PHOTODETECTOR CONFIGURATIONS

A device and method for detecting radiation comprising: a radiation-sensitive surface composed of an array of electrically inter-isolated radiation-sensitive elements (e.g. avalanche photodiode (APD), PIN diode, or scintillation sensor), each radiation-sensitive element is adapted to generate an electric current in response to absorbing radiation; an array of conversion circuits, each conversion circuit electrically coupled to a respective radiation-sensitive element and configured to generate an output signal indicative of the current generated by the radiation-sensitive element coupled thereto; and one or more summation arrangements, each summation arrangement coupled to a respective group of the conversion circuits, and configured to produce a summation result indicative of the radiation absorbed by respective group of the conversion circuits. The radiation-sensitive surface may be shaped as a dome-shape surface. The radiation-sensitive elements may be associated with radiation-sensitive planes such that all of the radiation-sensitive elements are directed toward a focal point on an inspected surface.

FIELD OF THE INVENTION

The present invention relates generally to radiation sensitive detectors, and more particularly, to methods and systems for efficiently evaluating objects under test using radiation sensitive detectors.

BACKGROUND OF THE INVENTION

Optical detectors, for example, composed of a photodiode followed by a transimpedance amplifier (TIA) are well known.

The performance of optical detectors is considerably affected by noise. Various techniques have heretofore been disclosed in the art in order to reduce the noise level at the TIA output. There is a need for improved techniques of employing radiation sensitive detectors.

BRIEF SUMMARY OF THE INVENTION

According to embodiments of the invention, there is provided a device for detecting radiation comprising: a radiation-sensitive surface composed of an array of electrically inter-isolated radiation-sensitive elements, each radiation-sensitive element is adapted to generate an electric current in response to absorbing radiation; an array of conversion circuits, each conversion circuit electrically coupled to a respective radiation-sensitive element and configured to generate an output signal indicative of the current generated by the radiation-sensitive element coupled thereto; and one or more summation arrangements, each summation arrangement coupled to a respective group of the conversion circuits, and configured to produce a summation result indicative of the radiation absorbed by respective group of the conversion circuits.

The radiation-sensitive elements may be formed of one or more of a group consisting of: avalanche photodiodes (APDs), PIN diodes, and scintillation sensors.

Each conversion circuit may comprise a transimpedance amplifier.

The summation arrangement may comprise a digital processing unit.

The device may be arranged with one or more openings configured for allowing radiation to pass through the one or more openings.

The radiation-sensitive surface may be shaped as a dome-shape surface.

Different radiation-sensitive elements may be associated with different radiation-sensitive planes such that all of the radiation-sensitive elements are directed toward a focal point on an inspected surface, an angle between a normal to a radiation-sensitive plane and the inspected surface is inversely proportional to the length of said normal, and the shortest distance between each radiation-sensitive plane and the inspected surface is equal to or greater than a preconfigured minimum working distance.

The radiation-sensitive segment may further be composed of a support arrangement to which the radiation-sensitive elements are mechanically attached and wherein the radiation-sensitive elements are arranged in one of the formats: adjacent to each other; spaced-apart from each other; and a combination thereof.

In accordance with an embodiment of the invention, a method for reducing noise in an inspection system is disclosed, comprising: irradiating an inspected surface with one or more electron beams; by a segmented radiation-sensitive surface composed of an array of electrically inter-isolated radiation-sensitive elements, detecting resulting radiation emanating from the inspected surface in response to the irradiating; converting, by an array of conversion circuits, each conversion circuit electrically coupled to a respective radiation-sensitive element, the currents produced by the radiation-sensitive elements due to detecting the emanated radiation, to respective voltage signals; producing, by one or more summation arrangements, each summation arrangement coupled to a respective group of the conversion circuits, a summation result indicative of the radiation absorbed by respective group of the conversion circuits; and analyzing the radiation detected by the segmented radiation-sensitive surface according to the resulting summation signal.

