Patent Publication Number: US-8993922-B2

Title: Laser processing method and device

Description:
TECHNICAL FIELD 
     The present invention relates to a laser processing method and laser processing apparatus for processing an object to be processed by irradiating the object with a laser beam. 
     BACKGROUND ART 
     Known as a conventional laser processing technique is one in which measurement means (a contact-type displacement meter, an ultrasonic rangefinder, or the like) for measuring the main surface height of the object to be processed is arranged in parallel with a condenser lens for converging a laser beam with a predetermined gap therebetween (see, for example, FIGS. 6 to 10 of Patent Document 1). In such a laser processing technique, while scanning the object with the laser beam along its main surface, the main surface height of the object is measured by the measurement means, and the condenser lens is driven along its optical axis such that the distance between the condenser lens and the main surface of the object becomes constant according to thus measured main surface at the time when the measurement point is positioned directly under the condenser lens. 
     Known as a technique for processing an object to be processed whose main surface has irregularities, on the other hand, is one in which the planarity in the whole part to be processed is measured by planarity measuring means (a planarity meter comprising a projector and a reflected light receiver) as a preparation for processing and then, with a blade replacing the planarity measuring means, the object is processed according to thus measured planarity (see, for example, Patent Document 2).
     Patent Document 1: Japanese Patent Application Laid-Open No. 2002-219591   Patent Document 2: Japanese Patent Application Laid-Open No. HEI 11-345785   

     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, the following problem to be overcome exists in the above-mentioned laser processing apparatus disclosed in Patent Document 1. When the laser beam irradiation is started from a position on the outside of the object to be processed, so as to perform processing while the laser beam and the object are moved along the main surface of the latter, the measurement means proceeds with measurement from the outside of the object to the inside thereof. When the condenser lens is driven according to the measured value of main surface height obtained by this measurement, the converging point of the laser beam may deviate from its predetermined position in end parts of the object to be processed. 
     Though the planarity of the main surface of the object to be processed can accurately be grasped when the technique disclosed in Patent Document 2 is used, respective means used for measuring and processing must be exchanged, which takes time and labor and may generate a deviation due to the exchange. 
     Therefore, it is an object of the present invention to provide a laser processing method and laser processing apparatus which can efficiently perform laser processing while minimizing the deviation of the laser beam converging point. 
     Means for Solving Problem 
     The present invention provides a laser processing method for irradiating an object to be processed with a first laser beam while converging the first laser beam with a lens such that a converging point is positioned within the object, and forming a modified region within the object along a line to cut in the object; the method comprising (1) a displacement acquiring step of irradiating the object with a second laser beam for measuring a displacement of a main surface of the object while converging the second laser beam with the lens, and acquiring the displacement of the main surface along the line to cut while detecting reflected light reflected by the main surface in response to the irradiation; and (2) a processing step of emitting the first laser beam and moving the lens and the object relative to each other along the main surface while adjusting a gap between the lens and the main surface according to the acquired displacement, so as to form the modified region along the line to cut. 
     Since the laser processing method of the present invention acquires a displacement of the main surface along the line to cut, and forms a modified region while adjusting the gap between the lens and the main surface according to thus acquired displacement, it can form the modified region at a predetermined position within the object. Since the lens converging the first laser beam for processing also converges the second laser beam for measurement, the displacement of the main surface can be acquired more accurately. 
     It will be preferred in the laser processing method of the present invention if the displacement of the main surface along the line to cut is acquired at a first time interval while moving the lens and the object relative to each other at a first speed along the main surface in the displacement acquiring step; and the modified region is formed while moving the lens and the object relative to each other at a second speed faster than the first speed along the main surface and adjusting the gap between the lens and the main surface at a second time interval shorter than the first time interval in the processing step. Since the displacement of the main surface is acquired at the first speed slower than the second speed for forming the modified region, the displacement of the main surface can accurately be acquired even when there is a large difference in level of the main surface, for example. Since the modified region is formed at the second speed faster than the first speed for acquiring the displacement of the main surface, the processing efficiency improves. When the first speed, second speed, first time interval, and second time interval are set such that the distance interval along the line to cut for acquiring the displacement of the main surface equals the distance interval along the line to cut for adjusting the gap between the lens and the main surface at the time of forming the modified region, for example, the gap between the lens and the main surface can be adjusted more faithfully according to the acquired displacement of the main surface. 
     It will also be preferred in the laser processing method of the present invention if the displacement acquiring step comprises a measurement preparatory step of holding the lens at a measurement initial position set such that a converging point of the second laser beam is located at a predetermined position with respect to the object; a first measurement step of starting emitting the second laser beam while holding the lens at the measurement initial position, moving the lens and the object relative to each other along the main surface, and releasing the lens from being held at the measurement initial position in response to reflected light of the second laser beam reflected by the main surface; and a second measurement step of adjusting the gap between the lens and the main surface after the release while detecting the reflected light of the second laser beam reflected by the main surface, so as to acquire the displacement of the main surface along the line to cut. After one end part of the line to cut is irradiated with the second laser beam while the lens is held at the measurement initial position, i.e., after the lens overhangs the object as the lens and the object move relative to each other, the lens is released from being held, and the displacement of the main surface is acquired, whereby the displacement can be acquired while excluding the influence of fluctuations in the shape of end parts in the object as much as possible. Since the quantity of reflected light varies depending on the distance from the reflecting surface, the lens can be released from being held assuming that a part where the quantity of the reflected light undergoes a predetermined change corresponds to an outer edge of the main surface of the object, for example. 
     It will also be preferred if the processing step comprises a processing preparatory step of setting a processing initial position for holding the lens with respect to the main surface according to the displacement of the main surface along the line to cut acquired by the displacement acquiring step, and holding the lens at thus set processing initial position; a first processing step of starting emitting the first laser beam while holding the lens at the processing initial position, and moving the lens and the object relative to each other so as to form the modified region in one end part of the line to cut; and a second processing step of releasing the lens from being held at the processing initial position after forming the modified region in the one end part of the line to cut, and moving the lens and the object relative to each other after the release while adjusting the gap between the lens and the main surface according to the displacement of the main surface along the line to cut acquired in the displacement acquiring step, so as to form the modified region. The modified region is formed in one end part of the line to cut while holding the lens at the processing initial position, and then is formed while the lens is released from being held and caused to follow the displacement of the main surface. Therefore, the modified region can be formed while excluding the influence of fluctuations in the shape of end parts in the object as much as possible. 
