Abstract:
Semiconductor wafer inspection device comprising a wager transport arm provided with at least one wafer support element, a wafer gripper, the gripper having two distant branches designed to take hold of the opposed edges of the wafer, the gripper being mounted so as to rotate on a shaft in order to be able to rotate the wafer between an approximately horizontal position and an approximately vertical position, and at least two inspection systems placed on one side of the wafer and on the other, in an approximately vertical position symmetrically with respect to the plane passing through the wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of inspection and verification of semiconductor wafers or substrates during or at the end of production, or during the production of integrated circuits. 
     2. Description of the Relevant Art 
     As a result of the tendency to increase the diameters of semiconductor wafers, they must be handled with extreme care and are increasingly fragile. Furthermore, the increasingly fine engraving of the patterns of a semiconductor wafer makes each component of the wafer more and more susceptible to production faults. 
     In conventional manner, semiconductor wafers are visually inspected by an operator. The human eye is capable of detecting relatively small faults on semiconductor wafers which have, to the untrained eye, the appearance of a mirror. The greater the production quality, the better the human eye is capable of locating small faults. However, the fact that engraving techniques are becoming increasingly fine means that the human eye is reaching its limits, in particular for specific types of fault. 
     Furthermore, the task of visually inspecting semiconductor wafers is slow and fastidious. In a clean room for producing semiconductor wafers, it is desirable to reduce the presence of humans. Visual inspection is also found to be costly. The inspection machines are generally slow and bulky, which is a significant disadvantage in a clean room whilst using electromagnetic radiation which requires specific protection for the operators. Finally, the visual inspection does not generate adequate statistical data relating to the positions, sizes or types of fault, which is indispensable for the statistical monitoring of methods and research into the causes of faults or problems. 
     SUMMARY OF THE INVENTION 
     The invention is intended to improve the situation. 
     The semiconductor wafer inspection device comprises a wafer transport arm which is provided with at least one wafer support element, a gripper for gripping wafers, at least one light source and at least one camera arranged at one side and at least two inspection systems which are arranged at one side and the other of the wafer in its substantially vertical position, symmetrically relative to the plane which extends through the wafer. Each camera of an inspection system can be positioned to capture the light reflected by the surface of the wafer opposite it. Each light source can be positioned to transmit an incident beam towards the surface. The gripper may comprise two remote limbs which are configured to hold opposite edges of the wafer. The gripper can be rotatably mounted on a shaft in order to be able to rotate the wafer between a substantially horizontal position and a substantially vertical position. 
     The method for inspecting semiconductor wafers comprises the following steps:
         a semiconductor wafer to be inspected is conveyed by at least one support element belonging to a transport arm,   the remote limbs which form part of a gripper grip opposite edges of the wafer,   the gripper rotates about a shaft which causes the wafer to move from a substantially horizontal position to a substantially vertical position, and   at least two inspection systems which are arranged at one side and the other of the wafer in its substantially vertical position, symmetrically relative to the plane which extends through the wafer, are actuated.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from a reading of the detailed description of embodiments set out by way of non-limiting example and illustrated with reference to the appended drawings, in which: 
         FIG. 1  is a schematic perspective view of a machine for inspecting flat discs, such as semiconductor wafers, an inspection system having been omitted for better understanding of the drawing; 
         FIG. 2  is a front elevation of the machine of  FIG. 1 , the cover and chassis elements having been omitted for better understanding of the drawing; 
         FIG. 3  is a plan view of the machine of  FIG. 1 ; 
         FIG. 4  is a schematic side view of the gripper in a first position; 
         FIG. 5  is a schematic side view of the gripper in a second position; 
         FIG. 6  is a cross-section view of a limb of the gripper; 
         FIGS. 7 and 8  are flow charts of steps of the method; and 
         FIG. 9  is a plan view of a flat disc inspection machine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Generally, current inspection machines are provided for a semiconductor wafer in a horizontal position resting on a plate, see US 2008/0007726, JP 100 56 046 and KR 2004 0024795. The same applies to document EP 1 194 803 which further proposes a complex catadioptric structure. 
