten) during a tungsten CMP process application.
After the wafer has undergone a CMP process, including layering the wafer with dielectrics or metals, the wafer proceeds to a photolithography process. Photolithography is a means for transferring shapes from a mask to the surface of a wafer, producing a patterned wafer. The steps involved in the photolithographic process will be briefly discussed. First, the wafer is chemically cleaned to remove any remaining foreign matter from the surface such as, for example, traces of organic, ionic and metallic impurities.
Silicon dioxide (SiO
2
) maybe deposited on the surface to serve as a barrier layer, whereupon a photoresist maybe applied to the surface. The wafer is spun at a high speed, called “spin coating,” to produce a thin uniform layer of photoresist on the surface. Traditionally, there have been two types of photoresist, positive and negative. Negative resists were common in the earlier years of integrated circuit processing, but more recently positive resists are the prevalent type of resist used in VLSI (very-large-scale-integration) fabrication processes. For positive resists, the wafer is exposed with UV (ultraviolet) light wherever the underlying material is to be removed. In this resist, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. The mask used contains an exact copy of the pattern which is to remain on the wafer.
The photoresist coating becomes photosensitive, or imagable, after a soft-baking step. During soft-baking, nearly all of the solvents are removed from the photoresist coating. If considerable solvent remains in the coating, the positive resist may be incompletely exposed.
The mask is a glass plate with a patterned emulsion of metal film on one side. The mask must be aligned with the wafer in the location that the pattern is to be transferred to the surface. The glass plate is a lens which directs the UV light onto the wafer. Exposure time can vary depending upon, for example, the sensitivity of the resist and the lens aperture. Once aligned, the photoresist is exposed through the pattern on the mask with a high intensity UV light, creating on the wafer surface a “printed” replica of the mask pattern.
The wafer is then placed in a developer solution until the resist becomes completely soluble (i.e., for positive resists), and then hard-baked to improve adhesion of the photoresist to the wafer surface and to cure the photoresist.
While not as common as in the CMP process, defects can and do occur during the photolithographic process. Briefly, the defects caused by photolithography can be grouped into three areas; (1) defects in the resist material, (2) problems with the pattern or equipment, and (3) errors in the printing or exposing processes. The photoresist material may become contaminated with a foreign particle, material or substance prior to spin-coating. As a result, the photoresist will contain small spurious particles causing an event. Additionally, residual resist (resist remaining after the developer step) creates “webbing” between the printed lines on the wafer during the exposure step.
A printing defect can result from anomalies on the pattern. Any damage to the pattern (e.g., scratches) can affect the wafer duplication. Lastly, printing errors can cause defects such as, for example, a “bridging” between two printed lines and large areas of unexposed photoresist.
Wafer manufactures ambitiously attempt to prevent defects due to the substantial threat of reduction in final wafer yields. Typically wafer production takes place in a clean room environment. A clean room is broadly defined as an uncontaminated or nearly uncontaminated room which is maintained for the manufacture or assembly of objects, e.g., semiconductor wafers. U.S. Federal standard 209E catagorizes clean rooms into classes defined by the levels of air cleanliness expressed in number of particles per cubic measure. For example, a class 100 room must have less than 100 particles per cubic foot; a class 10 room must have less than 10 particles per cubic foot, and so on. In general, as the class room number decreases, the number of particles per cubic measure allowed decreases.
Many of the particles present in the air, on clothing and on skin, if brought in
Jimenez William D.
Smith Travis L.
Zhou Hao
Farjami & Farjami LLP
Font Frank G.
Newport Fab LLC
Nguyen Sang H.
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