Optical inspection method and optical inspection system

An optical semiconductor wafer inspection system and a method thereof are provided for classifying and inspecting defects such as scratches, voids and particles produced in a flattening process by a polishing or grinding technique used for semiconductor manufacturing. The present invention is an optical semiconductor wafer inspection system and a method thereof characterized by obliquely illuminating a scratch, void or particle produced on the surface of a polished or ground insulating film at substantially the same velocity of light, detecting scattered light at the time of oblique illumination from the surface of an inspection target at different angles and thereby classifying the scratch, void or particle.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an optical semiconductor wafer inspection system and a method thereof for classifying and inspecting a defect such as scratch, void or particle as an example of extraneous material produced in a flattening process with a polishing or grinding processing technique used during semiconductor manufacturing.

As a conventional technique for classifying and inspecting defect such as scratch and particle on a semiconductor wafer, there is known a technique described in JP-A-2006-201179. That is, the technique described therein combines a high-angle detection optical system and middle-angle detection optical system which condense and receive scattered light generated from locations illuminated by a right-overhead illumination system and oblique illumination system and convert the scattered light to a luminous intensity signal, thereby sets a plurality of detection conditions and classifies defects on an inspection target based on a relationship between luminous intensity signals detected under the respective conditions.

BRIEF SUMMARY OF THE INVENTION

However, the above described conventional technique necessarily needs to be switched between the right-overhead illumination system and oblique illumination system as for the illumination system as the plurality of detection conditions and does not consider fixing the illumination system to the oblique illumination system and switching between the high-angle detection optical system and middle-angle detection optical system or between the high-angle detection optical system and low-angle detection optical system.

Therefore, the conventional technique does not consider carrying out processes by the high-angle detection optical system and low-angle detection optical system in parallel and increasing the inspection speed either. Here, the “middle-angle” and “low-angle” do not mean absolute angles but mean relative positions of angles lower than the position of high-angle.

It is an object of the present invention to solve the above described problems and provide an optical inspection system and an optical inspection method capable of speedily classifying defect such as scratch, void or particle which exists on the surface of an inspection target such as a semiconductor wafer.

In order to attain the above described object, the present invention provides an optical inspection method for an optical inspection system including a stage on which an inspection target is placed, an oblique illumination system that obliquely illuminates a surface of the inspection target placed on the stage, a high-angle detection optical system that directs light toward the surface at a high angle and detects high-angle scattered light generated from the inspection target with oblique illumination, and a low-angle detection optical system that directs light toward the surface at a low angle and detects low-angle scattered light generated from the inspection target with oblique illumination, wherein luminous intensity detected by the high-angle detection optical system is compared with luminous intensity detected by the low-angle detection optical system to classify a defect which exists on the inspection target.

The present invention provides an optical inspection system including a stage on which an inspection target is placed, an oblique illumination system that obliquely illuminates a surface of the inspection target and a detection optical system that detects scattered light generated from the inspection target through illumination by the oblique illumination system, wherein the detection optical system includes a high-angle detection optical system that directs light toward the surface at a high angle and a low-angle detection optical system that directs light toward the surface at a low angle, and a judgment section that compares luminous intensity detected by the high-angle detection optical system with luminous intensity detected by the low-angle detection optical system and classifies a defect on the inspection target.

The present invention can speedily inspect defects.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of an optical semiconductor wafer inspection system and a method thereof aimed at stable operation of a flattening process used in a semiconductor manufacturing process according to the present invention will be explained with reference now to the attached drawings.

As shown inFIG. 2, when forming an SiO2 film (processing target)22on an Si wafer21and applying CMP (Chemical Mechanical Polishing), this embodiment classifies a scratch23, void24and particle25generated on a wafer10.

However, the Si substrate21does not always exist below the SiO2 film22, but a wiring layer may exist instead.

In the CMP process, polishing is performed to flatten the surface of this SiO2 film22. Therefore, the scratch23which is a polishing scar is produced on the surface of the SiO2 film22as shown inFIG. 2.

Furthermore, the void24corresponds to a bubble (void) existing inside the SiO2 film22that emerges when the surface is polished. The particle25may be dust generated from within a semiconductor manufacturing system stuck to the surface of the SiO2 film22.

For a stable operation of the flattening process in this way, it is important to quickly carry out a defect classification process simultaneously with detection of defects, estimate the mechanism of occurrence of defects and take remedial actions.

Next,FIG. 1shows an example of the optical semiconductor wafer inspection system to implement the embodiment.

The optical semiconductor wafer inspection system includes a wafer10which is an inspection target placed on a stage15, position coordinates of which are measured and traveling in the XY direction of which is controlled, a light source2made up of, for example, an Ar laser having a wavelength of 488 nm and an oblique illumination optical system1made up of a reflecting mirror4.

Furthermore, the optical semiconductor wafer inspection system has a high-angle detection optical system5aand a low-angle detection optical system5bmade up of condensing lenses6a,6b, photoelectric transducers7aand7bmade up of a photomultiplier, CCD camera, CCD sensor and TDI sensor or the like respectively and A/D converters15aand15bthat convert analog luminous intensity signals outputted from the photoelectric transducers7aand7bto digital luminous intensity signals.