     The Applicant has realised that the inspection of semiconductor wafers in a horizontal position had various disadvantages. On the one hand, the surface flatness of the wafers is affected by gravity and therefore a deformation under the effect of its own weight, this being more significant as the diameter of the wafer increases. The wafers may have a diameter of 300 mm, even 450 mm. The solution generally selected involves using a reference surface having a surface flatness greater than that which can be measured, on which the wafer is positioned or held during the measuring operation. This reference surface is generally that of a solid support. The contact between this surface and the wafer is a source of contamination of the rear face. On the other hand, this method masks the rear face during the measuring of the front face. 
     Document US 2006/0018748 describes a photomask inversion mechanism which allows one face and then the other of a photomask to be inspected by an inspection machine. The inversion mechanism is unsuitable for industrial volumes in that it tends to divide by two the yield of the actual inspection machine. Furthermore, the photomask is carried only by its edges in the horizontal position during the inspection, which indicates that the photomask has a high thickness to length ratio, which is not the case for semiconductor wafers. 
     Document US 2007/0188745 describes a device for inspecting the two faces of a semiconductor wafer in an oblique position. A semiconductor wafer is arranged on an air support mechanism which allows the semiconductor wafer to be supported in a floating manner. The lower face of the semiconductor wafer is inspected by a scanning head which can be moved in translation along an axis, the scanning head having a length which is much smaller than the diameter of the semiconductor wafer. The semiconductor wafer is rotatably driven by support rollers against which the edge of the semiconductor wafer abuts. By combining the rotation of the wafer and the translation movement of the scanning head, the lower surface of the wafer can be progressively inspected, the inclined position providing a degree of support for the edge of the wafer on the rollers. However, the inspection of the upper face requires special means owing to the inclination of the wafer. The inspection of the lower face is particularly slow owing to the actual structure of the device. The wafer is at risk of vibrating and/or becoming deformed under the flow of air which holds it in a floating position. 
     Owing to the invention, the wafer which is held so as to be fixed in a substantially vertical position during the inspection has a particularly reduced risk of deformation. The term substantially vertical is understood to be to within +/−5°. The symmetrical structure of the device allows a regular laminar air flow within the device and reduces the risk of the semiconductor wafer becoming contaminated with dust or other harmful elements during the inspection (this cannot be the case during an inspection in the inclined position, since the air is unilaterally deflected). 
     Furthermore, the impact of the flow of air on the mechanical stability of the semiconductor wafer is reduced. The mechanical vibrations are reduced. The vertical position at the semiconductor wafer prevents a deformation under its own weight, which avoids use of a costly and heavy rectified plate. The risk of contamination of the lower surface of the semiconductor wafer is particularly reduced, in so far as the wafer is gripped by grippers which come into contact with the outer edge thereof. 
     The lower surface of the semiconductor wafer is available for an inspection which can take place in the same vertical position which increases the possibility of establishing correlations between the faults of the upper surface and the faults of the lower surface. Furthermore, the fact that the inspection of the rear surface is carried out in the same position as the inspection of the upper surface allows a significant time saving. The stability of the semiconductor wafer during the measurement operations reduces the risks of contamination, drastically reduces mechanical vibrations and increases the reliability of the measurements. 
     The opposite surfaces of the wafer are conventionally referred to as the upper surface and lower surface, even when the wafer is in a vertical position with reference to the horizontal position of the wafer supported by the fork, before and after the inspection in the vertical position. 
     The support elements provided to support a wafer upstream and downstream of the gripper may form a fork. The support elements may thus move the wafer into a horizontal position whilst remaining remote from the limbs of the gripper. The fork provides good mechanical stability of the semiconductor wafer and deformation under its own weight within acceptable limits during transport. The transport arms may comprise at least two articulation axes. The transport arm may be supported by a turret. The turret can support two wafer transport arms. The turret can be mounted for translation on a sliding member. The turret with two transport arms allows the movement of the wafer to be optimised between a store for wafers and the gripping gripper. 
     Each limb of the gripper may have a groove which is provided on a surface which is arranged opposite the other limb. The groove can be configured in accordance with the shape of the edge of the wafer. The groove may be generally V-shaped in axial section. The groove can ensure self-centering of the wafer. 
