Apparatus and method for inspecting the edge micro-texture of a semiconductor wafer

Abstract: The invention concerns an apparatus and a method for inspecting the edge micro-texture of a semiconductor wafer. The apparatus is characterized by a diffuse hemisphere having at least one access port and defining a normal axis; a laser target sphere mounted within the hemisphere; a laser source directing a laser beam to the laser target sphere and forming a laser spot on the laser target sphere, said laser beam being reflected from the laser target sphere; a wafer chuck presenting a wafer edge to the reflected laser beam, said wafer edge being tilted from the normal axis; and at least one camera for detecting radiation leaving the access port. The method is characterized in that a laser beam is directed to a laser target sphere which is mounted within a diffuse hemisphere; the laser beam forming a laser spot on the laser target sphere is reflected from the laser target sphere to the edge of a wafer; and radiation which leaves the hemisphere through at least one access port of the hemisphere is detected with at least one camera.

Description: It is well known that the visual, non-contact edge inspection of semiconductor wafers, e.g. silicon wafers, at different stages of wafer processing such as slicing, lapping, edge-shaping (grinding), etching, and polishing is generally performed by trained operators, under a bright light, from halogen or fluorescent lamps, with an naked eye, or using a magnifier lens (2X to 4X magnification). In this case, the submilimetric size of some edge defects makes the visual inspection with the naked eye inefficient. A better alternative to such methods would be to perform the inspection with a video camera microscope with a special illumination light source, such as a gooseneck fiber or a fiber optic ring, attached parallel and coaxial to the camera objective. Most wafer edge inspection equipment currently available uses video cameras with the microscope objective attached, and visible, fluorescent or halogen lamps (Hologenix), or red laser beams (Champman), as light sources for detecting/monitoring the defects and characterizing the edge texture roughness. For these instruments, the light beam impinges the wafer edge surface under one particular angle, 90 degrees or less, and the reflected light that contains wafer edge texture information is captured by the video camera. Generally speaking, the wafer edge illumination methods fall in two categories: 1) dark-field illumination (DF) and 2) bright-field illumination (BF). A bright-field illumination system examines directly reflected light (close to the normal incidence beam), while a dark-field illumination system examines light scattered off the wafer (under small angle incidence beam), almost parallel with the edge. Bright-field illumination means illuminating the edge at normal incidence angle, and looking at the resulting reflected image. This configuration can be described as a "co-axial" or "through-the-lens" geometry, the light source having a solid angle with respect of the part being observed. For example, a 25 mm light source (laser or light bulb), at a distance of 100 mm from the part being imaged, has a solid angle approximately equal to arctan 25/100 = 14 degrees. For flat specular surfaces, such as polished flat edges, the minimum dimension for a light source WBFmin, held at a distance D=(D1-D2) of the camera lens, should be twice the field-of-view width V, plus the camera lens entrance pupil diameter (aperture) A (Fig.1). Then, to completely fill the field-of-view on a planar, normally viewed specular object with uniform light, the BF co-axial illuminators should be larger than and external to the camera lens. When a specular surface deviates from planar, as in a real bevelded edge, for each one degree of full range of surface angle variation, the solid angle of the illumination source must be increased by double in order to appear fully and uniformly illuminated. For example, if a surface has +/-N degree of variance for a total range of 2N degree variance, the total solid angle of light source must be increased by 4N degree, and its diameter from WBFmin to WBF (Fig.2). Dark-field illuminaton uses light impinging the object under oblique low angles, and detection of the scattered or diffracted rays from the inspected surface, outside the direct specular field-of-view. This method is used to obtain information primarily from the scratches and particles that have a very small cross-section when inspected under normal angle, but increase their scattered cross-section under low angles. Since the dark-field uses scattered light, this method does not resolve the real size of the defect, because at this angle the incident spot size of the laser on the surface can be much larger than the smallest detectable defect. If the bright-field illuminator has its solid angle of illumination extended to the horizon in all directions, covering practically the dark-field also, the result would be a solid hemisphere, which includes both the specular and the scattered rays, with very small incident angles, almost parallel to the inspected surface. In the natural world, such continuous uniform illumination geometry is never achieved unless no observer is present, because the observer inevitably creates a discontinuity into the field surrounding the object under observation. In this case we have a continuous diffuse illumination, inside of a hemisphere, that is a theoretical, ideal, uniform, omnidirectional light source, known in optics as Coblentz sphere or "Total Integrated Sphere" (TIS) illuminator. The US-5,461,417 discloses a practical "Continuous Diffuse Illumination" (CDI) light source, based on this TIS principle. The basic feature of the US patent is the combination of a diffuse on-axis light source with adjacent off-axis light sources of equal brightness impinging light onto a diffusive hemisphere, to create a continuous diffuse illumination field. The CDI, described in the US patent, can create a continuous uniform diffuse illuminator if both the on-axis and off-axis light sources are active, at the same time; if only one type is on, the CDI changes itself in a bright-field illuminator, when only the on-axis source is on, or in a dark-field illuminator, when only the off-axis sources are on. It needs not only an extra mirror-like beamsplitter for separating the bright-field light rays from the reflected light incoming from the inspected sample, but a minimum of extra two dark-field light sources to be installed inside the diffusive hemisphere. A light switch between these BF and DF sources can only deliver light beams with fixed directions, diameters, and a specific power ratio. US-5,757,479 discloses an inspection apparatus which illiminates at least part of a sample with a diffused light from a diffused light source and inspects the sample based on light reflected from the sample. EP-605 055 A2 is related to a pyrometer comprising an Ulbricht sphere which is arranged in the radiation path of a measuring radiation, and a diffusor which is placed in the Ulbricht sphere. The present invention concerns an apparatus and a process for inspecting the edge micro-texture of a semiconductor wafer based on a TIS laser source. The invention is characterized by the features of the characterizing part of claims 1 and 7. The invention will be described hereinafter with reference to Fig. 3. Fig.3 shows a preferred embodiment of the claimed apparatus, namely a "Continuous Diffuse Variable Field Laser Illuminator" (CD-VFLI). The apparatus comprises a diffuse hemisphere 3 having a central access port 2 for a video camera 1, and an internal diffuse laser target sphere 4, mounted at 45 degrees from the camera port 2. It is also possible to provide more than one access port and one camera, e.g. two access ports and two cameras. The inspected wafer 5 is positioned under the hemisphere 3 at preferably 45 degrees from normal, which permits a better edge inspection angle, in order to get an evenly focused magnified image of the edge bevel. The wafer will be mounted on vacuum chuck 11 that can be tilted eventually away from the 45 degree position with a gimbal platform, and rotated with a rotary stage. A laser beam 10 with a variable beam diameter, and variable cross-section pattern, solid circle or annular-hollow circle, will impinge the target sphere 4 under a variable incidence angle (around 45 degrees from normal) and will be reflected toward the wafer 5 and the hemisphere 3. The focused laser beam spot can be moved on the target 4, or outside it, on the hemisphere 3. Changing the laser beam diameter, its cross-section pattern, or moving the laser spot position on the target sphere 4 or on the hemisphere 3 will continuously change the illuminator characteristic from bright-field only to dark-field only or to TIS. Thus, by defocusing the laser beam 10, which impinges the laser target sphere 4, from BF 6 to DF 7, one can continuously modify the characteristics of the illumination light source, from a bright-field source only (if the laser beam diameter is limited to WBF max 8), to a dark-field source only (if an annular beam pattern is selected, with a diameter between WBF max 8 and WDF max 9), or to a continuous diffusive uniform light source TIS (if the circular laser beam has a larger diameter than WDF max 9 and reaches the diffuse hemisphere 3). A second possiblility for changing the illuminator characteristics is to keep the laser beam diameter/pattern unchanged, but move the laser focused spot on the laser target sphere, to get a variable angle of incidence of the reflected rays impinging the examined sample, changing then the source characteristic from a bright-field to a dark-field, or to TIS.

