WO1993009424A1 - Process and device for detecting small ultimate particles on structured surfaces - Google Patents

Abstract

A process for detecting impurities on wafer masks uses a focussed laser beam (11) having a reduced numeric aperture for scanning the wafer mask (14). The geometrical distribution of the diffused light intensity in the space surrounding the scanning laser beam is determined by n detectors (17 to 24). Whereas the diffused light emanating from structures on the wafer mask may be described by cylindrical waves, the diffused light emanating from impurities expands as spherical waves. An irregular intensity distribution results, so that only a determined number of selected detectors generate a signal. From the number and position of the signal-generating detectors, the geometric distribution of the diffused light, therefore its origin, may be deduced.

Description

 Method and device for the detection of tiny particles on structured surfaces

The invention relates to a method and a device for detecting the smallest particles on structured surfaces by scanning the surface with a focused laser beam with a small numerical aperture, detecting the scattered light originating from the laser focus and analyzing the scattered light in an electronic evaluation circuit .

Structured areas in connection with the invention are in particular wafer disks during the manufacturing process of integrated circuits and exposure masks for producing the structures on the wafer disks.

Standardized ICs are manufactured in large quantities today. The wafer exposure processes necessary for the production are subject to a development process which makes it possible to produce ever smaller structures on the corresponding wafers. With this development, the importance of inspection systems increases, with the aid of which contamination on exposure masks can be detected. 75% of the committee in IC production today is based on undetected contaminants such as dust can also occur during the production of the wafers.

The first automatic inspection systems that were developed to control contamination were laser scanning systems. These work with the scattered light that arises during the scanning. A beam of light is focused on the area to be examined and the area is two-dimensionally focussed gropes. As long as unstructured wafers, masks or mirrors are examined, particles with a diameter of 0.15 μm to 0.2 μm can be detected with a detection reliability of 50% to 90%. The disadvantage of these scattered light measuring methods is their sharp drop in sensitivity to detection in the case of structured surfaces, since the scattered radiation which arises on the structures leads to a high radiation background.

Various devices have therefore been developed which, with the aid of linearly polarized light, can distinguish the scattered light which arises on the structures from the scattered light which arises on impurities is scattered, essentially maintains both the direction of polarization and the degree of polarization, while the degree of polarization of the light scattered by impurities changes. Known devices indicate a detection probability of 90% to 95% for particles with a diameter of 0.8 μm to 10 μm. In addition to the reduced sensitivity to detection, the devices require considerable optical design effort.

A modified laser scattering measurement method is given in US Pat. No. 5008558. The direction of incidence of the laser beam is then periodically changed by a small angle. Components are filtered out of the scattered light signal whose frequency corresponds to an integral multiple of the modulation frequency and which have a constant phase position with the modulation frequency over a certain time. This method is intended to detect small particles on the surface of a wafer and those in the space above the wafer.

The invention is based on the task of specifying a method with which reliable detection of even the smallest particles from a scattered light signal is possible even on structured surfaces without the need to use polarizing components and with which a detection sensitivity comparable to the scattered light measurement on unstructured surfaces is possible is achieved. This object is achieved according to the invention in a method of the type mentioned at the outset by the characterizing features of Claim 1. An advantageous development of the method results from the features of claim 2. A device for performing the method is specified in claims 4 and 5.

The invention is based on the knowledge that it is possible to detect the scattered light which has arisen on impurities due to its geometric intensity distribution. While the scattered light emanating from structures can be described by cylindrical waves, the scattered light of impurities propagates in the form of spherical waves. This results in a different spatial intensity distribution I _s / Θ,), which is used for detection. Equation (1) given below describes the light scattered on spheres and serves as a model for the scattering on impurities. Equation (2) describes the light scattered on cylinders and serves as a model for the scattering on structures.