     It will also be preferred in the laser processing method of the present invention if, in the displacement acquiring step, the first laser beam is emitted when acquiring the displacement of the main surface along the line to cut, so as to form the modified region along the line to cut. This forms the modified region while acquiring the displacement of the main surface at the same time, whereby a single scan can perform the measurement and processing. 
     It will also be preferred in the laser processing method of the present invention if the modified region formed in the displacement acquiring step is formed between the modified region formed in the processing step and the main surface. The modified region formed in the processing step is positioned farther from the laser light irradiation side than is the modified region formed in the displacement acquiring step, whereby the modified region can be formed widely in the laser beam emitting direction. 
     It will also be preferred in the laser processing method of the present invention if the line to cut includes first and second lines to cut; the displacement acquiring step moves the lens relative to the object in a first direction extending along the first line to cut, so as to acquire the displacement of the main surface along the first line to cut, and then moves the lens relative to the object in a second direction opposite from the first direction, so as to acquire the displacement of the main surface along the second line to cut; and the processing step forms the modified region along the first line to cut in the first direction, and then forms the modified region along the second line to cut in the second direction. Since the lens acquires the displacement along the first line to cut while moving in the first direction, and acquires the displacement along the second line to cut while moving in the second direction opposite from the first direction, displacements can be acquired as the lens moves back and forth with respect to the object. Since the respective modified regions are formed along the first and second lines to cut as the lens moves back and forth with respect to the object, modified regions can be formed more efficiently. 
     The present invention provides a laser processing apparatus for irradiating an object to be processed with a first laser beam while converging the first laser beam with a lens such that a converging point is positioned within the object, and forming a modified region within the object along a line to cut in the object; the apparatus comprising a lens for converging the first laser beam and a second laser beam for measuring a displacement of a main surface of the object onto the object; displacement acquiring means for acquiring the displacement of the main surface of the object by detecting reflected light reflected by the main surface in response to irradiation with the second laser beam; moving means for moving the object and the lens relative to each other along the main surface of the object; holding means for holding the lens such that the lens freely advances and retracts with respect to the main surface; and control means for controlling respective behaviors of the moving means and holding means; wherein, while emitting the second laser beam, the control means controls the moving means so as to move the object and the lens relative to each other along the main surface, the displacement acquiring means acquiring the displacement of the main surface along the line to cut; and wherein, while emitting the first laser beam, the control means controls the holding means so as to hold the lens while adjusting a gap between the lens and the main surface according to the displacement acquired by the displacement acquiring means, and controls the moving means so as to move the lens and the object relative to each other along the main surface, thereby forming the modified region. 
     Since the laser processing apparatus of the present invention acquires a displacement of the main surface along the line to cut, and forms a modified region while adjusting the gap between the lens and the main surface according to thus acquired displacement, it can form the modified region at a predetermined position within the object. Since the lens converging the first laser beam for processing also converges the second laser beam for measurement, the displacement of the main surface can be acquired more accurately. 
     It will be preferred in the laser processing apparatus of the present invention if, while emitting the second laser beam, the control means controls the moving means so as to move the object and the lens relative to each other along the main surface at a first speed, the displacement acquiring means acquiring the displacement of the main surface along the line to cut at a first time interval; and, while emitting the first laser beam, the control means controls the moving means so as to move the lens and the object relative to each other along the main surface at a second speed faster than the first speed, and controls the holding means so as to adjust the gap between the lens and the main surface at a second time interval shorter than the first time interval. Since control is made such that the displacement of the main surface is acquired at the first speed slower than the second speed for forming the modified region, the displacement of the main surface can accurately be acquired even when there is a large difference in level of the main surface, for example. Since control is made such that the modified region is formed at the second speed faster than the first speed for acquiring the displacement of the main surface, the processing efficiency improves. When the first speed, second speed, first time interval, and second time interval are set such that the distance interval along the line to cut for acquiring the displacement of the main surface equals the distance interval along the line to cut for adjusting the gap between the lens and the main surface at the time of forming the modified region, for example, the gap between the lens and the main surface can be adjusted more faithfully according to the acquired displacement of the main surface. 
     It will also be preferred in the laser processing apparatus of the present invention if the control means controls the holding means so as to hold the lens at a measurement initial position set such that a converging point of the second laser beam is located at a predetermined position with respect to the object; while starting the emission of the second laser beam with the lens being held at the measurement initial position, the control means controls the moving means so as to move the lens and the object relative to each other along the main surface, and controls the holding means so as to release the lens from being held at the measurement initial position in response to the reflected light of the second laser beam reflected by the main surface; and, after the release, the control means controls the holding means so as to adjust the gap between the lens and the main surface while detecting the reflected light of the second laser beam reflected by the main surface, the displacement acquiring means acquiring the displacement of the main surface along the line to cut. After one end part of the line to cut is irradiated with the second laser beam while the lens is held at the measurement initial position, i.e., after the lens overhangs the object as the lens and the object move relative to each other, the lens is released from being held, and the displacement of the main surface is acquired, whereby the displacement can be acquired while excluding the influence of fluctuations in the shape of end parts in the object as much as possible. Since the quantity of reflected light varies depending on the distance from the reflecting surface, the lens can be released from being held assuming that a part where the quantity of the reflected light undergoes a predetermined change corresponds to an outer edge of the main surface of the object, for example. 
     It will also be preferred in the laser processing apparatus of the present invention if the control means controls the holding means so as to set a processing initial position for holding the lens with respect to the main surface according to the displacement of the main surface along the line to cut acquired by the displacement acquiring means, and hold the lens at thus set processing initial position; while starting the emission of the first laser beam with the lens being held at the processing initial position, the control means controls the moving means so as to move the lens and the object relative to each other, thereby forming the modified region in one end part of the line to cut; and, after forming the modified region in the one end part, the control means controls the holding means so as to release the lens from being held at the processing initial position and adjust the gap between the lens and the object according to the displacement of the main surface acquired by the displacement acquiring means, and controls the moving means so as to move the lens and the object relative to each other, thereby forming the modified region. The modified region is formed in one end part of the line to cut while holding the lens at the processing initial position, and then is formed while the lens is released from being held and caused to follow the displacement of the main surface. Therefore, the modified region can be formed while excluding the influence of fluctuations in the shape of end parts in the object as much as possible. 
     It will also be preferred in the laser processing apparatus of the present invention if the displacement acquiring means emits the first laser beam when acquiring the displacement of the main surface, so as to form the modified region along the line to cut. This forms the modified region while acquiring the displacement of the main surface at the same time, whereby a single scan can perform the measurement and processing. 