Claims: Apparatus for inspecting the edge micro-texture of a semiconductor wafer (5) comprising a diffuse hemisphere (3) having at least one access port (2) and defining a normal axis, an illuminating source, a wafer chuck (11) and at least one camera (1), characterized by a laser target sphere (4) which is mounted within the hemisphere at an angle off-axis from the normal axis; a laser as illuminating source directing a laser beam (10) to the laser target sphere and forming a laser spot on the laser target sphere, the laser beam being reflected from the laser target sphere towards a wafer edge and the hemisphere; the wafer chuck presenting the wafer edge to the reflected laser beam, the wafer edge being tilted from the normal axis; and the camera detecting radiation leaving the access port. Apparatus as claimed in claim 1, characterized in that the wafer edge is tilted at approximately 45 degrees from the normal axis. Apparatus as claimed in claim 1, characterized in that the laser target sphere is mounted at approximately 45 degrees from the access port. Apparatus as claimed in claim 1, characterized by means for varying the diameter of the laser beam. Apparatus as claimed in claim 1, characterized by means for varying the cross section pattern of the laser beam. Apparatus as claimed in claim 1, characterized by means for moving the laser spot on the laser target sphere or on the hemisphere. Method for inspecting the edge micro-texture of a semiconductor wafer (5) by detecting reflected light, characerized by directing a laser beam (10) to a laser target sphere (4) which is mounted within a diffuse hemisphere (3) off-axis from a normal axis defined by the hemisphere, the laser beam forming a laser spot on the laser target sphere; reflecting the laser beam from the laser target sphere towards the edge of a wafer and the hemisphere; detecting radiation which leaves the hemisphere through at least one access port of the hemisphere with at least one camera (1); and providing the possibility of continuously changing an illuminator characteristic by changing the laser beam diameter or by changing the cross-section pattern of the laser beam or by moving the laser spot position on the target sphere or by moving the laser spot on the hemisphere. Method as claimed in Claim 7, comprising establishing a bright-field illuminator characteristic. Method as claimed in Claim 7, comprising establishing a dark-field illuminator characteristic. Method as claimed in Claim 7, comprising establishing a total integrated sphere (TIS) illuminator characteristic.

Inventors: NICOLESCO CLAUDIAN R, NICOLESCO, CLAUDIAN R.
Applicants: WACKER SILTRONIC
Application year: 1998
Main classification: G01N21
IPC codes: G01N21