I ₀ = intensity of the l I / o perpendicular ei n- (1) I _s = ^• I ₀ ^• / S (θ, f) | ² falling lights

Scattering intensity

(2) I _s = -) - ^■ I ₀ ^• [T (θ) [ ² iTr S (ΘY) = scattering function sphere

T (θ) = scattering function cylinder θ = elevation angle j = side angle l = laser wavelength r = distance from the laser focus

The two equations make it clear that, apart from constant factors, the intensities are essentially geometrical Distinguish spreading function. Both scattering functions depend on the height angle θ, ie the angle at which the scattering intensity is observed with respect to the optical axis of the incident light. Experimental investigations have shown that the differences in intensity of the scattered light when scattered on a sphere and scattered on a cylinder do not significantly depend on the height angle θ. However, the magnitude of the intensities is greater the smaller the angle between the vertical direction of irradiation and the direction of observation. The detectors should therefore be arranged as close as possible to the lens for focusing the laser beam.

The spreading function for a ball also depends on the side angle, i.e. the observation angle in a plane perpendicular to the optical axis of the incident light. Experimental investigations have shown that the dependence of the intensity distribution on the side angle is all the more measurable if the diameter of the laser focus is smaller than the diameter of the particles and the numerical aperture NA of the laser beam is as small as possible. However, the effect which is essential for the method according to the invention is that scattering radiation occurs in every direction when scattering on a particle.

The scattering function for a cylinder means that scattered radiation only occurs in the plane that is perpendicular to the cylinder axis and that contains the optical axis of the incident light. Of a plurality of detectors arranged around the optical axis of the incident light, only the detectors symmetrical to the cylinder axis and in the vicinity of the scattered radiation plane will generate a signal. The smaller the structures on the area to be examined, the more the actual scattered light distribution depends on the size of the laser focus and the numerical aperture NA. If the laser focus detects several structures with different edge directions at the same time, this results in an unsharp scattered radiation plane and detectors arranged asymmetrically to one another can also generate a signal at the same time. The method used for the method according to the invention However, the significant effect is that, when scattered on a structure, pronounced signal maxima occur at detectors which are essentially symmetrical to one another and no significant signal is produced at the other detectors.

In practice, limits have to be accepted both for the numerical aperture NA of the laser beam and for the diameter of the focus due to the optical correction of the imaging lens. In addition, the particles to be detected do not have a uniform size and the distribution and geometry of the structures on the surfaces to be examined are also very different. An undifferentiated scattered radiation background therefore always occurs. Experimental investigations have shown that this can be compensated for by a suitable reference voltage at the detectors and that the described dependencies in the geometric scattered light distribution can then be clearly demonstrated. A specific selection of the detectors generates a signal based on the scattered light. The geometrical distribution of the scattered light and thus its origin can be inferred from the number and position of the responding detectors. An increase in the detection sensitivity could be achieved if the cone of light to be received is limited by a defined aperture of the detectors. The aperture of the detectors is the numerical aperture NA of the laser beam

Adjust 360 NA according to the relationship α (θ) = - - 2 aresin, where α n cosθ is the aperture angle of the aperture based on the focus of the laser beam. A triangle has emerged as an advantageous approximation for the geometric shape of the aperture diaphragm, the base of which has been aligned parallel to the examined area. With a numerical aperture NA = 0.25 of the laser beam and a focal diameter of 5 μm, particles with a diameter from 0.3 μm could be reliably detected on a structured surface.

In the drawing, a device for performing the method according to the invention is shown schematically. It is described using the figures. These show in 1 shows a laser scanner with scattered light detectors, FIG. 2 shows a circuit arrangement for analyzing the detector signals.

1 shows the head of a laser scanner 10 with an objective 12 focusing a laser beam 11. The focus 13 of the laser beam 12 lies on the surface of a wafer 14. The optical axis 15 of the laser beam 11 is perpendicular to the wafer 14.