     It will also be preferred in the laser processing apparatus of the present invention if the moving means is adapted to move the object toward the lens, and the control means controls the moving means such that the modified region formed along the line to cut when the displacement acquiring means acquires the displacement is formed between the modified region formed later along the line to cut and the main surface. The modified region formed in the processing step is positioned farther from the laser light irradiation side than is the modified region formed in the displacement acquiring step, whereby the modified region can be formed widely in the laser beam emitting direction. 
     It will also be preferred in the laser processing apparatus of the present invention if the line to cut includes first and second lines to cut; the control means controls the moving means so as to move the lens relative to the object in a first direction along the first line to cut, while the displacement acquiring means acquires the displacement of the main surface along the first line to cut, and then the control means controls the moving means such that the lens moves relative to the object in a second direction opposite from the first direction, while the displacement acquiring means acquires the displacement of the main surface along the second line to cut; and, after forming the modified region along the first line to cut in the first direction, the control means controls the moving means so as to form the modified region along the second line to cut in the second direction. Since the lens acquires the displacement along the first line to cut while moving in the first direction, and acquires the displacement along the second line to cut while moving in the second direction opposite from the first direction, displacements can be acquired as the lens moves back and forth with respect to the object. Since the respective modified regions are formed along the first and second lines to cut as the lens moves back and forth with respect to the object, modified regions can be formed more efficiently. 
     Effect of the Invention 
     The laser processing method and laser processing apparatus of the present invention can efficiently carry out laser processing while minimizing the deviation of the converging point of a laser beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the configuration of the laser processing apparatus in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram showing a functional configuration of a control unit provided in the laser processing apparatus in accordance with the embodiment. 
         FIG. 3  is a view showing an object to be processed for explaining the embodiment. 
         FIG. 4  is a view for explaining the laser processing method in accordance with an embodiment of the present invention. 
         FIG. 5  is a chart for explaining the laser processing method in accordance with the embodiment. 
         FIG. 6  is a chart for explaining the laser processing method in accordance with the embodiment. 
         FIG. 7  is a flowchart for explaining the laser processing method in accordance with the embodiment. 
         FIG. 8  is a view for explaining the laser processing method in accordance with the embodiment. 
         FIG. 9  is a flowchart for explaining the laser processing method in accordance with the embodiment. 
         FIG. 10  is a flowchart for explaining the laser processing method in accordance with the embodiment. 
     
    
    
     EXPLANATION OF NUMERALS 
       1  . . . laser processing apparatus,  2  . . . stage,  3  . . . laser head unit,  4  . . . optical system main part,  5  . . . objective lens unit,  6  . . . laser emitting apparatus,  7  . . . control unit, S . . . object, R . . . modified region,  42  . . . processing objective lens,  43  . . . actuator,  13  . . . laser head,  44  . . . laser diode,  45  . . . light-receiving part. 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     The findings of the present invention can easily be understood in view of the following detailed descriptions with reference to the accompanying drawings, which are represented for the purpose of illustration only. Embodiments of the present invention will now be explained with reference to the accompanying drawings. When possible, parts identical to each other will be referred to with numerals identical to each other without repeating their overlapping explanations. 
     The laser processing apparatus in accordance with an embodiment of the present invention will be explained with reference to  FIG. 1 . As shown in  FIG. 1 , this laser processing apparatus  1  is one which irradiates a planar object to be processed S mounted on a stage  2  (moving means) with a processing laser beam L 1  (first laser beam) such that a converging point P is positioned within the object S, so as to form a modified region R caused by multiphoton absorption within the object S. The stage  2  is movable upward/downward and leftward/rightward and rotatable, whereas a laser emitting apparatus  6  mainly constituted by a laser head unit  3 , an optical system main part  4 , and an objective lens unit  5  is disposed above the stage  2 . The laser processing apparatus  1  also comprises a control unit  7  (control means), which outputs control signals for controlling behaviors of the stage  2  and laser emitting apparatus  6  (the movement of stage  2 , emission of laser beams from the laser emitting apparatus  6 , etc.) thereto. 
     The laser head unit  3  is detachably attached to an upper end part of the optical system main part  4 . The laser head unit  3  includes an L-shaped cooling jacket  11 . Embedded in a vertical wall  11   a  of the cooling jacket  11  is a cooling pipe  12  in a winding state, through which cooling water circulates. Attached to the front face of the vertical wall  11   a  are a laser head  13  which emits the processing laser beam L 1  downward, and a shutter unit  14  for selectively opening and closing an optical path of the processing laser beam L 1  emitted from the laser head  13 . This can prevent the laser head  13  and shutter unit  14  from overheating. For example, the laser head  13  uses an Nd:YAG laser and emits a pulsed laser beam having a pulse width of 1 μs or shorter as the processing laser beam L 1 . 
     In the laser head unit  3 , an adjuster  15  for adjusting the inclination of the cooling jacket  11  and the like is attached to the lower face of a bottom wall  11   b  of the cooling jacket  11 . The adjuster  15  is used for aligning an optical axis α of the processing laser beam L 1  emitted from the laser head  13  with an axis β which is set in the optical system main part  4  and objective lens unit  5  such as to extend vertically. Namely, the laser head unit  3  is attached to the optical system main part  4  by way of the adjuster  15 . When the inclination of the cooling jacket  11  or the like is adjusted by the adjuster  15  thereafter, the inclination of the laser head  13  or the like is adjusted in conformity to the movement of the cooling jacket  11 . As a consequence, the processing laser beam L 1  advances into the optical system main part  4  while in a state where its optical axis α coincides with the axis β. The bottom wall  11   b  of the cooling jacket  11 , the adjuster  15 , and a housing  21  of the optical system main part  4  are formed with through holes through which the processing laser beam L 1  passes. 
     On the axis β within the housing  21  of the optical system main part  4 , a beam expander  22  for enlarging the beam size of the processing laser beam L 1  emitted from the laser head  13 , an optical attenuator  23  for adjusting the output of the processing laser beam L 1 , an output observation optical system  24  for observing the output of the processing laser beam L 1  adjusted by the optical attenuator  23 , and a polarization adjusting optical system  25  for adjusting the polarization of the processing laser beam L 1  are arranged in this order from the upper side to the lower side. A beam damper  26  for absorbing the eliminated laser beam is attached to the optical attenuator  23 , and is connected to the cooling jacket  11  by way of a heat pipe  27 . This can prevent the beam damper  26  having absorbed the laser beam from overheating. 
