Illumination energy management in surface inspection

An apparatus and associated method for reducing thermal damage on a specimen during an inspection which includes a radiation source for supplying a beam of radiation, and a means for adjusting a first energy level of the beam of radiation to a second energy level as the beam of radiation is variably positioned from a first location on the surface of the wafer to a second location on the surface of the wafer.

BACKGROUND

The subject matter described herein relates to surface inspection, and more particularly to illumination energy management in surface inspection.

As design rules and process windows continue to shrink, integrated circuit (IC) manufacturers face challenges in achieving and maintaining yields and profitability while moving to new processes. The challenges have become more difficult because inspection systems are required to capture a wider range of physical defects on wafer surfaces. One such inspection system includes the use of lasers, which provide high sensitivity to detect small defects, and a relatively high throughput.

Lasers can cause surface damage to a semiconductor wafer, e.g., from thermal shock from the laser during a surface inspection process. In some inspection systems the wafer rotates about a central axis during the inspection process. Hence, the wafer surface near the central axis moves at a slower velocity than the wafer surface near the outer edge of the wafer. Accordingly, damage tends to occur near radial inner portions of a wafer surface because relatively more energy/mm2is imparted to the inner surface.

SUMMARY

Described herein are systems and accompanying methods for managing the amount of laser power applied to the surface of a semiconductor wafer during a surface inspection process.

In one aspect, the laser power can be adjusted as a continuous or discrete function of the radial distance of the laser beam spot from the center of the wafer.

In another aspect, a filter may be interposed between the laser origin and the wafer, such that the filter attenuates a portion of the laser power that varies as a function of the radial distance of the laser beam spot from the center of the wafer.

In yet another aspect, laser power can be managed by varying the spot size of the radiation beam incident on the surface of the wafer as a function of radial distance of the laser beam spot from the center of the wafer or by varying the speed of rotation of the wafer as a function of the radial distance of the laser beam spot from the center of the wafer.

Additional aspects are set forth in part in the detailed description which follows. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.

DETAILED DESCRIPTION

Described herein are exemplary systems and methods for illumination energy management in surface inspection. In the following description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments. However, it will be understood by those skilled in the art that the various embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the particular embodiments.

Various methods described herein may be embodied as logic instructions on a computer-readable medium. When executed on a processor the logic instructions cause a processor to be programmed as a special-purpose machine that implements the described methods. The processor, when configured by the logic instructions to execute the methods described herein, constitutes structure for performing the described methods.

FIG. 1is a simplified schematic view of a typical surface inspection system100. To simplifyFIG. 1, some of the optical components of the system have been omitted, such as components directing the illumination beams to the wafer. A wafer102is illuminated by a normal incidence beam104and/or an oblique incidence beam106. Wafer102is supported on a chuck108which is rotated by means of a motor110and translated in a direction by gear112so that beams104and/or106illuminate an area or spot102awhich is caused to move and trace a spiral path on the surface of wafer102to inspect the surface of wafer102. Motor110and gear112are controlled by controller114in a manner known to those skilled in the art.

The area or spot102ailluminated by either one or both beams104,106on wafer102scatters radiation from the beam(s). The radiation scattered by area102aalong directions close to a line116perpendicular to the surface of the wafer and passing through the area102ais collected and focused by lens collector118and directed to a photo-multiplier tube (PMT)120. Since lens118collects the scattered radiation along directions close to the normal direction, such collection channel is referred to herein as the narrow channel and PMT120as the dark field narrow PMT. When desired, one or more polarizers122may be placed in the path of the collected radiation in the narrow channel.

Radiation scattered by spot102aof wafer102, illuminated by either one or both beams104,106, along directions away from the normal direction116is collected by an ellipsoidal collector124and focused through an aperture126and optional polarizers128to dark field PMT130. Since the ellipsoidal collector124collects scattered radiation along directions at wider angles from the normal direction116than lens118, such collection channel is referred to as the wide channel. The outputs of detectors120,130are supplied to a computer132for processing the signals and determining the presence of anomalies and their characteristics.

Various aspects of surface inspection system100are described in U.S. Pat. No. 6,271,916 and U.S. Pat. No. 6,201,601, both of which are incorporated herein by reference. An exemplary surface inspection system is available from KLA-Tencor Corporation of San Jose, Calif., the assignee of the present application.

FIG. 2is a simplified schematic illustration of optical components200of surface inspection system100in accordance with an embodiment. It should be understood that optical components200can be included and integrated into surface inspection system100, with only the modifications necessary to encompass embodiments described herein. For clarity, some components of the surface inspection system have been omitted fromFIG. 2.

Optical components200of surface inspection system100direct illumination beam(s)104,106to wafer102. Accordingly, optical components200include at least one radiation light source, such as a laser202, and a filter or attenuator204that controls the energy level of incidence beam(s)104,106that are delivered to wafer102. As discussed in more detail below, in one embodiment, motion controller114controls the variable positioning of attenuator204to set the energy level of the laser power in system100. Motor110and gear112are also controlled by motion controller114to rotate and translate wafer102as appropriate to achieve the proper scanning motion.

AlthoughFIG. 2depicts a single laser202, a greater number of lasers may be included among optical components200, as appropriate for a particular application. Further, alternative radiation light sources, such as, for example, xenon lamps, light-emitting-diodes and the like, may be used instead of laser202.