The optical semiconductor wafer inspection system further has a stage controller14that controls the traveling of the stage15based on the position coordinates measured from the stage15, judgment sections17aand17bthat detect defects in synchronization with the traveling of the stage15and calculates luminous intensity signals thereof, and an overall control section9that controls the stage controller14, further controls the judgment sections17aand17band receives inspection results obtained from the judgment sections17aand17b.

Examples of the judgment sections17aand17binclude dedicated digital signal circuits that can perform pipeline processing in synchronization with scan clocks of the photoelectric transducers7aand7bthat perform scanning in synchronization with the traveling of the stage15.

Instead of performing the above described synchronization processing as the judgment sections17aand17b, there is also a method of storing the outputs of the A/D converters15aand15bin a memory and performing processing asynchronously, for example. In this case, the inspection speed is slower than the synchronous processing.

The illumination optical system and detection optical system will be explained by quotingFIG. 4.

FIG. 4(a) is a plan view of the arrangement of the illumination optical system and detection optical system seen from right above.

The low-angle detection optical system5bhaving a low angle is located at 180° with respect to the Y axis (minus direction: counterclockwise direction). The oblique illumination optical system1can be changed in three directions of 0°, 45° and 135° with respect to the Y axis.

In other words, the positional relationship between the low-angle detection optical system and oblique illumination system is arbitrarily selectable within a range except 0° to 45° within the XY plane on the surface.

FIG. 4(b) is a front view of the arrangement of the illumination optical system and detection optical system seen from right abeam.

The high-angle detection optical system5ais located at 90° with respect to the XY plane. The low-angle detection optical system5bis located at 12° with respect to the XY plane. The oblique illumination optical system1can be changed in three directions of 3°, 5° and 20° with respect to the X axis. YAG laser 355 nm can be used for the inclined illumination wavelength.

In other words, the oblique illumination system is arbitrarily selectable within a range of angle of elevation of 3° to 20° with respect to the XY plane (on the surface).

Furthermore, the low-angle detection optical system is kept to an angle of elevation of approximately 12° with respect to the XY plane (on the surface).

Next, the detection procedure will be explained.

Oblique illumination light12is irradiated onto the CMP plane of an insulating film22on the wafer10so as to prevent the oblique illumination light12from being directly irradiated onto the surfaces of the condensing lenses6aand6band prevent regularly reflected light of the oblique illumination light12from the wafer10from being directly irradiated onto the surfaces of the condensing lenses6aand6b.

While eliminating the regularly reflected light component generated from the insulating film22, only the scattered light (low-order diffracted light component) emitted from the scratch23, void24or particle25as the defect on the insulating film22is condensed by the condensing lenses6aand6bonto the light-receiving surfaces of the photoelectric transducers7aand7bmade up of a CCD, a TDI sensor or the like. While moving the stage15, the photoelectric transducers7aand7bsuch as a CCD and a TDI sensor are made to scan.

The outputs of the photoelectric transducers7aand7bare A/D-converted by the A/D converters16aand16bin synchronization with the scanning of the photoelectric transducers7aand7bsuch as a CCD and a TDI sensor.

The outputs of the A/D converters16aand16bare inputted to the judgment sections17aand17band the judgment sections17aand17bcalculate scattered light luminous intensity information and defect position information from the scratch23, void24or particle25as the defect on the insulating film22in synchronization with the movement of the stage15and the scanning of the photoelectric transducers7aand7band record the information in the overall control section9as the inspection result.

As the method of judging a defect, the existence of a defect is judged, for example, when the luminous intensity level of the scattered light from the insulating film22is a certain threshold or above.

Next, classification after the existence of a defect is judged will be explained.

The principle of classification for implementing the above described embodiment according to the present invention will be explained usingFIG. 3.

In the case of the scratch33which is a defect on the insulating film32, since this is a shallow concave defect, the luminous intensity signal of the high-angle detection optical system5ais smaller than the low-angle detection optical system5birrespective of the defect size.

In the case of the void34, since this is a deep concave defect, exposure at the edge of the hole seen from the direction of the low-angle detection optical system5bis small and the luminous intensity signal of the high-angle detection optical system5ais larger than the low-angle detection optical system5birrespective of the defect size.

In the case of the particle25, since this is a high convex defect, the luminous intensity signal of the high-angle detection optical system5ais substantially the same as the luminous intensity signal of the low-angle detection optical system5birrespective of the defect size.

Therefore, it is possible to classify the scratch33, void34and particle35according to the following relationship.

Luminous intensity signal of high-angle detection optical system5a<luminous intensity signal of low-angle detection optical system5b

Luminous intensity signal of high-angle detection optical system5a>luminous intensity signal of low-angle detection optical system5b

Luminous intensity signal of high-angle detection optical system5a≦luminous intensity signal of low-angle detection optical system5b

As shown above, when the luminous intensity signal of a defect is small, it is not possible to judge whether the defect is the small-sized particle35or large-sized scratch33using only one detection optical system, whereas it is possible to classify the defect by observing relative intensities of the luminous intensity signal of the high-angle detection optical system5aand the luminous intensity signal of the low-angle detection optical system5b.