     For observing the object S mounted on the stage  2 , a light guide  28  for guiding an observation visible ray is attached to the housing  21  of the optical system main part  4 , whereas a CCD camera  29  is disposed within the housing  21 . The observation visible ray is guided by the light guide  28  into the housing  21 , successively passes through a field stop  31 , a reticle  32 , a dichroic mirror  33 , and the like, and then is reflected by a dichroic mirror  34  disposed on the axis β. The reflected observation visible ray advances downward on the axis β and irradiates the object S. On the other hand, the processing laser beam L 1  is transmitted through the dichroic mirror  34 . 
     The reflected light beam of the observation visible ray reflected by a surface S 1  of the object S advances upward on the axis β, and is reflected by the dichroic mirror  34 . The light beam reflected by the dichroic mirror  34  is further reflected by the dichroic mirror  33 , so as to pass through an imaging lens  35 , etc., thereby entering the CCD camera  29 . An image of the object S captured by the CCD camera  29  is displayed on a monitor (not depicted). 
     The objective lens unit  5  is detachably attached to the lower end part of the optical system main part  4 . Since the objective lens unit  5  is positioned by a plurality of positioning pins with respect to the lower end part of the optical system main part  4 , the axis β set in the optical system main part  4  and the axis β set in the objective lens unit  5  can easily be aligned with each other. By way of an actuator  43  (holding means) using a piezoelectric device, a processing objective lens  42  is mounted to the lower end of the housing  41  of the objective lens unit  5  while in a state where the optical axis coincides with the axis β. The housing  21  of the optical system main part  4  and the housing  41  of the objective lens unit  5  are formed with through holes through which the processing laser beam L 1  passes. The peak power density of the processing laser beam L 1  converged by the processing objective lens  42  is at least 1×10 8  (W/cm 2 ) at the converging point P. 
     In order to position the converging point P of the processing laser beam L 1  at a predetermined depth from the surface S 1  of the object S, a laser diode  44  for emitting a rangefinding laser beam L 2  (second laser beam) and a light-receiving part  45  (light-receiving means) are disposed within the housing  41  of the objective lens unit  5 . The rangefinding laser beam L 2  is emitted from the laser diode  44 , and is successively reflected by a mirror  46  and a half mirror  47 , and then by a dichroic mirror  48  disposed on the axis β. The reflected rangefinding laser beam L 2  advances downward on the axis β, and passes through the processing objective lens  42 , thereby irradiating the surface S 1  of the object S. On the other hand, the processing laser beam L 1  passes through the dichroic mirror  48 . 
     The reflected light beam of the rangefinding laser beam L 2  reflected by the surface S 1  of the object S reenters the processing objective lens  42 , and advances upward on the axis β, so as to be reflected by the dichroic mirror  48 . The reflected light beam of the rangefinding laser beam L 2  reflected by the dichroic mirror  48  passes through the half mirror  47 , so as to enter the light-receiving part  45 , and is converged on a four-divided position detecting device (displacement acquiring means) in which a photodiode is equally divided into four. According to the converged image pattern of the reflected light beam of the rangefinding laser beam L 2  converged onto the four-divided position detecting device, it can be detected where the converging point of the rangefinding laser beam L 2  due to the processing objective lens  42  is positioned with respect to the surface S 1  of the object S. Information about the converged image pattern of the reflected light beam of the rangefinding laser beam L 2  converged on the four-divided position detecting device is outputted to the control unit  7 . According to this information, the control unit  7  outputs a control signal for indicating to the actuator  43  a position for holding the processing objective lens  42 . 
     Physically, the control unit  7  comprises an interface for exchanging signals with the stage  2  and laser emitting apparatus  6 , a CPU (central processing unit), and a storage device such as memory or HDD. According to a program stored in the storage device, the CPU performs a predetermined information processing operation and outputs results of the information processing as control signals to the stage  2  and laser emitting apparatus  6 . 
       FIG. 2  shows a functional configuration of the control unit  7 . Functionally, as shown in  FIG. 2 , the control unit  7  comprises a laser emission controller  701 , a stage movement controller  702 , an actuator controller  703 , a converging point calculator  704 , an end part determiner  705 , a displacement acquiring/reproducing part  706  (displacement acquiring means), and a displacement storage  707 . The laser emission controller  701  is a part which outputs signals for controlling emissions of the processing laser beam L 1  and rangefinding laser beam L 2  to the laser head  13  of the laser head unit  3  and the laser diode  44  of the objective lens unit  5 , respectively. The stage movement controller  702  is a part which outputs a control signal for controlling the movement of the stage  2  thereto. The actuator controller  703  is a part which outputs a control signal for controlling the driving of the actuator  43  of the objective lens unit  5  to the actuator  43 . The displacement acquiring/reproducing part  706  is a part which reads the amount of expansion/contraction of the actuator  43  from the control signal outputted from the actuator controller  703  to the actuator  43 , and stores the amount of expansion/contraction into the displacement storage  707 . The displacement acquiring/reproducing part  706  is also a part which reads the amount of expansion/contraction stored in the displacement storage  707  and outputs the amount of expansion/contraction to the actuator controller  703 . The actuator controller  703  outputs a control signal such that the actuator  43  is driven by the outputted amount of expansion/contraction. The amount of expansion/contraction varies depending on the displacement of the main surface S 1  of the object S, and thus can be taken as an amount indicative of the displacement of the main surface S 1 . The converging point calculator  704  is a part which calculates the distance between the object S and the converging point of the rangefinding laser beam L 2  according to an astigmatism signal outputted from the light-receiving part  45  of the objective lens unit  5 . The end part determiner  705  is a part which determines whether the processing objective lens  42  is at a position corresponding to an end part of the object S or not according to the quantity of light received by the light-receiving part  45 . Operations of the individual functional constituents will be explained later. 
     An outline of a laser processing method carried out by thus configured laser processing apparatus  1  will now be explained. First, the object S is mounted on the stage  2 , and the stage  2  is moved such that the converging point P of the processing laser beam L 1  is positioned within the object S. The initial position of the stage  2  is determined by the thickness and refractive index of the object S, the numerical aperture of the processing objective lens  42 , etc. 
