Micro Positioning Apparatus

Abstract

A workpiece is positioned in the order of tenths of microns, or less, with respect to a fixed assembly, with automatic stabilization of coordinates by apparatus having a solid base with two movable carriages, displacable each in a different orthogonal direction, which drive a reference block bearing the piece to be positioned. To compensate for malpositioning a correction means is provided consisting of an assembly of at least three concentric frames, the first one (C1) being fixed on the upper carriage and the other two being coupled to each other by resilient means and position transducers, such as piezoelectric ceramic or megnetostrictive elements. The transducer displaces the corresponding frame along a direction where no resilient means are present. Displacement is measured by four interferometers whose beams illuminate reflecting faces of the reference blocks and fringes; or photoelectric microscopes aimed for instance at an engraved marking on the block of some tens microns wide are used.

Description

The present invention relates to a positioning apparatus intended to accurately position a part with automatic stabilization of its coordinates. It especially concerns devices that, while controlling a slow displacement of the said part along one of its coordinates, provide for automatic stabilization of the other coordinates by means of an opto-electric regulating loop. These cartesian and/or polar coordinates are determined with regard to a reference system a mobile element of which supports the part to be positioned.

Such apparatus are used for instance for the photographical reproduction of several consecutive views on a same photosensitive surface, the position of the said views having to be located with a very high accuracy with reference to the picture taking system. The displacement of the photographical plate is usually carried out along two orthogonal coordinates by means of a mechanical system of crossed carriages provided with high accuracy screw-jack controls. Such a system however does not allow an accuracy better than one micron. Furthermore that accuracy varies with time owing to wear of control components generating backlash, and also with unequal thermal expansion of the different parts of the structure. On the other hand, it is very difficult to maintain one of the coordinates constant in the cartesian plane while the other is varied, for a parasitic rotation of one of the carriages is always likely to take place.

To remedy these shortcomings the present invention has for one object the design of an accurate positioning device of a piece with automatic stabilization of its coordinates.

Subject matter of the invention, the positioning device consists of at least three concentric frames (C1, C2, C3) coupled two by two along different coordinates by flexion blades (L1, L2). Between them are pressed together biased piezoelectric ceramic transducers (X1, X2, Y1, Y2) placed along the coordinate not occupied by the blades (L1, L2), the inner frame (C3) supporting a reference block (A) on which is placed the part (P) to be located, position transducers (L, M, S, F, E) solid with said block (A) providing comparator circuits with voltages whose difference with regard to those previously chosen is applied as an error voltage to the corresponding groups of ceramic transducers (X1, X2 or Y1, Y2). Further, crossed carriages (V - W) are provided on the base of the device, to drive the reference block along either of two orthogonal directions.

Characteristics as well as features of the present invention will be better understood from the following description, of a non-restrictive example with reference to the accompanying drawings in which

- FIG. 1 shows a general perspective view of an apparatus according to the invention;

- FIG. 2 is a view from underneath of a particular embodiment of the mechanical part of the device of FIG. 1;

- FIG. 3 is a block diagram of the optical locating system;

- FIG. 4 shows the diagram of a complete particular embodiment of the positioning apparatus;

- FIG. 5 is a view from above of a variant of carrying out and use of the positioning apparatus.

As the position of a part related to a system may vary with the time for instance because of small temperature variations of the ambient, or also because of its displacement along one of its coordinates, the said displacement generally being accompanied by parasitic rotations, a very accurate permanent localization of the said part is achieved according to the present invention, and the results obtained are then used by means of a stabilizing loop allowing the correction of the fixed coordinates of the part in a way maintaining them equal to a previously chosen mean value, with an accuracy better than one-tenth of one micron.

To this end a device is used such as that represented on FIG. 1. This device comprises a correcting system C, controlled by piezoelectric ceramic transducers and fixed on the upper carriage of for instance two crossed carriages V - W placed one above the other on the base Z of the equipment and intended to displace the optical block A by translation along two orthogonal directions respectively.

The displacements of this latter with regard to the base Z are measured by four interferometers I1, I2, I3, I4 whose beams illuminate the reflecting faces A1 and A2 of block A. The deviations with regard to the ideal displacement of block A produce error voltages that, when applied to the ceramic transducers of correction device C, result in the reduction of the amplitude of these shortcomings.