In the example above, the classification of the scratch33, void34and particle35on a semiconductor wafer has been explained, but it is also possible to classify a scratch or particle produced on, for example, a hard disk in a manufacturing process of the hard disk.

The above described classification will be explained by quoting the flowchart shown inFIG. 5.

The process starts in step501and a stage scan starts (step502). Signals condensed by the high-angle detection optical system and low-angle detection optical system are A/D-converted (step503). In step504, defects are detected from the A/D-converted signals by the defect judgment section and their luminous intensity levels are calculated. In a comparison between the luminous intensity of high-angle detection and the luminous intensity of low-angle detection calculated (step505), if the luminous intensity of high-angle detection<luminous intensity of low-angle detection, the defect is judged to be a scratch (step506).

On the contrary, in the comparison between the luminous intensity of high-angle detection and the luminous intensity of low-angle detection calculated (step507), if the luminous intensity of high-angle detection>luminous intensity of low-angle detection, the defect is judged to be a void (step508).

On the other hand, in the comparison between the luminous intensity of high-angle detection and the luminous intensity of low-angle detection calculated (step509), if the luminous intensity of high-angle detection≦luminous intensity of low-angle detection, the defect is judged to be a particle (step510).

The above described judgment is repeated until all defects detected during stage scanning are judged. (step511)

When repeated stage scanning on the entire area to be inspected on the wafer is completed (step512), the process ends (step513). Since the above described judgment is made in synchronization with stage scanning, the inspection process is speedily carried out.

Furthermore, since detection processes of the high-angle detection optical system and the low-angle detection optical system are carried out in parallel, this contributes to speed enhancement of the inspection.

The above described classification will be explained in further detail by quotingFIGS. 6 to 10.

FIG. 6shows scatter diagram data of a luminous intensity ratio between particles and voids. This is a data example where the particle35and void34are actually classified.

The relative intensities of the luminous intensity signal of the high-angle detection optical system5aand the luminous intensity signal of the low-angle detection optical system5bare defined as the luminous intensity signal of the low-angle detection optical system5b÷luminous intensity signal of the high-angle detection optical system5a=luminous intensity ratio.

Assuming the threshold is 0.6 in the scatter diagram of the luminous intensity ratio, it is apparent that a defect equal to or greater than the threshold can be classified as the particle35, a defect equal to or less than the threshold can be classified as the void34. This threshold varies depending on the gain of each photoelectric transducer7a,7b, but the ability to classify defects based on a certain threshold is invariable irrespective of the size of the particle or void.

This principle will be explained usingFIGS. 7 to 10.

FIG. 7shows a general relational expression between a reflection factor and an angle of incidence of light on a certain plane. Here, P-wave (Rp) and S-wave (Rs) mean that an electric field vector of light is parallel and perpendicular to the plane of incidence respectively.FIG. 8shows a graph of the reflection factor on an interface with n=1.7 between matter and air as an example. This graph is equal to a relationship when the numerical value (Expression 1) in the relational expression ofFIG. 7is assumed.
(nI=1.0, nII=1.7)  [Expression 1]

The shape of the graph generally varies depending on the refractive index of matter or the like, but the reflection factor increases as the angle of incidence of light approximates to parallel to the plane. When there is no polarization of the electric field vector of the incident light, the reflection factor of light becomes Rp+Rs of the graph. Furthermore, inFIG. 8, the angle perpendicular to the plane is assumed to be 0° and the angle parallel to the plane is assumed to be 90°. If this is applied toFIG. 4,

The reflection factor “Rp+Rs” inFIG. 8becomes approximately 0.07 at 0°. Furthermore, the reflection factor becomes 0.35 at 78°. The reflection factor increases approximately five-fold. There is also an approximately five-fold difference between the particle distribution center inFIG. 6and void distribution center.

This principle will be explained when applied to the state where the scatter diagram data of the luminous intensity ratio inFIG. 6is acquired.

Like the state of the void scattered light shown inFIG. 9, there are not many blocking planes in the scattering direction of the high-angle detection optical system5ain the 90° direction shown inFIG. 4. This direction corresponds to the 0° direction shown inFIG. 8.

On the contrary, there is an interface in the scattering direction of the low-angle detection optical system5bin the 12° direction shown inFIG. 4, where there is a situation in which scattered light is significantly reflected, hardly reaching the low-angle detection optical system5b. This direction becomes the 78° direction shown inFIG. 8. In this way, the luminous intensity ratio shown inFIG. 6is assumed to decrease in the case of the void.

On the other hand, in the state of the particle scattered light shown inFIG. 10, there is nothing that blocks in the scattering detection of the high-angle detection optical system5ain the 900 direction or in the scattering detection of the low-angle detection optical system5bin the 12° direction. This causes the luminous intensity ratio shown inFIG. 6to increase in the case of the particle.

As described above, it is appreciated that keeping the angle of the low-angle detection optical system5bto approximately 12° or below makes it possible to classify the particle and void.