     Subsequently, the processing laser beam L 1  is emitted from the laser head  13 , the rangefinding laser beam L 2  is emitted from the laser diode  44 , and the stage  2  is moved such that the processing laser beam L 1  and rangefinding laser beam L 2  converged by the processing objective lens  42  scan a desirable line (line to cut) of the object S. Here, the light-receiving part  45  detects the reflected light beam of the rangefinding laser beam L 2 , and the control unit  7  controls the actuator  43  in a feedback fashion such that the converging point P of the processing laser beam L 1  is always positioned at a predetermined depth from the surface S 1  of the object S, whereby the position of the processing objective lens  42  is minutely adjusted along the axis β. 
     Therefore, even when the surface S 1  of the object S wobbles, for example, a modified region R caused by multiphoton absorption can be formed at a position located at a predetermined depth from the surface S 1 . Forming the linear modified region R within the planar object S as such can generate a cleavage from the linear modified region R acting as a start point, whereby the object S can be cut easily with a high precision along the linear modified region R. 
     The laser processing method using the laser processing apparatus  1  of this embodiment will be explained more specifically. The explanation of the laser processing method will also illustrate operations of the laser processing apparatus  1 . The laser processing method in accordance with this embodiment can be divided into a displacement acquiring step of acquiring the displacement of the surface S 1  (main surface) of the wafer-like object S, and a processing step of emitting the processing laser beam L 1  so as to form a modified region. Each of the displacement acquiring step and processing step will be explained. 
     First, the wafer-like object S used in this explanation will be explained with reference to  FIG. 3 . In the object S, 2n lines to cut C 1  to C 2n  are set. The lines to cut C 1  to C 2n  are subjected to laser processing in groups of two each. In the case of the lines to cut C 1  and C 2 , for example, the displacement of the line to cut C 1  is acquired from a point X 1  to a point X 2  on an extension of the line to cut C 1 , and then the displacement of the line to cut C 2  is acquired from a point X 3  to a point X 4  on an extension of the line to cut C 2 . When displacements are acquired as such, the stage  2 . (see  FIG. 1 ) can efficiently be moved such that the processing objective lens  42  (see  FIG. 1 ) shifts from the point X 1  to the point X 2  and then in reverse from the point X 3  to the point X 4 . After the displacements concerning the lines to cut C 1  and C 2  are acquired, while reproducing the amount of movement of the actuator  43  according to thus acquired displacements, a modified region is formed along the line to cut C 1  from the point X 1  to X 2  on the extension of the line to cut C 1 , and then a modified region is formed along the line to cut C 1  from the point X 3  to X 4  on the extension of the line to cut C 2 . 
     (Displacement Acquiring Step) The displacement acquiring step of acquiring the displacement of the surface S 1  along the lines to cut C 1  to C n  in the wafer-like object S will now be explained. 
     The explanation will be made with reference to  FIGS. 4(A) to 4(C) .  FIGS. 4(A) to 4(C)  are views showing the cross section II-II of  FIG. 3 . For easier understanding, the hatching indicating the cross section is omitted in  FIGS. 4(A) to 4(C) . As shown in  FIG. 4(A) , the object S is attracted and secured to the stage  2  by way of a dicing film  2   a . The dicing film  2   a  is secured with a dicing ring (not depicted). 
     As shown in  FIG. 4(A) , the stage  2  moves such that the processing objective lens  42  is placed at a position corresponding to a point Q 1  on the line to cut C 1  in the object S. The actuator  43  holding the processing objective lens  42  is in a state expanded by 25 μm from the most contracted state. This amount of expansion, i.e., 25 μm, is set as one half of the maximum amount of expansion of the actuator  43 , i.e., 50 μm. In this state, the stage  2  is moved up/down so that a reflected light beam of the observation visible ray is in focus. In this in-focus state, the rangefinding laser beam L 2  is emitted, an astigmatism signal is obtained according to a reflected light beam of the rangefinding laser beam L 2 , and the value of the astigmatism signal is taken as a reference value. 
     Subsequently, as shown in  FIG. 4(B) , the stage  2  is moved such that the processing objective lens  42  is placed at a position corresponding to the point X 1  on an extension of the line to cut C 1  while keeping the amount of expansion of the actuator  43  in the state of  FIG. 4(A) . The position of the processing objective lens  42  with respect to the object S in the vertical direction shown in  FIG. 4(B)  is the initial position (measurement initial position). Thereafter, the stage  2  moves such that the processing objective lens  42  shifts in the direction of arrow F in  FIG. 4(B)  (measurement preparatory step). 
     The rangefinding laser beam L 2  is reflected less by the dicing film  2   a  so that the total quantity of light reflected thereby is smaller, whereas the total quantity of reflected light increases in the object S. Namely, the total quantity of reflected light beam of the rangefinding laser beam L 2  detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) increases, whereby it can be determined that the processing objective lens  42  is located at a position intersecting the line to cut C 1  in the object S when the total quantity of reflected light beam exceeds a predetermined threshold. Therefore, when the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) is greater than the predetermined threshold, the processing objective lens  42  is assumed to be located at one end of the line to cut C 1 , and expansion amount control of the actuator  43  is started so as to release the expansion amount of the actuator  43  from being held at this time, such that the astigmatism signal becomes the reference value (first measurement step). 
     Hence, when the processing objective lens  42  moves in the direction of arrow F in  FIG. 4(B) , the state shown in  FIG. 4(C)  is attained. As shown in  FIG. 4(C) , area G 1  (one end part) becomes a transition area in which the processing objective lens  42  shifts from the held state to a state capable of following the displacement of the surface S 1  of the object S, whereby the amount of movement of the actuator  43  does not correspond to the displacement of the surface S 1  in this area. In the next area G 2  in which the amount of expansion of the actuator  43  is released from being held and the expansion amount control for the actuator  43  is carried out such that the astigmatism signal becomes the reference value, the amount of movement of the actuator  43  corresponds to the displacement of the surface S 1 . Therefore, the track G of the change in expansion amount of the actuator  43  corresponds to the displacement of the surface S 1 . Thereafter, when the processing objective lens  42  is located at the other end of the line to cut C 1  as shown in  FIG. 4(C) , the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) decreases. Hence, when the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) becomes smaller than a predetermined threshold, the processing objective lens  42  is assumed to be located at a position corresponding to one end of the line to cut C 1 , the amount of expansion of the actuator at this time is held, and the recording of the track G is terminated. Information of the track G is stored into the displacement storage  707  (second measurement step). 