A funnel-shaped holder 16 is coupled to the objective 12, into which a plurality of detectors 17 to 24 that receive scattered light are inserted. The detectors are aligned so that their

Axes are directed to the focus 13. The inclination with respect to the optical axis 15 of the laser beam is denoted by θ.

A diaphragm in front of the detector limits the scattered light cone to an angle α (θ). In particular, a triangular diaphragm 25 is used, the base of which is aligned parallel to the surface of the wafer 14.

2 shows a top view of the arrangement of eight detectors 17 to 24 evenly distributed around the focus 13. The arrangement assumes that the scattered light is taken up by light guides and that photomultipliers are provided as photoelectric detectors . The output signals of the photomultipliers are each fed to one input of a differential amplifier, at the other input of which an adjustable reference voltage is applied. The outputs of the differential amplifiers are input into a pulse decoder, which uses them to form a signal code that corresponds to the number and position of the signaling detectors. By comparing with a predetermined signal code, it can be decided in the circuit whether a particle has been detected and whether an output signal is generated.

A particle is the scattering center when more than five detectors emit a signal. A structural element as a scattering center is present if only two detectors opposite one another emit a signal. In between, however, there may also be transitional give shape. If a structural element forms a corner, for example, two pairs of detectors opposite one another can simultaneously emit a signal. All other signal combinations are then to be interpreted as particle scattering.

Claims

Expectations 1. Method for the detection of very small particles on structured surfaces by scanning the surface point by point with a focused laser beam with a small numerical aperture NA, detection of the scattered light originating from the laser focus and analysis of the scattered light in an electronic evaluation circuit, characterized in that - the scattered light distribution by the optical axis of the laser beam is measured by a plurality of n> 3 symmetrically arranged detectors, the recording direction of which is inclined by an angle θ with respect to the optical axis of the laser beam and whose received scattered light cone is limited by a defined aperture is, - the angle of inclination θ is essentially determined by the geometrical dimensions of the laser beam optics, ~ the aperture of the detectors that of the laser beam according to 360 NA Relationship α (θ) = 2arc sin is adapted, n cosθ with α = aperture angle of the aperture based on the focus of the laser beam, - the larger the number n of detectors is chosen, the smaller the numerical aperture of the laser beam the detector signals are compared with a reference voltage, the size of which can be set as a function of the structures present on the surface and the particle size to be detected, the signals exceeding the reference voltage are fed to a decoder circuit in which the detector assigned signals are logically linked to one another, a particle being considered recognized if - if n <5 all detectors generate a signal simultaneously, - if n> 5 more than 5 detectors simultaneously generate a signal. 2. The method of claim 1, d a d u r c h g e k e n n ¬ z e i c h n e t that the detectors «β« is approximately assigned a triangular aperture, the base of which is aligned parallel to the structured surface to be scanned. 3. Device for the detection of tiny particles on structured Surfaces with a laser scanning device (10) for point-by-point scanning of the surface (14) with a focused laser beam (11) with a small numerical aperture, a scattered light detector and an electronic evaluation circuit, that is, that - The laser beam (11) focusing lens (12) of the laser scanning device (10) in a symmetrical distribution concentric to the optical axis (15) of the laser beam (11) a plurality of n> 3 scattered light detectors (17-24) is assigned, wherein - the recording direction of the detectors is inclined at an angle θ with respect to the optical axis (15) of the laser beam (11) and is directed at the focus (13) of the laser beam, - The aperture of the detectors is adapted to that of the laser beam (11) and - The number of detectors is greater, the smaller the aperture of the laser beam (11), - The evaluation circuit is an adjustable reference voltage, a circuit for comparing each detector signal with the reference voltage, a decoder circuit for generating a signal code assigned to the detectors and logic ¬ circuit which, if a defined sig- nalcodes generates an output signal for the detection of a particle. 4. Apparatus according to claim 3, d a d u r c h g e k e n n ¬ z e i c h n e t that the aperture of the detectors is triangular, the base of the triangle (25) is aligned parallel to the surface to be scanned (14).