FIG. 2 is a top plan view of a particular structure of the mechanical part of correction device C. This mechanical part comprises three concentrical frames C1, C2, C3 made mutually solid by means of resilient blades L1, L2. These frames are preferably made of a low thermal expansion coefficient metal such as invar for instance. Piezoelectric ceramics X1, X2, Y1, Y2 are placed on the inner faces of these frames on both sides of the corresponding concentric frame, their axes being the mediatrices of the sides of the said frames comprising resilient blades such as L1 and L2. The contact between these transducers and the outer face of the corresponding concentrical frame is made for instance by means of a metallic ball B clasped between the said face and that of the transducer. This arrangement thus controls the displacement of the frame C3 with regard to frame C1, and thus with the base Z of the equipment, either along direction X, or along direction Y. These displacements are carried out independently from one another, in one direction or in the other. The property of piezoelectric ceramics to expand or to contract with respect to a mean expansion upon application of an electric signal is well known and need not be emphasized here.

If there is no control signal, the same bias voltage is applied to both ceramics and it produces the same expansion of the said ceramics. This force thus avoids backlash of the mechanical assembly of two frames one in the other. The control signal applied to either of both groups of ceramics is such that it brings about an increase of expansion for one of the two ceramics of this group, and a reduction of the expansion of the other, causing the displacement of the inner frame regarding the reference frame C1. A control signal applied to the group X1, X2 thus involves a displacement along the X-axis of frame C2, and so indirectly of frame C3, while such a signal acting on group Y1, Y2 produces a displacement along the Y-axis of the said frame C3. It is easy to understand that this arrangement permits correction of the positions along X and Y at any time using as control signals for the ceramic transducers the error voltages produced by comparing the continuously accomplished measurements of the coordinates and the previously chosen mean values. Thus an automatic stabilization of one or the other coordinate X or Y, or both, is obtained. Further, inaccuracies that may result from vibrations of frequency comprised in the frequency band of the correcting device are avoided.

To correct the position errors owing to possible spurious rotations of part P, the latter is placed upon a reference block A placed on a stand C4 suspended within frame C3 by means of resilient blades L3. Ceramic transducers R, acting upon these blades L3 through balls B, allow slight rotation of support C4 regarding frame C3, and thus regarding reference frame C1. These ceramic transducers R are controlled by the error signal arising from the comparision of two simultaneous measurements along the same coordinate, carried out close to each extremity of a same side of reference block A.

Thus, having placed part P to be positioned upon reference block A, for instance by means of fixed marks on the said block A, the latter is first roughly positioned on support C4, for instance by means of the mechanical crossed carriages system V, W of FIG. 1. This positioning is carried out with respect to the base Z of the equipment supporting the coordinate measurement system. The yield of the comparison of these measurements with the previously chosen mean values in then applied as an electric control signal to the correcting ceramic transducers X1, X2, Y1, Y2, R, a bias voltage being on the other hand applied continuously to the latter ones, thus defining a mean operating point. The corrections thus accomplished are checked continuously resulting in a coordinate stabilization. These corrections being mutually independant, a displacement of part P and block A can be carried out along one of the coordinates while maintaining the other fixed.

Thus, the joint action of piezoelectric transducers X1, X2, Y1, Y2, R introduces very fine mutually independant corrections along axes X and Y and/or to rotation.

In a particular embodiment of the device, the coordinate measuring system is an optical system based on laser interferometry. A basic diagram of this system is represented on FIG. 3. A laser L, preferably of the stabilized monomode type, simultaneously illuminates four Michelson interferometers through three partially reflecting mirrors M1, M2, M3 and two opaque mirrors M4 and M5. Each of these interferometers comprises a fixed-flat mirror F, a mobile-flat mirror composed of one of the faces A1 or A2 of reference block A, and a partially reflecting mirror S. Reference block A advantageously is an optical element whose reflecting faces A1 and A2 have a very good flatness, the 90 degree angle formed by these cases being defined with an accuracy better than .+-. 0.5 arc second.

When, because of the movement of the crossed carriages, reference block A moves along axis X or Y with respect to stand C4, hence to the base Z whereon the optical system is fixed, the two interferometers directed along the displacement direction transmit to the corresponding receiving systems E a light intensity sinusoidally varying with the displacement.

Receiving systems E comprise for instance photoelectric receivers followed by circuits capable of discriminating between the direction of the movement. In such circuits, a reversible counter determines the accurate position of part P, independently of the successive backward and forward movements of reference block A, with an absolute precision better than 0.3 micron. Conventional circuits not represented here then compare the measured values with the predetermined values. The resulting error signals are then applied to the piezoelectric transducers in order to accomplish a simultaneous position correction. In the application of these devices to the photographic production technique of microcircuit masks where a same image is impressed upon a photographic plate along lines and columns, the impression of a line of images may be accomplished for instance by stabilizing the column coordinate and also correcting the parasitic rotations while guiding the plate displacement along the said line, the displacement measurements accomplished being usable for displacement speed regulation along this line.