     Though the processing objective lens  42  having reached a position corresponding to one end of the line to cut C 1  is detected according to the fact that the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) exceeds a predetermined threshold in the foregoing explanation, this is not restrictive, whereby other criteria may also be employed. An example of such criteria will be explained with reference to  FIGS. 5(A) and 5(B) .  FIG. 5(A) , whose ordinate and abscissa indicate the total light quantity detected by the four-divided position detecting device of the light-receiving part  45  (see  FIG. 1 ) and time, respectively, is a chart recording the change in the total light quantity detected by the four-divided position detecting device of the light-receiving part  45  (see  FIG. 1 ). In this case, as mentioned above, it is determined that the processing objective lens  42  is located at a position corresponding to one end of the line to cut C 1  at the time when the light quantity exceeds a predetermined threshold T 1 . 
     From the graph of  FIG. 5(A) , at predetermined intervals (e.g., at individual sampling points), the amount of change in difference obtained by subtracting the previous total light quantity value from the current total light quantity value is calculated. Thus obtained values are plotted in  FIG. 5(B)  whose ordinate and abscissa indicate the amount of change and time, respectively. In this case, a part exhibiting a positive peak seems to be a point where the change in the total light quantity is the largest, i.e., a part corresponding to the vicinity of the center of an edge (outer edge) of the object S. Therefore, the tracking of the actuator  43  can be started after the differential peak shown in  FIG. 5(B)  stops changing after the total light quantity shown in  FIG. 5(A)  becomes the threshold T 1 . 
     Though the processing objective lens  42  having reached a position corresponding to the other end of the line to cut C 1  is detected according to the fact that the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) becomes smaller than a predetermined threshold in the foregoing explanation, this is not restrictive, whereby other criteria may also be employed. An example of such criteria will be explained with reference to  FIGS. 6(A) and 6(B) .  FIG. 6(A) , whose ordinate and abscissa indicate the total light quantity detected by the four-divided position detecting device of the light-receiving part  45  (see  FIG. 1 ) and time, respectively, is a chart recording the change in the total light quantity detected by the four-divided position detecting device of the light-receiving part  45  (see  FIG. 1 ). In this case, as mentioned above, it is determined that the processing objective lens  42  is located at a position corresponding to one end of the line to cut C 1  at the time when the light quantity becomes smaller than a predetermined threshold T 2 . 
     From the graph of  FIG. 6(A) , at predetermined intervals (e.g., at individual sampling points), the amount of change in difference obtained by subtracting the previous total light quantity value from the current total light quantity value is calculated. Thus obtained values are plotted in  FIG. 6(B)  whose ordinate and abscissa indicate the amount of change and time, respectively. In this case, a part exhibiting a negative peak seems to be a point where the change in the total light quantity is the largest, i.e., a part corresponding to the vicinity of the center of an edge (outer edge) of the object S. Therefore, the amount of expansion/contraction of the actuator  43  can be fixed at that corresponding to this part, and the recording of the amount of expansion/contraction can be stopped. 
     Operations of the laser processing apparatus  1  in the displacement acquiring step will be explained with reference to the flowchart shown in  FIG. 7 . The stage controller  702  of the control unit  7  outputs a control signal to the stage  2  so as to move the processing objective lens  42  to a point Q 1  on the C 1  (step S 01 ). In response to the output of this control signal, the stage  2  moves. The actuator controller  703  of the control unit  7  outputs a control signal to the actuator  43  so as to make the latter expand by 20 μm. In response to the output of this control signal, the actuator  43  expands by 20 μm. In this state, the stage  2  is moved up/down so that the observation visible ray is in focus therewith, and a focal position of the observation visible ray is set (step S 02 ). 
     The laser emission controller  701  of the control unit  7  outputs a control signal to the laser diode  44  so as to make the latter emit the rangefinding laser beam L 2  (step S 03 ). In response to the output of this control signal, the laser diode  44  emits the rangefinding laser beam L 2 , whereas its reflected light beam reflected by the surface S 1  of the object S is received by the four-divided position detecting device in the light-receiving part  45 . In response to the light received, signals are outputted to the converging point calculator  704  and the end part determiner  705 . 
     The converging point calculator  704  holds the value of astigmatism signal in this state as a reference value (step S 04 ). Subsequently, the stage movement controller  702  outputs a control signal to the stage  2  such that the processing objective lens  42  moves to a position corresponding to X 1  on an extension of the line to cut C 1  in the object S (step S 05 ). The stage  2  moves in response to the output of this control signal, whereby the processing objective lens  42  shifts to the position corresponding to X 1  on the extension of the line to cut C 1  in the object S. 
     Subsequently, the stage movement controller  702  outputs a control signal to the stage  2  so as to move the processing objective lens  42  in the direction of arrow F in  FIG. 4(B) . In response to the output of the control signal, the stage  2  moves, whereby the processing objective lens  42  starts moving in the direction of arrow F. 
     According to the signal outputted from the light-receiving part  45 , the end part determiner  705  of the control unit  7  determines whether the processing objective lens  42  is located at an end part of the object S or not (step S 06 ). When it is determined that the processing objective lens  42  is located at an end part of the object S, the end part determiner  705  outputs an instruction signal to the actuator controller  703  so as to make the latter start the expansion/contraction of the actuator  43  such that the astigmatism signal equals the held reference value. The actuator controller  703  outputs the control signal to the actuator  43  so as to make the latter start expanding/contracting in order for the astigmatism signal to equal the held reference value (step S 07 ). In response to the output of this control signal, the actuator  43  expands/contracts according to the displacement of the surface S 1  of the object S, and holds the processing objective lens  42  such that the astigmatism signal becomes the held value (such that the distance between the processing objective lens  42  and the surface S 1  of the object S becomes constant). Therefore, the track G of the amount of expansion/contraction of the actuator  43  is formed in conformity to the displacement of the surface S 1  of the object S (see  FIG. 4(C) ). The displacement acquiring/reproducing part  706  in the control unit  7  starts recording the amount of expansion/contraction of the actuator  43  (step S 08 ). 
     According to the signal outputted from the light-receiving part  45 , the end part determiner  705  determines whether the processing objective lens  42  is located at the other end part of the object S or not (step S 09 ). When it is determined that the processing objective lens  42  is located at an end part of the object S, the end part determiner  705  outputs an instruction signal to the actuator controller  703  so as to make the latter stop the expansion/contraction of the actuator  43 . In response to the output of this instruction signal, the actuator controller  703  outputs a control signal to the actuator  43  so as to make the latter stop expanding/contracting and attain a held state (step S 10 ). In response to the output of this control signal, the actuator  43  stops expanding/contracting. In response to the output of the control signal from the actuator controller  703  to the actuator  43 , the displacement acquiring/reproducing part  706  terminates the recording of the amount of expansion/contraction of the actuator  43  (step S 11 ). When the processing objective lens  42  is located at the point X 2  on an extension of the line to cut C 1 , the stage movement controller  702  outputs a control signal to the stage  2  so as to make the latter stop moving (step S 12 ). Thereafter, an average value of the amounts of expansion/contraction of the actuator  43  stored as those recorded before a predetermined time from when the recording is terminated among the amounts of expansion/contraction of the actuator  43  stored in the displacement storage  707  is calculated, and the amount of expansion/contraction of the actuator  43  is fixed so as to become this average value (step S 13 ). 