     At least one of the limbs of the gripper is pivotably mounted along an axis which is substantially perpendicular relative to the plane of the wafer held between the limbs. Preferably, the two limbs are pivotably mounted along parallel axes or along a common axis. The transport elements may have a lateral spatial requirement less than the opening between the limbs of the gripper. The risks of mechanical interference between the transport elements and the gripper are thus limited. 
     Each source of light may comprise a monitor having a diagonal which is greater than the maximum opening between the limbs of the gripper. Excellent illumination of the opposing surfaces of the semiconductor wafer is thus ensured. The light source may comprise an LCD or plasma screen. The camera can be arranged above the corresponding source of light. 
     The device may comprise a control unit which is configured to control the display of parallel lines by the light sources. The control unit can be configured to control an alternating arrangement of vertical and horizontal lines displayed by the light sources. 
     The control unit may comprise at least one output which is connected to a camera and at least one output which is connected to the corresponding light source in order to synchronise the camera and the light source. Advantageously, the control unit controls the two cameras and the two symmetrical sources of light with parameters such as the exposure times or the illumination mode specific to the characteristics of each face. 
     The control unit can be configured to control an illuminated oval zone of the light source and a dark outer edge. The light reaching the surface of the semiconductor wafer is thus optimised, increasing the proportion of incident light. In the same manner, the proportion of incident or diffused light captured by the cameras is reduced. The saturation decreases. The contrast is thereby improved. 
     The control unit can be configured to control the display of different colours simultaneously by the sources of light in order to optimise the detection quality. 
     The control unit may be configured to control an illumination which is alternated by the light sources. The proportion of diffused light reaching the cameras is thus reduced. When the semiconductor wafer is illuminated by a light source, a portion of the incident beam can pass through the semiconductor wafer and be subject to phenomena of diffraction and reach the camera located at the opposite side. Of course, this phenomenon is dependent on the reflectivity of the semiconductor wafer. In the case of a particularly reflective wafer, the phenomenon is relatively harmless and the illumination may be simultaneous. In contrast, in the case of a relatively non-reflective semiconductor wafer, the alternating illumination allows better resolution of the detection. 
     The control unit can be configured to control the taking of images by the cameras in an alternating or simultaneous manner, in particular in synchronisation with alternating or simultaneous illumination. Each image can be taken for an exposure time of approximately from 100 to 3000 milliseconds. The transfer time by the cameras can be in the order of from 5 to 600 milliseconds. The simultaneous or alternating taking of images allows the duration of the inspection to be optimised compared with an inspection of one face and then the other. 
     Furthermore, the cameras may be provided with an electronic or mechanical shutter. The synchronisation between the taking of images with a camera and the illumination by the corresponding light source may replace the electronic or mechanical shutter whilst ensuring that quality images are taken. In this instance, the duration of the illumination may be between 100 and 3000 milliseconds. 
     The cameras may be provided with an objective lens, referred to as a pivoting lens. It is possible better to observe the edges of the semiconductor wafer owing to the pivoting of the focal plane. 
     The LCD or plasma screen of the monitor may comprise a frosted panel which allows a reduction in fringe residues which are linked to the generation of a harmonic which is connected to the periodicity of the pixels of the screen and the presence of dead zones. The frosting may be carried out using acid or by mechanical frosting of the sanding or polishing type. In this manner, a fault inspection precision is obtained in the order of a nanometer perpendicularly relative to the surface observed. 
     In  FIGS. 1 to 3 , the inspection machine  1  has been illustrated with the cover open. More precisely, in  FIG. 1 , the front cover and one of the lateral covers are open. In  FIG. 2 , the front cover is open. In  FIG. 3 , the top cover is open. Of course, in the operating state, the inspection machine  1  is provided with covers thereof closed. The covers are opaque in order to prevent the introduction of parasitic light which is capable of interfering with the cameras. Furthermore, in  FIG. 1 , one of the two screens, the corresponding camera and the support of the gripper have been omitted so that the other components can be better seen. In the same manner, in  FIG. 2 , the gripper and the gripper support have been omitted, the substrate being presented there in the inspection position which is substantially vertical. 
     As can be seen in  FIGS. 1 to 5 , the inspection machine  1  comprises a frame  2 , for example, of the mechanically welded type, which forms an inspection chamber  3  and a supply chamber  4  which are separated by a partition  5  through which an aperture  6  extends. The frame  2  is covered by the covers. 