FIG. 4 represents a detailed diagram of a particular embodiment of a device according to the invention. The optical part of this realization is practically the same as that above described with reference to FIG. 3, reference mirrors F1, F2, F3, F4 in this instance being fixed upon piezoelectric transducers K1, K2, K3, K4 controlled by means of potentiometers P5, P6, P7, P8.

The correction device C under which are placed crossed plates W and V comprises three groups of concentric parts C1, C2, C3. These parts can be suspended one regarding the other by flexion blades L1, L2 and displaced by piezoelectric transducers X1, X2, Y1, Y2, each comprising two electrically independent stacked transducers, springs T.sub.x and T.sub.y being used to support frames C1, C2 C3 against the said transducers.

At the beginning of the operating process, block A is brought by means of plates V - W to the proximity of the position of origin defined for instance by three photoelectric microscopes D1, D2, D3. These microscopes can locate the position of an engarved mark some tens of microwide, with an accuracy of between some microns to some hundredths of micron according to the sensitivity of the equipment. The feed accuracy of the screws of plates V, W should allow in a first time to position block A with regard to the microscopes within a few microns. The position of origin can then be defined within less than 0.1 micron by means of piezoelectric transducers X1, X2, y1, Y2 controlled by potentiometers P1, P2, P3, P4. This ultimate operation is accomplished with the maximum sensitivity of the microscopes corresponding to an accuracy of some hundredths of one micron.

The position of origin of block A once defined, potentiometers P5, P6, P7, P8 are adjusted in order to displace very finely reference mirrors F1, F2, F3, F4 and observe on voltmeters U1, U2, U3, U4 the corresponding shifting of interference fringes. If the displacement of block A is then to be made along X, switches J.sub.x are thrown over from position 1 to 2 in order to apply the amplified output voltage of receivers R1 and R2 to transducers Y1 and Y2.

During this displacement, the translation or rotation deviations referred to the ideal displacement along X result at the output of receivers R1, R2 in a slight shifting of the fringe part corresponding to a positive or negative error voltage with regard to zero. Amplified by elements Q1 and Q2, this error voltage is applied to transducers Y1 and Y2 in order to correct these shortcomings and reduce its amplitude in a proportion equal to the regulation rate of the system.

During this constant Y-displacement, a second receiver R6 connected with receiver R5 of interferometer I4 allows according to a known measurement process the displacement along X by totalizing the number of passing interference fringes. For this purpose, a logic circuit B.sub.2 for discrimination between the directions of the movement controls a reversible counter G2 which adds or subtracts the finges provided by R5 according to the displacement of block 1 along X in one direction or the other. The result of this measurement can be displayed on a direct view display device E.sub.x.

When during the displacement along X the value read on E.sub.x is sufficiently near to the desired value, plate V is stopped, and the ceramic transducers controlled by P3 and P4 proceed to the fine position adjustment. In this case, voltmeters U5 and u4 allow indeed near the zero of the last fringe to define a whole number of fringes with a fractionnary excess whose precision is of the order of magnitude of only some hundredths of a micron.

The same procedure is also applied to carry out a given displacement along Y, X being constant. The original position of block A can also be located by means of only two microscopes locating the position of marks engraved on the upper horizontal face of the block. But in this instance, the microscopes are mounted vertically above the block and may be difficult to integrate into a general system.

Some minor modifications excepted, these devices may also be used for other applications such as the position marking of a body of revolution revolving around its axis. A device according to the present invention and adapted to this application is represented on FIG. 5. This device can moreover be used to measure the deformations of a revolving body using a technique called holographic interferometry and consisting of comparing the body to be examined with its hologram. The accurate positioning of this body P is tantamount to making coincide its axis of revolution O with the rotation axis of plate T it is placed onto, and to correct the errors inherent to the unavoidable shortcomings of said plate T.

The mechanical part of the device of FIG. 5 is obtained starting from that of FIG. 2 by replacing the assembly formed by frame C3, stand C4 and ceramic transducers R with a stand C5 supporting the rotation axis of plate T. The carraiges V, W are hidden in the view of FIG. 5A mechanical system with crossed carriages 100 and 200 mounted on plate T allows a first centering with a mechanical comparator N mounted on stand C5 by exploring a narrow region of the periphery of part P while plate T is rotating. This first centering is achieved with a precision of about a few microns. A precision better than one tenth of a micron is obtained by using additionally displacement transducers such as two Michelson interferometers illuminated by a same laser source L. The movable mirror of these interferometers, arranged one along the X, and the other along the y axis, is formed by the point whereon the laser beam impinges in the so-called reference area of the part to be examined, the said area being reflecting. Two lenses H converge the laser beam on the same point of rotation axis O of part P, this convergence being obtained strictly by means of a conventional laser beam centring apparatus momentarily substituted for part P to be studied.