     (Processing Step) The processing step of emitting the processing laser beam L 1  and rangefinding laser beam L 2  will now be explained. 
     The explanation will be set forth with reference to  FIGS. 8(A) to 8(C)  showing the cross section II-II of  FIG. 3  as with  FIGS. 4(A) to 4(C) . For easier understanding, the hatching indicating the cross section is omitted in  FIGS. 8(A) to 8(C) .  FIG. 8(A)  shows a state where the processing objective lens  42  has started forming a modified region on the line to cut C 1 . Before attaining the state of  FIG. 8(A) , the stage  2  is further raised by a predetermined distance (hereinafter referred to as processing height), such that the distance between the surface S 1  of the object S and the processing objective lens  42  is set shorter by the processing height. Here, assuming that the focal point of the visible range and the converging point of the laser beam coincide with each other, the processing laser beam L 1  is converged at a position corresponding to the value of product of the processing height from the surface S 1  and the refractive index of the object S at the laser wavelength within the object S. When the object S is a silicon wafer having a refractive index of 3.6 (at a wavelength of 1.06 μm) and a processing height of 10 μm, for example, the processing laser beam L 1  is converged at a position of 3.6×10=36 μm. 
     The actuator  43  is fixed at the amount of expansion set in  FIG. 4(C) , whereby the processing objective lens  42  is disposed at the initial position (processing initial position). Before the state of  FIG. 4(C)  shifts to the state of  FIG. 8(A) , the processing laser beam L 1  and the rangefinding laser beam L 2  are emitted. The stage  2  moves such that the processing objective lens  42  shifts in the direction of arrow H (processing preparatory step). 
     The rangefinding laser beam L 2  is reflected less by the dicing film  2   a  so that the total quantity of light reflected thereby is smaller, whereas the total quantity of reflected light increases in the object S. Namely, the total quantity of reflected light beam of the rangefinding laser beam L 2  detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) increases, whereby it can be determined that the processing objective lens  42  is located at a position intersecting the line to cut C 1  in the object S when the total quantity of reflected light beam exceeds a predetermined threshold. Therefore, when the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) is greater than the predetermined threshold, the processing objective lens  42  is assumed to be located at one end of the line to cut C 1 , and expansion amount control of the actuator  43  is started so as to release the expansion amount of the actuator  43  from being held at this time. The amount of expansion is controlled according to the track G of the expansion amount of the actuator  43  acquired as explained with reference to  FIGS. 4(A) to 4(C) . More specifically, the displacement acquiring/reproducing part  706  generates reproducing information according to the information of track G stored in the displacement storage  707 , and the actuator controller  703  outputs a control signal to the actuator  43  according to the reproducing information outputted from the displacement acquiring/reproducing part  706  to the actuator controller  703 . Therefore, when the processing objective lens  42  moves in the direction of arrow H in  FIG. 6(A) , the state shown in  FIG. 8(B)  is attained. As shown in  FIG. 8(B) , the modified region R is formed by a fixed processing height in area J (one end part). After the modified region R is formed by the fixed processing height in this area J, the processing objective lens  42  moves along the line to cut C 1 , thereby forming the modified region R with the processing laser beam L 1  (first processing step). 
     When the processing objective lens  42  further moves in the direction of arrow H in  FIG. 8(A)  from the state shown in  FIG. 8(B) , the processing objective lens  42  is located at the other end of the line to cut C 1  as shown in  FIG. 8(C) . When the processing objective lens  42  reaches a position outside of the object S, the state opposite from that explained with reference to  FIG. 8(A)  is attained, whereby the total quantity of the reflected light beam of the rangefinding laser beam L 2  detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) becomes smaller. Therefore, when the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) becomes smaller, the processing objective lens  42  is assumed to be located at a position corresponding to one end of the line to cut C 1  (in the state corresponding to FIG.  8 (C)), the amount of expansion of the actuator at this time is held. The stage  2  moves such that the processing objective lens  42  reaches the position of X 2  in  FIG. 8(C)  while keeping the amount of expansion of the actuator  43 , so as to be ready for the processing of the next line to cut C 2  (second processing step). 
     Though the processing objective lens  42  having reached the position corresponding to one end of the line to cut is detected according to the fact that the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) becomes greater than a predetermined threshold in the above explanation, other criteria can be employed as explained in the displacement acquiring step. Though the processing objective lens located at the position corresponding to the other end of the line to cut is detected according to the fact that the total light quantity detected by the four-divided position detecting device in the light-receiving part  45  (see  FIG. 1 ) becomes smaller than a predetermined threshold in the above explanation, other criteria can be employed as explained in the displacement acquiring step. 
     Operations of the laser processing apparatus  1  in this processing step will be explained with reference to the flowchart shown in  FIG. 9 . 
     The stage movement controller  702  of the control unit  7  outputs a control signal to the stage  2  so as to make the latter rise by the processing height (step S 21 ). In response to the output of this control signal, the stage  2  rises by the processing height. 
     The laser emission controller  701  of the control unit  7  outputs control signals to the laser head  13  and laser diode  44  so as to make them emit the processing laser beam L 1  and the rangefinding laser beam L 2 , respectively (step S 22 ). In response to the output of the control signals, the processing laser beam L 1  and the rangefinding laser beam L 2  are emitted. 
     The stage controller  702  of the control unit  7  outputs a control signal to the stage  2  so as to move the processing objective lens  42  in the direction of arrow H in  FIG. 8(A)  (step S 23 ). In response to the output of this control signal, the stage  2  starts moving. 
