FR2530354A1 - Device for exposing, while moving, a track on a photosensitive film. - Google Patents

Abstract

The device comprises a table 16 for supporting the film, fitted with means 18 enabling it to move in a first direction at a constant speed and in a second direction perpendicular to the first and means for measuring movement in the said directions. A laser 10 supplies an actinic radiation for the resin and an optical system 12 forms a point image of the laser beam on the surface. The optical system comprises, in succession along the path of the beam emitted by the laser, an acousto-optic modulator 20 enabling the beam to be blocked and an acousto-optic deflector 22 enabling the beam to be deflected in the second direction and associated with a frequency-ramp generator. The light emission is synchronised from information supplied by the means for measuring movement and which control the frequency-ramp generator and the modulator in response to the movement of the table in the first direction.

Description

Device for impressing a line on a photosensitive layer in flight
The subject of the invention is a device for impressing a layer of photosensitive resin along a determined path, in flight, that is to say while this layer is moving. The invention finds a particularly important, although not exclusive, application, constituted by the manufacture of reticles intended subsequently for the production of integrated circuits.

 Are already known devices for generating reticles or masks for integrated circuits of the kind which include a support table for the layer, provided with means making it possible to move it in a first direction at constant speed and in a second direction perpendicular to the first and means for measuring displacement in said directions ;. a laser providing actinic radiation for the resin and an optical system for forming a point image of the laser beam on the surface, as well as means for synchronizing the emission of light. US Pat. No. 4,095,891 describes a device of this type. genre: the monochromatic light beam emitted by the laser is fixed and illuminates an object plane, the image of which is formed by an objective on the layer, moved by the table in both directions to form the required path.

 At present, this solution has not found industrial application and the machines for generating reticles used in the manufacture of circuits with very high integration belong to one or the other of two types. In the first type, each line of the circuit is exposed to the light coming from a flash lamp through a slot of variable dimensions. These machines have the disadvantage of requiring very long durations, often exceeding several hours, for. make complex circuits. The second, more recent type uses an electron beam. But machines of this kind are very complex, therefore very expensive, and their implementation involves many easements, such as the need for an empty chamber and the frequent change of filament.

 The present invention starts from an entirely different approach and proposes a device of the kind defined above, the optical system of which successively comprises, on the path of the beam (put by the laser, an acousto-optical modulator making it possible to block the beam and an acousto-optic deflector which deflects the beam in the second direction and which is associated with a frequency range generator. Synchronization means controlled from information supplied by the displacement measurement means are provided to control the generator frequency ramp and the modulator in response to the movement of the table in the first direction, so as to impress the portion of the plot contained in a column and then successively in adjacent columns.

 In this device, writing is carried out by synchronized movements of the support table in a first direction and by deflection of the laser beam transversely to this first direction. It is thus possible to achieve a much higher scanning frequency than with mechanical devices. Performing the following sweep of successive adjacent columns makes it possible to use acousto-optical deflectors despite their low number of resolution points, which does not currently exceed approximately 1500, which at first sight should lead to dismiss them. Thanks to these devices, it is possible to achieve a high localization precision, of the order of 0.1 pue, corresponding to that of the means of measuring the movement of the table. Finally, it should be noted that it is these means of displacement of the table which controls synchronization, while most printing devices using a printer associated with a mechanical deflector, such as a rotating mirror, use the mechanical element at higher speed, i.e. the rotating mirror , to synchronize slow mechanical elements.

 The synchronization means will generally be provided to trigger the generator control ramp with a variable delay so as to correct the positioning errors of the table in the direction of movement of the latter at constant speed. These means could also be provided to trigger the sending to the modulator of the data representative of the tracing along a line with a delay or a predetermined advance to correct the positioning errors of the table in the second direction.

 These correction operations can only be carried out due to the favorable characteristics of the acousto-optical means, the synchronization and delays of which can be controlled to within a few nanoseconds.

In an advantageous embodiment of the invention, the residual errors in positioning the table in the direction of movement of the latter at constant speed are corrected by modifying the carrier frequency applied to the modulator. This modification, which can be very rapid, typically in a period of less than 100 ns, makes it possible to make up for the errors which can be described as "local" due to random variations in the speed relative to the average speed of. travel,
The invention will be better understood on reading the following description of particular embodiments, given by way of nonlimiting examples. The description refers to the accompanying drawings, in which
FIG. 1 is a perspective block diagram showing the relative arrangement of the main components of a device according to the invention,
FIG. 2 shows a possible constitution of the optical system of the device of FIG. 1,
FIG. 3 is a block diagram showing a possible constitution of the means for synchronizing and controlling the deflector and the modulator,
- Figure 4 is a diagram showing
the control delays which occur during the implementation of the means shown in FIG. 3,
FIG. 4 is a time diagram showing the control of the deflector and of the modulator,
- Figure 5 is a diagram of an optical system according to a variant of
reading.

The device whose main elements concerned by the invention are shown in FIG. 1 is intended to generate reticles intended for the manufacture of circuits
integrated, reticles reproducing on a large scale (5 or 10 for example layout of the circuit. This device comprises a laser 10 intended to form, through an optical system
12, a point image on a surface 14 coated with a layer of resin placed in the image plane of the system.