During measurement, voltages collected by photoelectric receivers G and corresponding to impinging point displacements of the optical beams on the body P to be studied, are applied to the corresponding piezoelectric transducers so as to automatically compensate for parasitic displacements along axes X and Y. The fixed mirrors F of the used interferometers are advantageously assembled on piezoelectric ceramic transducers K, thus allowing to adjust their position during the initial setting so as to be always centred on an interference finge on the body P to be studied, whatever be its diameter at the impinging point.

Devices have thus been described permitting a very accurate positioning of parts, as well as an automatic stabilization of this positioning, the absolute precision obtained being less than one tenth of one micron. It is understood that such a precision can however only be obtained if the influence of external perturbations is sufficiently weak. These devices are especially convenient for instance for the realization of microcircuit masks, as well as for the measurement of deformations of a revolution body rotating around its axis. The present description being only given as a non-restrictive example, the invention involves all the variants of realization, and especially those using position sensing systems other than systems founded upon laser interferometry.

In that respect and without departing from the principles of the invention, variants of realization may be designed which use other means than that described for presetting the initial position of the reference block A. In the above disclosure, photoelectric microscopes have been mentioned which were aimed at a special marking on the block. These microscopes can be replaced by other means such as capacitive or reactive sensors, the sensor and the reference block forming for instance a capacitor which is varied as the reference block is moved with respect to the sensor. A sensor consisting of a cluster of optical fibres can also be used, part of this cluster guiding a light beam to the corresponding face of the reference block whereas another part of the cluster of fibres is connected to optical receivers receiving the reflected light from said face of the block reference. A displacement of the block results in a modification of the received light intensity which modification is a function of the spacing between the block and the sensor.

Such an alteration of the reflected light by the reference block can also be sensed by a microscope set so as to have the aimed surface of the block reference in its focal plane. If the reference block is moved, the assembly consisting of the microscope and the block is no longer set and the intensity of the reflected light is varied as a function of the slight displacement of the block.

Another means can also be used to sense this displacement of the block from its preselected initial position. The assembly of the block and of an interferometer is illuminated with white light, that is with a non-monochromatic light, i.e. exhibiting a wide spectrum, and produces so called white fringes which are detected and whose position is a function of the displacement of the block constituting the movable mirror of the interferometric system.

The positioning system has been described as far as the measurements and the corrections are made in a plane. But obviously the invention is not limited to such a case and its principles apply also if the measurements and corrections are made with respect to the three axes of the cartesian coordinate system for instance. It is only sufficient to provide a fourth frame which supports the reference block A and for that frame, for instance, the transducer means operates along the z coordinate whereas the resilient blades operate along the x or y axis. Two interferometers should also be added which receive the light reflected for instance by the third face of the reference block perpendicular to this z axis, e.g. for instance, here, the upper surface of the block.

In the description, the resilient blades are mentioned as flexion blades. These blades can also be torsion blades.

In the description also, the displacements contemplated are independant translations along either the x or y axis at a time. It is possible to consider more complex movements produced by a simultaneous displacement along the x and y axes.

In the above disclosure, a complete description of an interferometer, and also a complete description of the transducers used, which are piezoelectric ceramics or magnetostrictive elements as well, have not been given, since they are known. A mention of a barium titanate actuator for correcting an angular error in the mirror parallelism in an interferometric system is given in the "Journal of Research of the National Bureau of Standards -- C -- Engineering and Instrumentation -- Vol 65C, No. 2 April--June 1961" "An Automatic Fringe Counting Interferometer for Use in the Calibration of Line Scales" by Herbert D. Cook and Louis A. Marzetta.

Claims

What we claim is:

1. In a positioning apparatus for accurately positioning a piece (P) with respect to a fixed assembly and automatic stabilization of its coordinates having a base (Z);

movable carriage means (V, W) located on said base;

a reference block (A, C3) carrying the piece (P) to be positioned;

means (11-14) disposed to detect, with respect to said base, any deviation of position of said reference block (A) with respect to a pre-selected initial position of said reference block and generating error voltages upon such deviation;

said positioning apparatus comprising

correction means (C) disposed on said carriage means (V, W) and supporting said reference block (A), said correction means comprising a pair of frames (C2, C3) of different sizes, the inner frame (C3) being smaller and located concentrically within the outer frame (C2);