     The inspection chamber  3  has a symmetrical structure relative to a vertical plane which extends through the centre of  FIGS. 2 and 3 . The inspection machine  1  comprises a supply  8  of filtered air of the laminar type which allows a movement of air to be generated in the direction from the top to the bottom of the chamber  3  as illustrated by the arrows  7 . The supply  8  of air also forms the upper wall of the chamber  3 . The floor of the measuring chamber is constituted by a stack of 2 grids, one of which can be offset from the other, which allows the flow of air leaving this output to be controlled and thereby allows the excess pressure in the measuring chamber to be controlled. 
     The inspection machine  1  comprises two video screens  9  and  10  which are mounted symmetrically, in particular relative to a vertical plane which extends through the centre of the inspection machine  1  or which extends through the substrate  11  to be inspected which is held in a vertical position, see  FIGS. 1 and 2 . Each screen  9 ,  10  rests on a support  12 , for example, of the articulated type which allows orientation of the screen  9 ,  10  along an axis which is substantially parallel relative to the plane of the substrate  11 , for example, a substantially horizontal axis, and adjustment in translation of the position of the screen  9 ,  10  relative to the measured surface. The screens  9  and  10  are mounted opposite each other remotely and are orientated slightly in an upward direction, for example, with an angle of between 10 and 30°. The screens  9  and  10  may be of the LCD or plasma type. The screens  9 ,  10  have a height which is greater than 1.6 times the diameter of the substrate to be inspected, for example, a height of 54 cm for a substrate with a diameter of 300 mm and a height of 72 cm for a substrate with a diameter of 450 mm. 
     The sides of the illumination screen are conventionally referred to as the height and width. The height is intended to be understood to be the smallest dimension of the display zone of the screen, with reference to the orientation of the screen when the screen is used as a conventional video display device. 
     The inspection machine also comprises two cameras  13 ,  14  which are located in the inspection chamber  3 . The cameras  13 ,  14  can be supported by the supports  12 . A support  12  is common to a screen  9  or  10  and a camera  13  or  14 . The camera  13  is hidden in  FIG. 1  by a pillar of the frame  2 . The cameras  13 ,  14  can also be adjusted in terms of position, in particular in terms of height, width and length, the length corresponding to the horizontal distance relative to the substrate  11 . Furthermore, the cameras  13  and  14  can be adjusted in terms of angular orientation. The cameras may be of the CCD type (Charge Coupled Device) or CMOS type (Complementary Metal Oxide Semiconductor). The screen  9  and the camera  13  form a first inspection system. The screen  10  and the camera  14  form a second inspection system. The first and second inspection systems are symmetrical. The respective positions of the screen  9 , the substrate  11  and the camera  13  at one side, the screen  10 , the substrate  11  and the camera  14  at the other side of the inspection chamber  3  are selected so that each screen  9 ,  10  transmits an incident beam which reaches the substrate  11  at its corresponding face  11   a ,  11   b , respectively, and the camera  13 ,  14  captures the beam reflected by the surface  11   a ,  11   b . The faces  11   a  and  11   b  are parallel. The incident beam does not completely reach the substrate  11 . The relative positions are selected so that the surface  11   a ,  11   b  is sufficiently illuminated to allow the camera  13 ,  14  to detect a light signal which is representative of faults in the surface  11   a ,  11   b . The luminosity and contrast of the screen  9 ,  10  are adjusted to high levels to promote the detection of faults by the cameras  13 ,  14 . Furthermore, the inactive surfaces of the chamber  3  have maximum absorption of the wavelengths used. That is to say, the inactive surfaces of the inspection chamber  3  are black. The interference of the cameras  13 ,  14  is thereby limited. 
     Since the cameras  13 ,  14  are inclined relative to the normal to the surfaces  11   a ,  11   b , they have a slight distortion, the distance between the upper end of the surface  11   a  and the objective lens of the camera being less than the distance between the lower end of the surface  11   a  and the objective lens of the camera  13 . The same applies to the camera  14  relative to the surface  11   b  facing it. To this end, the cameras  13 ,  14  may comprise a pivoting objective lens which allows the distortion to be reduced and better observation to be achieved for the peripheral zone of the surfaces  11   a  and  11   b.    
