The present invention relates to an auto-focusing method and apparatus and, in particular, it concerns an auto-focusing method and apparatus for a scanning microscope. 
By way of introduction, high-resolution scanners, such as scanning microscopes, are used to detect defects in wafers.  The resolution of such scanners is typically better than 500 nanometers.  Resolution is generally defined as the size of the smallest distinguishable feature.  In order for the process to be effective the process needs to be accurate and fast.  Prior art high-resolution scanners employ an auto-focus method as follows.  First, the scanning head of the scanner moves to a new viewing region above the surface of the sample to be read.  A measurement beam is passed through the optics of the scanner and is reflected off of the new viewing region back through the optics of the scanner.  The focus of the scanner is then adjusted based upon analysis of the measurement beam.  The scanner then reads from the new viewing region.  Therefore, there is a time delay while adjusting the focus of the scanner. 
With respect to an unrelated art, namely, low-resolution scanning systems, a method for reducing time delay associated with focusing adjustments for low-resolution scanning systems is taught by U.S. Pat.  No.  6,325,289 to Mazzone.  Mazzone teaches an apparatus and process for focusing a laser beam for reading optical codes.  The apparatus includes an optical reader which enables optical codes to be read on objects carried on a supporting plane which is provided with a conveyor for moving the objects along a feeding direction.  The optical reader includes a laser beam scanner, a data processing unit connected to the scanner, a first scanner for generating at least a first scan in a first scan plane, so as to measure the instantaneous distance of at least one point on the surface of each of the objects from the scanner, a second scanner for generating a plurality of second scans in a second scan plane, different from the first, so as to read the optical code on the object; and a data processing unit for adjusting the laser beam focal setting according to the distance measured with the first scan.  The above apparatus employs a polygon mirror arrangement to perform the reading and measuring scans without moving the laser.  Polygon mirror arrangements are not suitable for high-resolution scanning systems.  Polygon mirror arrangements are typically employed for scanners having a maximum resolution of approximately half a millimeter.  Therefore, the system taught by Mazzone cannot be applied to high-resolution scanning systems.  Additionally, in accordance with one of the embodiments taught by Mazzone, the measurement and reading scans are not performed at the same time, thereby wasting time.  In accordance with another embodiment taught by Mazzone, the measurement and reading scans are performed at the same time using separate parallel optical arrangements.  Separate parallel optical arrangements may be suitable for low-resolution requirements of reading optical codes, such as bar codes, but high-resolution applications generally require the measurement scans to be performed using the same optics with which the reading scan is performed. 
There is therefore a need for a system and method for reducing delays associated with focusing adjustments. 
The present invention is an auto-focus scanning microscope construction and method of operation thereof. 
According to the teachings of the present invention there is provided, an auto-focus method for focusing an optical arrangement of a sub-micron optical system, the optical system including a head, at least part of the optical arrangement being included in the head, the optical arrangement being configured for reading from or writing to a surface of a medium, the head performing a scanning motion relative to the surface, the method comprising: (a) illuminating at least part of a first viewing region of the surface by the optical arrangement; performing a focus range measurement of a second viewing region of the surface, the step of configuring and the step of performing being performed substantially at the same time, wherein a centroid of the first viewing region is offset from a centroid of the second viewing region at least in a direction which is parallel to a direction of relative motion of the medium and the head; (b) providing relative movement between the medium and the head in order for the optical arrangement to illuminate at least part of the second viewing region; and (c) adjusting the focus of the optical arrangement based upon the focus range measurement. 
According to a further feature of the present invention, the step of performing is performed by sending a measurement ray through at least part of the optical arrangement. 
According to a further feature of the present invention: (a) the optical arrangement includes a lens which is included in the head; and (b) the step of performing is performed by sending a measurement ray through the lens. 
According to a further feature of the present invention, the measurement ray passes through an optical center of the lens. 
According to a further feature of the present invention, the first viewing region and the second viewing region are non-overlapping. 
According to the teachings of the present invention there is also provided a sub-micron optical system for reading from or writing to a surface of a medium, comprising: (a) an optical arrangement and a head, at least part of the optical arrangement being included in the head; (b) a drive mechanism configured for providing relative movement between the medium and the head in a drive direction; (c) an auto-focus measurement system; and (d) an auto-focus mechanical system configured to adjust the focus of the optical arrangement based upon at least one focus range measurement taken by the auto-focus measurement system, wherein: (i) the optical arrangement and the auto-focus measurement system are configured, such that, the optical arrangement illuminates at least part of a first viewing region of the surface substantially at the same time that the auto-focus measurement system performs a focus range measurement of a second viewing region of the surface; and (ii) a centroid of the first viewing region is offset from a centroid of the second viewing region at least in the drive direction. 
According to a further feature of the present invention, the auto-focus measurement system is configured to direct a measurement ray through at least part of the optical arrangement. 
According to a further feature of the present invention: (a) the optical arrangement includes a lens which is included in the head; and (b) the auto-focus measurement system is configured to direct a measurement ray through the lens. 
According to a further feature of the present invention, the auto-focus measurement system is configured to direct the measurement ray through an optical center of the lens. 
According to a further feature of the present invention, the first viewing region and the second viewing region are non-overlapping. 
According to the teachings of the present invention there is also provided an auto-focus method for focusing an optical arrangement of an optical system, the optical system including a head, the optical arrangement including a lens, the lens being included in the head, the optical arrangement being configured for reading from or writing to a surface of a medium, the head performing a scanning motion relative to the surface, the method comprising: (a) illuminating at least part of a first viewing region of the surface by the optical arrangement; (b) performing a focus range measurement of a second viewing region of the surface, the performing including sending a measurement ray through the lens, the step of configuring and the step of performing being performed substantially at the same time, wherein a centroid of the first viewing region is offset from a centroid of the second viewing region at least in a direction which is parallel to a direction of relative motion of the medium and the head; (c) providing relative movement between the medium and the head in order for the optical arrangement to illuminate at least part of the second viewing region; and (d) adjusting the focus of the optical arrangement based upon the focus range measurement. 
According to a further feature of the present invention, the measurement ray passes through an optical center of the lens. 
According to a further feature of the present invention, the first viewing region and the second viewing region are non-overlapping. 
According to the teachings of the present invention there is also provided an optical system for reading from or writing to a surface of a medium, comprising: (a) an optical arrangement and a head, the optical arrangement having a lens which is included in the head; (b) a drive mechanism configured for providing relative movement between the medium and the head in a drive direction; an auto-focus measurement system configured to direct a measurement ray through the lens; and (c) an auto-focus mechanical system configured to adjust the focus of the optical arrangement based upon at least one focus range measurement taken by the auto-focus measurement system, wherein: (i) the optical arrangement and the auto-focus measurement system are configured, such that, the optical arrangement illuminates at least part of a first viewing region of the surface substantially at the same time that the auto-focus measurement system performs a focus range measurement of a second viewing region of the surface; and (ii) a centroid of the first viewing region is offset from a centroid of the second viewing region at least in the drive direction. 
According to a further feature of the present invention, the auto-focus measurement system is configured to direct the measurement ray through an optical center of the lens. 
According to a further feature of the present invention, the first viewing region and the second viewing region are non-overlapping.