first resilient coupling means (L2, L2) interposed between and interconnecting the inner (C3) and the outer (C2) of said frames at opposite sides thereof for resisting relative motion between said frames in a first direction, and maintaining alignment in said first direction while permitting deflection in a direction transverse to said first direction, the first resilient means having their respective terminal ends fixed to respective frames;

second resilient coupling means (L1, L1) interposed between and interconnecting the outer of said frames (C2), and said base at opposite sides of said outer frame for resisting relative motion between said outer frame and said base (C1; Z) in the transverse direction, while permitting deflection in said first direction, the second resilient means (L1, L1) having one end fixed to said outer one of said frames and the other to said base;

first position transducer means (Y1, Y2) interposed between said concentric frames (C2, C3) and acting in said transverse direction;

second position transducer means (X1, X2) interposed between said outer frame (C2) and the base (C1; Z) and acting in said first direction;

said reference block (A; C3) being supported by said inner frame;

said deviation detection and error signal generating means being interconnected with said first and second transducer means and controlling said transducer means to effect resilient deformation of the respective resilient means to position the inner frame, and with it, said reference block, to reduce the deviation to zero.

2. Apparatus according to claim 1, comprising further means to compensate for rotational movement, said further means comprising

a third frame (C4);

third resilient deflection means coupling said inner frame (C3) and said third frame, said third frame supporting said reference (A) carrying said piece (P) to be positioned with accuracy;

third position transducer means acting on said third resilient deflection means to slightly rotate said third frame with respect to said inner frame, and thus with respect to said base upon being energized to compensate for spurious rotation of said reference block.

3. Positioning apparatus according to claim 1 wherein the position detection means provided to detect any deviation of said reference block from its preselected initial position comprises optical means consisting of a monochromatic source of light, interferometer means (I1-I4) illuminated by said source reflecting on corresponding faces of said reference block; and receiving means to receive information carried by said optical means and to derive therefrom data related to said deviation of said reference block.

4. Positioning apparatus according to claim 1 wherein said receiving means comprises photoreceiver means, logical means, said logical latter means being responsive and connected to the output from said photo receiving means and controlling a reversible counter to derive data related to the direction of said deviation of said reference block; and

comparator means to compare said data with reference data corresponding to said preselected position of said reference block, said logic means being responsive to the drift of the fringes produced by said interferometer means.

5. Positioning apparatus according to claim 1 wherein said transducer means are piezoelectric ceramics; means biasing the piezoelectric ceramics to balance the respective positions of the frames in the absence of any correcting signal, said ceramics being disposed so as to move the frame to which they are coupled in a direction different from that controllable by said resilient means and independently therefrom, said ceramics receiving said error voltages.

6. Positioning apparatus according to claim 1 wherein the inner frame comprises a rotatable plate (T);

a stand (C5 - FIG. 5) bearing the rotation axis of the rotatable plate (T);

a piece (P) to be accurately positioned having an axis of revolution;

crossed carriage means mounted on said plate (T) for pre-adjusting the position of said piece (P) with respect to said plate (T);

mechanical comparator means (N) mounted on said stand (C5), to sense, concurrently while said plate (T) is rotating about its axis, a part of the peripheral outline of said piece (P);

and optical means to accurately position said piece (P) controlling and connected to said transducer means, whereby to bring into coincidence the axis of said plate (T) with the axis of said revolution piece (P).

7. Positioning apparatus according to claim 1 wherein said transducer means are magnetostrictive elements.

8. Positioning apparatus according to claim 3 wherein said reference block has orthogonal mirror faces, operating as moving mirrors of the associated interferometer means.

9. Positioning apparatus according to claim 4 wherein the logic means includes a reversible counter;

and means are provided distinguishing between the two possible directions of count and along which said reference block may be moved with respect to a reference axis, said distinguishing means controlling said reversible counter.

10. Apparatus according to claim 3 comprising piezoelectric ceramic transducer means, the reference mirrors of said interferometer means being respectively mounted on said piezoelectric ceramic transducer means;

and potentiometer means connected to and controlling said piezoelectric ceramic transducer means to provide for fine adjustment thereof.

11. Apparatus according to claim 1 comprising potentiometer means connected to and coupled to said transducer means controlling the resilient coupling means to said frames to provide for fine adjustment of the position of said frames.

12. Apparatus according to claim 1 further comprising photoelectronic microscopes (D1, D2, D3) located on said base to determine, accurately, the initial position of said reference block;

and engraved markings of about 10 microns width on said reference block, said microscopes being aimed at said engraved markings.

13. Apparatus according to claim 12 comprising reference lines formed on said base, said reference lines extending in orthogonal directions and forming reference lines for said photoelectric microscopes and for reference mirrors of an interferometer;

said carraige means being movable in said orthogonal directions.