Attenuator204may be, for example, an addressable array of selected neutral fixed-density filters; a continuously variable neutral density filter; a plurality of polarizers that includes at least one rotatable polarizer; a rotating polarization retarder placed in front of a polarizer and the like, all of which are known in the art. Beam106passes through attenuator204, which produces an attenuated, collimated beam with a desired power level.

Referring now toFIG. 2in one operational embodiment, the dosage or energy level D of laser202is automatically adjusted as a function of the radial scan distance of the laser beam spot or scan spot from the center of wafer102. As illustrated in Equation 1, with a given laser power P, wafer rotational speed ω, and wafer translational speed Vx, dosage D (light power density×exposure time) is approximately proportional to:
P/(ω×r+Vx)  (1)
where r is equal to the current scan radius of the scan spot from the center of wafer102. The effective dosage D, therefore, increases as the radius of the scan radius decreases, thus reaching a maximum at r=0.

In accordance with one embodiment, as the scan radius r approaches 0, the laser power P is simultaneously ramped down. In operation, laser controller206of surface inspection system100can be made to drive laser202in a laser power feedback loop, thus ramping a laser power profile as a function of scan radius r. Alternatively, a calibration table can be provided from which a correction factor for discrete scan radius can be determined. The calibration table can be system specific.

The loss of signal and thus the potential loss of signal/noise ratio (S/N) is compensated by a simultaneously adjustable noise filter and amplitude correction. The S/N can be determined as set forth in Equation 2, where Rtis equal to the tangential spot size and Rris equal to the radial spot size.

In this embodiment, a constant S/N, that is, a constant energy level, can be maintained while ramping down laser power P in approximate proportion to scan radius r, if Rr, Rt, ω are maintained as a constant. That is:
P∝r  (3)
It has been shown that the maximum dosage D may be reduced by a factor of up to 10 as the scan radius r approaches 0 without negatively affecting the sensitivity of surface inspection system100.

In another embodiment, dosage D can be adjusted by varying the rotational speed ω of the wafer as a function of the radial distance of the beam spot from the center of the wafer. As shown in Equation 4, if P, Rr, Rtare maintained constant, then rotational speed ω is approximately proportional to the scan radius from the center of wafer102as follows:

Accordingly, in operation, controller114can cause the speed of motor110to vary the rotational speed of wafer102, while simultaneously translating wafer102under incidence beam(s)104,106. In this manner, as r approaches 0, the rotational speed of wafer102increases to reduce the energy/mm2imparted to the inner surface of wafer102.

In another embodiment, dosage D can be adjusted by varying the spot size as a function of the radial distance of the beam spot from the center of wafer102. As shown in Equation 5, if P, Rr, ω are maintained constant, then spot size Rtis approximately proportional to the scan radius from the center of wafer102as follows:

Accordingly, in operation, laser controller206can operate to continuously refocus laser beam(s)104,106using a focusing device, such as a lens assembly and the like, to cause the spot size of the beam to vary. In this manner, as r approaches 0, the spot size can be increased to reduce the energy/mm2imparted to the inner surface of wafer102.

In some embodiments, dosage D is adjusted by interposing a filter or attenuator204between the laser origin and wafer102, where the filter attenuates a portion of the laser power that varies as a function of the radial distance of the beam spot from the center of wafer102.

In one operational embodiment, attenuator204of surface inspection system100can be configured for the selection of or conditioning of filters, polarizers, and the like, to pass or reject specific wavelengths to set appropriate attenuation levels. In this embodiment, scan motion can be synchronized with attenuation value and proper amplitude correction, such that dosage D is automatically adjusted as a discrete function of the radial distance r of the scan spot from the center of wafer102.

In another operational embodiment, attenuator204is motorized such that it can be variably positioned by motion controller114between laser202and wafer102. In this manner, incidence beam(s)104,106travels through attenuator204as the beam is delivered to wafer102. By manipulating and thus varying the distance of attenuator204from the origin or source of the radiation from laser202, the power level of beam(s)104,106is also made variable. In this embodiment, scan motion can be synchronized with attenuation value and proper amplitude correction, such that as the scan radius r approaches 0, the proper attenuation value is provided by the adjustment of the position of attenuator204relative to laser202.

FIG. 3is a flowchart which provides a general method for managing the radiation or laser power by employing the various embodiments described herein. Referring toFIG. 3, at operation310a surface scan is initiated. In various embodiments initiating a surface scan includes causing a surface of a wafer to be impinged by a beam of radiation at a first location such that the beam of radiation imparts a first level of energy to the surface. At operation315the radiation energy level imparted to the surface is varied as a function of the radial distance from the center of the wafer as the radiation beam moves from the first location on the surface of the wafer to a second location on the surface of the wafer.

In one embodiment, varying the energy level includes varying the power level of the radiation beam as a function of radial distance of the radiation beam from the center of the wafer.

Alternatively, varying the energy level may include varying the speed of rotation of the wafer as a function of the radial distance of the radiation beam from the center of the wafer, or varying a spot size of the radiation beam as a function of radial distance of the radiation beam from the center of the wafer.

In another alternative embodiment, adjusting the first energy level to the second energy level includes varying the position of a filter relative to the origin of the radiation beam as a function of radial distance of the radiation beam from the center of the wafer.

Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be all referring to the same embodiment.