In accordance with an embodiment of the present invention, a radiation-sensitive system is disclosed comprising: An array of electrically inter-isolated radiation-sensitive elements forming a radiation-sensitive surface, wherein each radiation-sensitive element is adapted to generate an electric current in response to absorbing radiation; an array of conversion circuits, each conversion circuit electrically coupled to a respective radiation-sensitive element and configured to generate an output signal indicative of the current generated by the radiation-sensitive element coupled to said conversion circuit; and at least one summation arrangement coupled to a group of the conversion circuits, wherein the conversion circuits that belong to said group are electrically coupled to radiation-sensitive elements that constitute a contiguous segment of the radiation-sensitive surface comprising at least part of the array of radiation-sensitive elements, said summation arrangement configured to produce a summation result indicative of the whole radiation absorbed by said segment of the radiation-sensitive surface.

In some embodiments, the radiation-sensitive surface comprises an opening that allows a primary radiation beam to pass through and irradiate an inspected surface. Resulting radiation emanated from the inspected surface is then detected by the radiation-sensitive surface. In some of these embodiments, this technique is employed in various evaluation applications such as defect detection, defect review and critical dimension inspection in semiconductor wafers and in masks for manufacturing semiconductor wafers.

In some embodiments, the radiation-sensitive surface comprises an arrangement of individually cut sections, wherein a collection of section sides, one of each section, forms an opening in the radiation-sensitive surface having either a partial or a complete polygon shape.

In some embodiments, the conversion circuits comprise transimpedance amplifiers.

In some embodiments, the radiation-sensitive elements are mechanically attached to a support arrangement, e.g. a ceramic substrate such that all the elements are directed to a common focal point on an inspected surface, an angle between the normal to a radiation-sensitive plane and the inspected surface is inversely related to the length of said normal to the radiation-sensitive plane, and the shortest distance between each radiation-sensitive plane and the inspected surface is equal to or greater than a preconfigured minimum working distance.

In accordance with an embodiment of the present invention, there is also provided a method for reducing noise in an object evaluation system. The method comprises the steps of: irradiating an object under evaluation, detecting by several adjacent electrically inter-isolated radiation-sensitive elements the radiation emanating from the object due to the irradiation, converting currents generated in response to the detection to respective voltage signals, and summing the voltage signals.

In accordance with an embodiment of the present invention, there is also provided a radiation-sensitive device comprising radiation-sensitive elements that are mechanically attached to a support arrangement, e.g. a ceramic substrate such that all the elements are directed to a common focal point on an inspected surface, an angle between the normal to a radiation-sensitive plane and the inspected surface is inversely related to the length of said normal to the radiation-sensitive plane, and the shortest distance between each radiation-sensitive plane and the inspected surface is equal to or greater than a preconfigured minimum working distance.

In some embodiments, there is at least one traversing opening in the radiation-sensitive device. In some of these embodiments, the at least one traversing opening is formed to allow radiation to pass therethrough toward the focal point, thereby allowing a responsive radiation to emanate therefrom toward the radiation-sensitive elements. In some of these embodiments, this technique is employed in various evaluation applications such as defect detection, defect review and critical dimension inspection in semiconductor wafers and in masks for manufacturing semiconductor wafers.

In some embodiments some of the radiation-sensitive element in the device is electrically coupled to a respective conversion circuit, said respective conversion circuit configured to generate an output signal indicative of the electrical current generated by the radiation-sensitive element coupled to the respective conversion circuit, and there is at least one segment of the radiation-sensitive device comprising at least part of the radiation-sensitive elements, wherein all the output signals resulting from the radiation-sensitive elements comprised in said segment are conveyed to a summation arrangement configured to produce a summation result indicative of the whole radiation absorbed by said segment of the radiation-sensitive device. In some of these embodiments, the conversion circuits comprise transimpedance amplifiers.

In some embodiments, the radiation-sensitive device comprises an arrangement of individually cut sections, wherein a collection of section sides, one of each section, forms an opening in the radiation-sensitive surface having either a partial or a complete polygon shape.