     The inspection machine  1  comprises a gripper  15  for retaining the substrate  11 . The gripper  15  which can be seen in  FIGS. 1 and 3  is illustrated in greater detail in  FIGS. 4 and 5  which provide the substrate  11  with a horizontal receiving position and vertical inspection position, respectively. The gripper  15  comprises a base  16  which rests on the frame  2 , a turret  17  and two limbs  18  and  19 . The base  16  may generally have a rectangular parallelepipedal shape. The turret  17  is articulated to the base  16  along a substantially horizontal axis which extends via the aperture  6 . The turret is capable of ensuring a rotation of at least 90°. A rotation of 180° may allow a substrate  11  to be inverted which may be found to be advantageous in some applications. The rotation of the turret  17  may be provided by an electromechanical driving device which is arranged in the base  16 , for example, a step motor. 
     The limbs  18  and  19  are symmetrical relative to a plane normal with respect to the substrate  11  when the substrate  11  is carried by the limbs  18  and  19 . Each limb  18 ,  19  is articulated to the turret  17  along its own axis, which is offset relative to the pivot axis of the turret  17  and normal with respect to the substrate  11 . In a variant, the limbs  18  and  19  may be coaxial. In another variant, one of the limbs is stationary relative to the turret  17  and the other limb is articulated. The turret  17  comprises a member for actuating the limbs  18 ,  19 , for example, in the form of two step motors or one step motor and a gear mechanism which allows the limbs  18  and  19  to remain symmetrical regardless of their angular position. The limbs  18  and  19  can pivot between two operating positions, an open position used to move the substrate  11  closer or further away and a position in engagement with the outer edge  11   c  of the substrate  11 .  FIGS. 4 and 5  illustrate the position in engagement. 
     More specifically, each limb  18 ,  19  has a bent shape so that the spatial requirement of the turret  17  is less than the diameter of the substrate  11 . That is to say, the limbs  18 ,  19  are in the form of a circumflex accent. The limbs  18 ,  19  each have an inner face  18   a ,  19   a  which is opposite the inner face of the other limb  19 ,  18  and which is provided to come into contact with the outer edge  11   c  of the substrate  11 . The inner face  18   a ,  19   a  has an elongate groove  20  parallel with the pivot axis of the turret  17 . The groove  20  which can be seen in  FIG. 6  may have a cross-section which is V-shaped, or alternatively in the form of a semi-circle or ogive, to co-operate in an appropriate manner with the outer edge  11   c  of the substrate  11  and to provide retention both in the horizontal position of the substrate  11  illustrated in  FIG. 4  and in the vertical position illustrated in  FIG. 5  and in intermediate positions with a slight clamping action which reduces to negligible values the deformation of the substrate  11 , in particular the buckling in the inspection position. 
     The inspection machine  1  comprises a substrate handling member  21  which is provided to bring a substrate  11  to the gripper  15  before inspection and to unload the substrate from the gripper  15  after the inspection. The handling member  21  is arranged in the supply chamber  4 . The handling member  21  may be in the form of a robot which is provided with an operating element which is capable of passing through the aperture  6  provided in the partition  5 . 
     The inspection machine  1  comprises two containers  22 ,  23  which can be removed to store a plurality of substrates  11 . The containers  22 ,  23  are supported by a wall of the chamber  4  at the side opposite the internal partition  5 . The containers  22 ,  23  may be of the self-closing type so as to close during a separation with respect to the inspection machine  1 . In the same manner, the wall of the handling chamber  4  is provided with an aperture in the region of the containers  22 ,  23 , preferably provided with an automatic shutter which closes the supply chamber  4  before the containers  22 ,  23  are completely removed. The contamination of the substrates  11  and the chambers of the inspection machine  1  by dust is thereby limited. 
     The inspection machine  1  comprises a prealignment member  24  for the substrates  11 . The prealignment member  24  may be arranged along the partition  5  at a longitudinal end of the supply chamber  4 . The prealignment member  24  and the supply chamber  4  are separated by a partition  25  through which there extends an opening  25   a  which allows the passage of a substrate  11  which is carried by the handling member  21 . Furthermore, the inspection machine  1  comprises a control and processing unit  26  which can be in the form of an electronic scanner. The control unit  26  is arranged at the end of the supply chamber  4  opposite the prealignment member  24  with a separation partition  27 . The processing unit  26  can also be in contact with the partition  5 . The control unit  26  is connected to the screens  9  and  10 , to the cameras  13  and  14 , to the gripper  15  and to the handling member  21 . 