     According to the signal outputted from the light-receiving part  45 , the end part determiner  705  of the controller  7  determines whether the processing objective lens  42  is located at an end part of the object S or not (step S 24 ). When it is determined that the processing objective lens  42  is located at an end part of the object S, the end part determiner  705  outputs an instruction signal to the actuator controller  703  so as to make the latter start the expansion/contraction of the actuator  43  and hold the converging point position of the rangefinding laser beam L 2 . The actuator controller  703  outputs the control signal to the actuator  43  so as to make the latter start expanding/contracting in order for the astigmatism signal to equal the held reference value (step S 25 ). In response to the output of this control signal, the actuator  43  holds the processing objective lens  42  according to the amount of expansion/contraction (track G) recorded beforehand (step S 26 ). Therefore, the modified region R is formed at a position corresponding to the displacement of the surface S 1  of the object S (see  FIG. 8(B) ). 
     According to the signal outputted from the light-receiving part  45 , the end part determiner  705  determines whether the processing objective lens  42  is located at the other end part of the object S or not (step S 27 ). When it is determined that the processing objective lens  42  is located at the end part of the object S, the end part determiner  705  outputs an instruction signal to the actuator controller  703  so as to make the latter stop the expansion/contraction of the actuator  43 . In response to the output of this instruction signal, the actuator controller  703  outputs a control signal to the actuator  43  so as to make the latter stop expanding/contracting and attain a held state (step S 28 ). In response to the output of this control signal, the actuator  43  stops expanding/contracting. When the processing objective lens  42  is located at the point X 2  on an extension of the line to cut C 1 , the stage movement controller  702  outputs a control signal to the stage  2  so as to make the latter stop moving (step S 29 ). Thereafter, an average value of the amounts of expansion/contraction of the actuator  43  stored as those recorded before a predetermined time from when the recording is terminated among the amounts of expansion/contraction of the actuator  43  stored in the displacement storage  707  is calculated, and the amount of expansion/contraction of the actuator  43  is fixed so as to become this average value (step S 30 ). 
     The above-mentioned preparatory step and processing step are performed for all the lines to cut C 1  to C n  in the object S, whereby respective modified regions R are formed along the lines to cut C 1  to C n . 
     Though this embodiment relates to a case where one stage of the modified region R is generated, a plurality of stages of modified regions may be formed. The latter case will be explained with reference to  FIGS. 10(A) and 10(B)  in comparison with  FIGS. 4(A) to 4(C) . In  FIGS. 4(A) to 4(C) , the displacement of the surface S 1  of the object S is acquired according to the focal position of the observation visible ray converged by the processing objective lens  42 . When the stage  2  is raised by the processing height in  FIG. 4(A)  and the resulting astigmatism signal is taken as the reference signal, the converging point of the laser beam can be positioned within the object S. When the stage  2  is moved in this state while being irradiated with the processing laser beam L 1  and the rangefinding laser beam L 2 , the state shown in  FIG. 10(A)  is obtained. Namely, while a modified region N 1  is formed by the processing laser beam L 1  within the object S, a track K conforming to the displacement of the surface S 1  of the object S can be acquired if the amount of expansion/contraction of the actuator  43  is recorded. Subsequently, the stage  2  is further raised, and laser processing is performed while reproducing the amount of expansion/contraction of the actuator  43  recorded as the track K as explained with reference to  FIGS. 8(A) to 8(C) , whereby a modified region N 2  is formed within the object S at a position conforming to the displacement of its surface S 1 . 
     Performing laser processing while recording the displacement of the surface S 1  of the object S as such can form the modified region more efficiently. When acquiring the displacement of the surface S 1  of the object S, the converging point of the rangefinding laser beam L 2  is formed within the object S. Therefore, the beam diameter of the rangefinding laser beam L 2  on the surface S 1  of the object S becomes greater, and thus can reduce the influence caused by the state of the surface S 1  (such as streaks of grinding scars caused by back-grinding). 
     Though the moving speed of the stage  2  in the displacement acquiring step and that in the processing step are set identical to each other, it will also be preferred if they differ from each other. More specifically, the moving speed of the stage  2  in the displacement acquiring step (first speed) is set slower than the moving speed of the stage  2  in the processing step (second speed). In this case, the sampling period for acquiring the displacement of the main surface S 1  in the displacement acquiring step (first time interval) is set longer than the sampling period for driving the actuator  43  in the processing step (second time interval). For example, the displacement acquiring step moves the stage  2  at a speed of 60 m/s and uses a sampling period of 1 ms, whereas the processing step moves the stage  2  at a speed of 300 m/s and uses a sampling period of 0.2 ms. Since the product of the moving speed and the sampling period becomes a pitch (distance interval in the direction of the line to cut) in each operation, the pitch for acquiring the main surface S 1  in the displacement acquiring step and the pitch for driving the actuator  43  in the processing step become the same, whereby the recording pitch coincides with the reproducing pitch. Such a setting lowers the moving speed of the stage  2  in the displacement acquiring step, so as to be able to follow the main surface S 1  of the object S even when there are large irregularities. This does not lower the processing efficiency, since the processing speed in the processing step is the same. 
     In this embodiment, the amount of expansion/contraction of the actuator  43  is acquired along the line to cut as one corresponding to the displacement of the surface S 1  of the object S, and a modified region is formed while adjusting the gap between the processing objective lens  42  and the surface S 1  by making the actuator  43  expand/contract according to thus acquired amount of expansion/contraction, whereby the modified region can stably be formed at a predetermined position within the object S. Since the processing objective lens  42  converging the processing laser beam L 1  converges the rangefinding laser beam L 2 , deviations can be prevented from occurring due to exchanges of lenses and so forth, whereby the displacement of the surface S 1  can be acquired more accurately. 
     The processing objective lens  42  and the object S move relative to each other and, after the processing objective lens  42  overhangs the object S, the processing objective lens  42  is released from being held at the initial position, so as to acquire the displacement of the surface S 1 , whereby the displacement can be acquired while excluding the influence of fluctuations in the shape of end parts in the object S as much as possible. 
     The modified region is formed in one end part of the line to cut while holding the processing objective lens at the initial position, and then is formed in conformity to the displacement of the surface S 1  acquired beforehand while releasing the processing objective lens  42  from being held, whereby the modified region can be formed while excluding the influence of fluctuations in the shape of end parts in the object S as much as possible. 
     Since the modified region can stably be formed along the line to cut, when cutting/separating a wafer, which is an object to be processed, into chips by expanding the dicing film  2   a  and so forth after forming the modified region, the wafer can always be cut stably with a favorable cutting quality even when cutting a large amount of wafer. 
     INDUSTRIAL APPLICABILITY 
     The laser processing method and laser processing apparatus of the present invention can efficiently carry out laser processing while minimizing the deviation of the converging point of a laser beam.