In accordance with an embodiment of the present invention, there is also provided an arrangement of individually cut radiation-sensitive sections arranged to form a radiation-sensitive surface, wherein a group of section sides, one of each section, form an opening in the radiation-sensitive surface having either a partial or a complete polygon shape.

In some embodiments, the above opening is formed to allow radiation to pass therethrough toward an object under evaluation so as to result in radiation emanating from the object under evaluation toward the radiation-sensitive surface. In some of these embodiments, this technique is employed in various evaluation applications such as defect review and critical dimension inspection in semiconductor wafers and in masks for manufacturing semiconductor wafers.

In some embodiments, at least one of the radiation-sensitive sections comprises an array of electrically inter-isolated radiation-sensitive elements, each electrically coupled to a respective conversion circuit, said respective conversion circuit configured to generate an output signal indicative of the electrical current generated by the radiation-sensitive element coupled to the respective conversion circuit, and there is at least one segment of the array comprising at least part of the radiation-sensitive elements contained in the array, wherein all the output signals resulting from the radiation-sensitive elements comprised in said segment are conveyed to a summation arrangement configured to produce a summation result indicative of the whole radiation absorbed by said segment of the array. In some of these embodiments, the conversion circuits comprise transimpedance amplifiers.

In some embodiments, at least one of the radiation-sensitive sections comprises an array of electrically inter-isolated radiation-sensitive elements that are disposed to form an array of radiation-sensitive planes wherein each radiation-sensitive plane comprises one or more radiation-sensitive elements, said at least one of the radiation-sensitive sections further comprises a support arrangement to which the radiation-sensitive elements are mechanically attached, and wherein the support arrangement is structured such that all the radiation-sensitive elements are directed toward a focal point on an inspected surface, an angle between the normal to a radiation-sensitive plane and the inspected surface is inversely proportional to the length of said normal, and the shortest distance between each radiation-sensitive plane and the inspected surface is equal to or greater than a preconfigured minimum working distance. In some of these embodiments, this technique is employed in various evaluation applications such as defect review and critical dimension inspection in semiconductor wafers and in masks used for manufacturing semiconductor wafers.

In all the above embodiments, the involved radiation-sensitive devices and elements may comprise avalanche photodiodes (APDs), PIN diodes, or scintillation sensors. In some of these embodiments, the involved arrays may comprise radiation-sensitive elements of more than one type. For example, APD combined with a scintillator, or a PIN diode combined with a scintillator, can be used.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide new techniques that improve signal to noise ratio in inspection systems that employ optical detectors. Embodiments of the invention will be presented with respect to their use in inspection systems for evaluating semiconductor wafers and masks for manufacturing semiconductor dies, and in various related applications such as defect detection, defect review and critical dimension inspection. These techniques exploit inherent properties of photodetectors, such as capacitance-area relation and efficiency-‘incidence angle’ relation.

Referring toFIG. 1Athere is shown a block diagram that schematically illustrates a part of a wafer inspection system100, in accordance with an embodiment of the present invention. InFIG. 1A, a Scanning Electron Microscope (SEM), represented by a SEM column104, emits a primary electron beam108. In other embodiments, other beam sources may be employed, e.g. a laser emitting a single primary photon beam or multiple beams, an electron source emitting multiple beams.

Electron beam108impinges on an inspected surface112at an incidence point117. In the described embodiment inspected surface112belongs to a semiconductor wafer, which is mounted on a stage116. When the electrons in beam108penetrate the wafer they are scattered into an onion shaped volume. In this scattering process low energy electrons, so called secondary electrons or SE are generated. High energy electrons, so called backscattered electrons or BSE are also generated. A detection system (shown in part inFIG. 1Aas device120) collects the radiation that emanates from the volume around incidence point117(depicted inFIG. 1Aby dashed arrows118). The electrons118are detected by being absorbed by the radiation-sensitive surface (not specifically shown inFIG. 1A) of a radiation-sensitive device120which is shown in cross section view. In the middle of radiation-sensitive device120, there is a traversing opening126that allows electron beam108to pass through radiation-sensitive device120toward semiconductor wafer112.