     The handling member  21  comprises a turret  28  which is capable of moving in translation relative to the frame  2  along an axis parallel with the partition  5 . In this manner, the handling member  21  can move close to the opening  25   a  in the direction towards the prealignment member  24  in one position and move in the region of the aperture  6 , opposite the gripper  15 , in another position, or opposite the container  22  or opposite the container  23 . The turret  28  can move along a sliding member  29  which is fixedly joined to the frame  2 . The handling member  21  comprises an arm  30  having two articulation axes, supported by the turret  28 , and a fork  31  supported by the end of the arm  30  opposite the turret  28 . The articulation axes of the arm  30  may be substantially vertical. That is to say, the arm  30  is provided with two mutually parallel articulation axes normal with respect to the plane of a substrate  11  resting on the fork  31 . 
     The fork  31  may be in the form of a plate having a substantially constant thickness and a generally rectangular contour with a large cut-out provided from one short side, allowing two teeth to remain. The cut-out may generally have a slightly flared U-shape. The teeth of the fork  31  form a substrate transport element. The spacing between the teeth of the fork  31  may be adapted to the diameter of the substrate to be handled, for example, of between 150 and 250 mm for a substrate having a diameter of 300 mm or between 225 and 400 mm for a substrate having a diameter of 450 mm. The fork  31  is provided with a movement in a horizontal plane owing to the movement of the turret  28  on the slide  29  and to two rotations allowed by the two articulation axes of the arm  30 . The handling member  21  comprises a mechanism for vertical movement, in particular in translation in order to adjust the height of the fork  31  and consequently the substrate  11  carried by the fork  31 . 
     During operation, see  FIG. 7 , the control unit  26  controls the handling member  21 , the gripper  15 , the screens  9  and  10  and the cameras  13  and  14 . The handling member  21  is presented opposite the container  22  which contains a plurality of substrates to be inspected. The fork  31  passes below a substrate  11  then raises the substrate  11  by a few millimeters and withdraws from the container  22  supporting the substrate  11 . The handling member  21  then moves the substrate  11  as far as the prealignment member  24  which brings about appropriate positioning of the substrate  11 , for example, using three fingers which are moved with a radial movement and which come into contact with the outer edge  11   c  of the substrate  11 . Then, the fork  31  ensures that the substrate  11  is taken and moves it through the aperture  6  in order to move it between the limbs  18  and  19  of the gripper  15 . The fork  31  is located very slightly below the limbs  18  and  19  so that the substrate  11  is located in the region of the limbs  18  and  19 . The limbs  18  and  19  are clamped on the outer edge  11   c  of the substrate  11 . The fork  31  is lowered to become disengaged from the substrate  11  which is now held between the limbs  18  and  19 , in particular in the grooves  20 . The handling member  21  withdraws the fork  31 , for example, into the handling chamber  4 . 
     The substrate  11  held between the grippers  18  and  19  in a substantially horizontal starting position, is rotated through a quarter turn in order to move it into the substantially vertical position illustrated in  FIG. 1 . The control unit  26  then proceeds with the inspection itself by controlling the illumination by the screen  9  of the face  11   a  of the substrate  11  fixed in position by the gripper  15 . The screen  9  displays substantially vertical lines which are alternately luminous and black, then substantially horizontal lines  35  which are alternately luminous (white or colour) and black, for q times, with q being between 1 and 20. Simultaneously, the camera  13  takes images, for example, for a duration of between 100 and 3000 milliseconds. The camera  13  may take a succession of images for each type of line. Then, the screen  9  is switched off and the screen  10  illuminated to illuminate the face  11   b  of the substrate  11 . The screen  10  displays lines which are similar to those of the screen  9 , in particular vertical lines  34 , see  FIG. 2 . The camera  14  simultaneously takes one or more images. The images taken by the cameras  13  and  14  are transferred to the control unit  26  which brings about a processing operation in order to verify the presence of faults, in particular faults in the surface flatness or appearance of the faces  11   a  and  11   b  of the substrate. This sequential operating mode may advantageously be replaced with a simultaneous mode, where the screen/camera system inspecting the upper face and that inspecting the lower face operate in an independent and simultaneous manner. 
     In one embodiment, the illumination is provided by the whole surface of the screens  9  and  10 . The Applicant has realised that it was advantageous to limit the illumination to an oval zone  32  on the screens  9  and  10  corresponding to the geometric projection of the faces  11   a  and  11   b  of the substrate  11  on the screens  9  and  10 , respectively. In this instance, the vertical lines  34 , then horizontal lines  35  are displayed in the oval zone  32 , the outer edge  33  of the screen remaining black. The quantity of light diffused in the inspection chamber  3  is reduced and the interference is reduced for the cameras  13  and  14 , which can then provide a signal with improved quality. 
     Then, since the stationary phase of the substrate  11  in a substantially vertical position has ended, the turret  17  of the gripper  15  controlled by the control unit  26  rotates substantially through a quarter of a turn, to place the substrate  11  in a substantially horizontal position. The fork  31  of the handling member  21  moves below the substrate  11  at a safe distance, for example, in the order of a few millimeters, then moves vertically and climbs at low speed as far as the region of the lower face  11   b  of the substrate  11 . The limbs  18  and  19  then move from the engaged position to the open position, the substrate  11  resting on the fork  31 . 
     The fork  11  leaves the inspection chamber  3  and, moving through the supply chamber  4 , places the substrate  11  in the container  22  or  23 . The cycle can then be repeated. Of course, in order to increase the productivity of the inspection machine, the handling member  21  can be controlled to take a substrate  11  and convey it to the prealignment member  24  during the steps during which the substrate  11  previously carried to the gripper  15  is being inspected by the cameras  13  and  14 . 
     As can be seen in the flow chart of  FIG. 7 , the steps of illumination by the screen  9 ,  10  and observation by the camera  13 ,  14  may be repeated until sufficiently precise data are obtained. The number of sub-steps p can be between 1 and 10. 
     In the embodiment of  FIG. 9 , the handling member  21  can be provided with a turret  28  which supports two arms  30 ,  33 , each of which is provided with a fork  31 ,  32 . The productivity of the inspection machine  1  can be improved by following the flow chart of  FIG. 8  in so far as the fork referred to as the upstream fork, can be dedicated to the handling steps prior to the inspection by the cameras  13  and  14  whilst the additional fork referred to as the downstream fork can be dedicated to the handling steps following the inspection by the cameras  13  and  14  to move the substrate  11  inspected from the gripper  15  into the container  22  or  23 . 
     Several steps can be carried out simultaneously depending on the respective durations of each step and in particular the duration of the inspection by the cameras  13  and  14 . More specifically, the upstream fork may remove a substrate from the prealignment member  24  whilst the preceding substrate is being inspected by the cameras  13  and  14 , the upstream fork waiting for the preceding substrate to be removed by the downstream fork. As soon as the downstream fork has removed the preceding substrate  11  from the processing chamber  3 , the upstream fork can introduce the following substrate into the processing chamber  3 . That is to say, the duration between two inspection steps by the cameras  13  and  14  is reduced, leading to a higher yield. 
     Furthermore, the upstream fork has two operations to carry out, bringing a substrate  11  to the prealignment member  24 , then bringing the substrate  11  to the gripper  15 , whilst the downstream fork has one handling operation to carry out: bringing the inspected substrate  11  to the downstream container  23 . The control unit  26  can give priority to the upstream fork  31 , which again allows the cycle time to be reduced slightly. In this manner, the downstream fork may remain with an inspected substrate awaiting storage whilst the upstream fork carries out another operation, for example, removing a substrate from the container  22  in order to bring it to the prealignment member  24 , or removing a substrate  11  from the prealignment member  24 . 
     Furthermore, the control unit  26  can be configured to simultaneously operate the sources of light formed by the screens  9  and  10 . 
     The containers  22  and  23  may be used, one as an upstream container and the other as a downstream container. The containers  22  and  23  may be used, one after the other, a substrate  11  removed from the container  22  returning there after inspection, optionally in the same position.