Radiation-sensitive device120comprises of an array of adjacent electrically inter-isolated photodiode elements indicated by reference numeral130(only one numeral130is depicted inFIG. 1Afor the sake of simplicity). In one embodiment, each element130comprises avalanche photodiode (APD). In other embodiments, other radiation-sensitive device types are employed such as PIN diode, and scintillation sensor. A combination of devices can be used. A combination of an APD or PIN diode with a scintillator can be used. When a scintillation sensor is employed, it can be coupled to any type of a radiation-sensitive device, including photomultiplier tube (PMT), either directly or through a light guide. While absorbing radiation118, each element130generates an electric current approximately proportional to the radiation absorbed by the element. Conductive wires132convey the generated currents to a conversion stage136comprising an array of conversion circuits, one per each element130. Conversion stage136is further described below. Conversion stage136then outputs, through an output142, one or more voltage signals representing the above currents, for analysis and/or evaluation in a subsequent stage not shown inFIG. 1A.

Radiation-sensitive device120is segmented such that each segment comprises a contiguous group of adjacent elements that are indicated by a dashed ellipse140. In one embodiment, each segment140comprises three APD elements, as illustrated inFIG. 1A. However, this is an indicative number that may also represent more elements that cannot be shown in a cross section view. Each segment140accounts for a respective individual voltage signal at output142, as described below. An example top view of segments is shown inFIGS. 3A and 3Bdescribed below.

FIG. 1Bis a partial zoom out of wafer inspection system100shown inFIG. 1A.FIG. 1Billustrates a group of three conversion circuits within conversion stage136, each conversion circuit indicated by reference numeral148. Conversion stage136comprises four such groups, each connected to a respective segment140through part of wires132. A group148of conversion circuits are respectively connected to the group of three APD elements in segment140that is shown inFIG. 1B, such that each conversion circuit148converts the current produced by an APD element to a respective voltage output signal at the output of that conversion circuit. In the described embodiment, each conversion circuit148comprised a transimpedance amplifier (TIA). In other embodiments other conversion circuit types may be employed.

Conversion circuits148are followed by a summation arrangement152comprising an operational amplifier156that receives the voltage output signals produced by conversion circuits148through respective resistors160. A feedback resistor164then determines the voltage level at the output of summation arrangement152. This output voltage thus constitutes a summation signal which provides a summation result indicative of the overall radiation absorbed by segment140shown inFIG. 1B. It follows from the above that conversion stage136produces, through output142, four summation signals, each related to a different segment140. In some embodiments other summation arrangement are employed such as a digital processing unit, which first converts the output signals of conversion circuits148to digital values and then sums them numerically. In typical embodiments, the digital processing unit is implemented in hardware, software or a combination thereof.

The motivation for producing summation signals, as explained above, is to improve the Signal to Noise Ratio (SNR) at output142, assuming that most of the noise is a readout noise resulting due to the input noise of conversion circuits148rather than shot noise of photodiodes130. This improvement can be explained, with reference toFIG. 1B, by comparing between the resulting SNR related to segment140in the following two cases:

(A) Segment140consists of a single photodiode element connected to a single conversion circuits148(this case is not shown inFIGS. 1A and 1B).

Let us assume that the SNR in case A is V/N where V stands for the area of segment140and N stands for Noise Intensity (N may be calculated, for example, as the Root of Sum of Squares (RSS) value) of the current collected by segment140. Let us calculate now the SNR in case B. As the desired signal is proportional to the area of segment140, it is not meaningfully affected by the split and therefore it is equal to V. However, the capacitance of each photodiode element130is ⅓ of the capacitance of segment140since a photodiode capacitance is proportional to the photodiode area. Consequently, the noise voltage at the output of each conversion circuits148is (⅓)*N. The noise voltage at the output of summation arrangement152is the rout-mean-square of the noise at the outputs of the three conversion circuits148, i.e. (1/√3)*N. Hence the SNR in case B is √3*V/N, i.e. √3 higher than in case A. In the general case the resulting SNR is Aix higher, where X is the number of elements per segment.

The radiation-sensitive elements130may share a common support surface, for example, a ceramic substrate. The radiation-sensitive elements130may be placed adjacently to each other, up to a physical touch. In such an adjacent placement of the radiation-sensitive elements130, the radiation is collected in a continuous manner across the radiation-sensitive surface.

FIGS. 2A and 2Bdepict two views of a multiplane radiation-sensitive device220which is part of a wafer inspection system200, in accordance with an embodiment of the present invention.FIG. 2Adepicts a cross-sectional side view andFIG. 2Bdepicts a bottom view. The following explanations relate toFIGS. 2A-2Btogether. In the following, only device220is described since the rest of system200was already included inFIG. 1A. Device220comprises a ceramic substrate224, which constitutes a support arrangement for an array of electrically inter-isolated radiation-sensitive elements230. In the side of ceramic substrate224that faces inspected surface112there are three concentric octagonally shaped grooves225,226,227, each comprising eight inclined planes directed to incidence point117. In other embodiments, different groove numbers and shapes may be realized. Electrically inter-isolated photodiode elements230, which are principally the same as photodiodes130in inFIG. 1A, are mechanically attached to the inclined planes in ceramic substrate224(only part of elements230are indicated by reference numeral for the sake of drawing clarity). As a result, all elements230are directed toward incidence point117, which thereby constitute a focal point in the sense that radiation118impinges on elements230substantially perpendicularly.

The elements230attached to any groove side constitute a contiguous radiation-sensitive plane. In device220, the planes of grooves225and226are one element planes. Each of the eight planes of groove227comprises a contiguous radiation-sensitive segment of device220comprising four elements230, indicated inFIG. 2Bby a dashed ellipse240.

In the described embodiment, all the elements in each plane240are coupled through conversion circuits, such as circuit148, to a summation arrangement, such as arrangement152, for improving the detection SNR as explained above with regard toFIGS. 1A and 1B. The rational for this is the following: The planes disposed farther from point117are of larger area and receive lower radiation density. Consequently the dominant noise affecting the SNR while detecting their received radiation in readout noise, the SNR is typically low and their larger area allows partitioning to several elements. In other embodiments segments230may comprise part of a plane or several planes. The geometrical and spatial arrangement of the planes may be set to collect radiation in predetermined collection angles.

As shown inFIG. 2A, ceramic substrate224is formed such that the angle θ between the normal to a radiation-sensitive plane and the inspected surface is inversely proportional to the length of said normal. This is done so as to keep the Working Distance operational parameter at acceptable value. The shortest distance between each plane and the inspected surface equal to or greater than a predefined Working Distance indicated inFIG. 2Aby reference numeral234. In the example ofFIG. 2A, the radiation sensitive device220is illustrated as the lower part of the SEM column, and thus the Working distance is shown as reflecting the distance between the wafer112and device220.

The Working Distance in SEM is the distance from the lower SEM lens to the inspected object at which the beam is focused. For various applicational requirements, the Working Distance needs to be minimized. For certain applications, the radiation sensitive array must have certain thickness and this requirement may limit the ability to minimize the Working Distance.

According to an embodiment of the invention, the radiation-sensitive surface is shaped as a dome-shape surface. The Forming of the radiation-sensitive surface as a dome-shaped surface would allow high sensor response for low collection angles; however, implementing a dome-shaped surface would require thicker carrying substrate. While in general, thicker carrying substrate may be useful, there are certain operational constrains that require thinner carrying substrate. For example, for certain SEM imaging applications, the Working Distance operating parameter (illustrated by numerical reference234inFIG. 2A) is impacting the imaging resolution. Higher resolution may require shorter Working Distance.

The structure illustrated inFIG. 2Aallows for achieving high sensor response, while using limited radiation-sensitive device′ thickness. The structure illustrated inFIG. 2Aallows for efficient collection of signals at low radiation angles.

According to yet another embodiment of the invention, radiation sensitive device with thicker substrate can be used. For example, the radiation sensitive device220may be aligned with the bottom part of SEM column104or be set at a higher distance from the wafer112.

FIG. 3Aillustrates a top view of a radiation-sensitive device320a, which is part of a wafer inspection system (not shown inFIG. 3A) like those shown in the previous figures. Radiation-sensitive device320acomprises an arrangement of four individually cut sections325a,325b,325cand325d. A collection of section sides, one of each section, forms a square opening326ain radiation-sensitive device320a, which allows radiation to pass through. In other embodiments other polygonal shapes of opening326aare formed, by employing various segment numbers and by using unequal edge sizes. Each section comprises multiple electrically inter-isolated photodiode elements330. Each dashed ellipse340indicates a segment comprising two elements, which would yield a summation signal at the system output, as explained with regard to the previous FIGs. In other embodiments, other figures and shapes of element per segment are employed. The figures and shapes of element per segment may not be uniform over the same radiation-sensitive sections. In some embodiments the radiation-sensitive surface of at least part of sections325ato325dis formed to have a plurality of planes all directed to the same focal points as described above with regard toFIGS. 2A and 2B. In other embodiments, sections325a,325b,325cand325dare inclined such that a dome-shape structure is realized.

FIG. 3Billustrates a top view of a radiation-sensitive device320b, which differs from radiation-sensitive device320ain that it comprises only three sections325a,325band325c. Consequently, the created opening326bhas a partial polygon shape.

The realization of a dome-shape device may require the use of a support arrangement to which the radiation-sensitive elements are mechanically attached. For example, the support arrangement may comprise a ceramic substrate. The realization of the dome-shape device is presented herein as integrated with the electrical segmentation of the radiation-sensitive elements, each with its conversion circuit and with a summation arrangement. The dome shape device may be realized without the electrical segmentation of the of the radiation-sensitive elements. Further, the dome shape device may be realized by the use of spaced-apart radiation-sensitive elements.

The above description has focused on the specific system and device components that are essential for understanding certain features of the disclosed techniques. Conventional components that are not needed for this understanding have been omitted fromFIGS. 1A to 3Bfor the sake of simplicity but will be apparent to persons of ordinary skill in the art. The configurations shown inFIGS. 1A to 3Bare example configurations, chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable configurations can also be used. For example, in some embodiments, radiation-sensitive devices as described above may comprise several openings, in various locations on the radiation-sensitive device, for allowing irradiation by more than a single primary beam and/or for detecting reflected radiation in separate radiation-sensitive devices. In some embodiments, radiation-sensitive devices may be employed that comprise any type of radiation-sensitive elements, as well as different element types in the same radiation-sensitive device.

FIG. 4is a flowchart400that schematically illustrates a method for reducing noise in a wafer inspection system, in accordance with an embodiment of the present invention. The method begins with an irradiating step404, in which a primary electron beam108emitted from SEM column104irradiates an object112under evaluation, which is, in an embodiment, a semiconductor wafer under inspection. In a detecting step408that follows, a radiation-sensitive segment such as140,240, and340, of a radiation-sensitive device such as120,220,320aand320b, comprising several adjacent electrically inter-isolated photodiode elements such as130,230, and330, absorbs resulting radiation that emanates from object112. Next, in a converting step412, conversion circuits148convert the currents produced by the photodiode elements due to detecting the emanated radiation, to respective voltage signals.

The method proceeds to a summing step416, in which summation arrangement152produces a summation signal, which is approximately proportional to the radiation emanating from object112and detected by the radiation-sensitive segment. Flowchart400ends with an analyzing step, in which a processing stage that follows summation arrangement152analyzes and/or evaluates the summation signal.

Flowchart400is an example flowchart, which was chosen purely for the sake of conceptual clarity. In alternative embodiments, any other suitable flowchart can also be used for illustrating the disclosed method. Method steps that are not mandatory for understanding the disclosed techniques were omitted fromFIG. 3for the sake of simplicity.

Although the embodiments described herein mainly address semiconductor wafers inspection systems, the methods and systems exemplified by these embodiments can also be applied to systems that comprise any suitable type of particle and wave radiation, and to any suitable application that involves radiation detection such as imaging and viewing.