 The surface 14 is carried by a table 16 with crossed trays. First drive means, a fraction of which, 18, is shown make it possible to move the upper plate, which directly carries the layer 14, in the direction y.

Other driving means (not shown) make it possible to move the layer 14 in the direction x orthogonal to y. The table is provided with displacement measuring means which can be of the kind described in patent application FR 81 07708, the precision of which is at least equal to the resolution sought.

 The laser 10 will be chosen so as to be very actinic for the resin to be impressed. In practice, it will frequently be possible to adopt a helium-cadmium laser having a power of a few milliwatts.

The optical system 12 comprises, in addition to components which will be described later, an acousto-optical modulator
20, whose essential role is to order by all or
nothing the light intensity which arrives on the layer 14, and an acousto-optic deflector 22. The deflector 22
is placed so as to impose on the beam passing through it an angular deflection adjustable in the direction x.

 The acousto-optical deflectors available at present time have a number of resolution points which can hardly exceed 1500, owing to the limited dimensions of the crystal which they comprise.

In fact, we can generally be satisfied with a very lower number of resolution points, 512 or even 256 for example. If, for example, we are trying to achieve an addressable reticle in steps of 0.5zm, with an accuracy of + Olim, we can achieve this result by scanning successive lines, such as line 24, comprising 256 points, therefore having a length of 128pu. By a television type scan (advancing at constant speed of the table in the y direction and scanning in the x direction using the deflector 22, then rapid reverse), for a given position of the table along x, an strip or column 26 of 128 # m wide. The table is then moved in the direction x by a distance of 126 # m, a new column is impressed during a movement in the opposite direction along y, and so on.

 The modulator 20 is advantageously used, not only to control the passage of the light beam, but also to compensate for the linearity defects of the movement of the table 16 in the direction y. To do this, it suffices to control the modulator 20 by a circuit which provides, not a constant frequency, but a modifiable frequency so as to compensate for a difference corresponding to two or three resolution points: in the case mentioned above of a resolution of 0.5pal we can anticipate a deflection on - + 0.7iimenviron. This result is easily achieved provided that the modulator 20 is controlled by a circuit providing a high carrier frequency (greater than 200 MHz) and therefore modifiable over a few tens of megahertz without loss of efficiency. It is easy to arrive at a resolution better than 0.01 vm.

 The practical arrangement of the optical system can be that shown diagrammatically in FIG. 2. In FIG. 2, where the elements corresponding to those of FIG. 1 are designated by the same reference number, there is a laser 10, for example a heliuFcadmium laser Providing a monochromatic radiation at 442 nanometers, followed by an afocal 28 of adaptation on the input of the acousto-optic modulate-ur 20. This modulator, commercially available, can comprise as acousto-optic material "flint" or paratellurite which has the advantage of a lower control power. These same materials can be used for the deflector 22. For the modulator, it is also possible to provide other materials, such as, in particular, lead molybdate. The modulator 20 also includes a transducer for applying the control frequency to the materials. and it is associated with a high frequency generator controlled circuit, which will be described later.

 The application of the high frequency voltage to the transducer causes the appearance of a diffracted light order, which is transmitted to the rest of the optical system, while the non-diffracted light is retained. Such a modulator makes it possible to achieve a transmission efficiency of the order of 90%.

 The modulator 20 is followed by a lens 30 for additional opening of the beam, then a prism 32 for returning the beam to 900 and a lens 34 constituting an afocal with the lens 30. The beam then passes to the deflector 22 which is followed by a new afocal optical system, constituted for example by two doub-lets 36 and 44. The system also comprises two cylindrical lenses 38 and 40, respectively upstream and downstream of the deflector 22 as well as two other prisms with total reflection 41 and 42 of folding of the optical path. The cylindrical lenses 38 and 40, whose role will appear later, have their curvature in a plane perpendicular to the scanning plane of the deflector 22.

Between the doublets 36, 44 is placed a field correction lens associated with a diaphragm 43 for blocking stray light, that is to say non-diffracted light. The pupil 22 -------- is conjugated to the entrance pupil of the scanning objective 45 by the doublet 44
- constituting an afocal with the doublet 36. All these members can be carried by a base 47 on which the plates move.

Iy (Yk) ~ Ix (x - Vx.t) . 5(t) dt (1)
Dans cette formule : Iy est la distribution d'intensité suivant la direction y k est l'indice de la ligne considérée, correspondant A un balayage
par le déflecteur,
S(t) est le signal provenant du modulateur, qui peut prendre
les valeurs 0 et 1,
Ix est la distribution d'intensité suivant la direction x,
Vx est la vitesse de balayage suivant x.In the device which has just been described, we can write the ex-
Position E (xv) of layer 14 as an expression
E (x, y)

Iy (Yk) ~ Ix (x - Vx.t). 5 (t) dt (1)
In this formula: Iy is the intensity distribution along the direction yk is the index of the line considered, corresponding to a sweep
by the deflector,
S (t) is the signal from the modulator, which can take
the values 0 and 1,
Ix is the intensity distribution along the direction x,
Vx is the scanning speed along x.

 The scanning speed is fixed by the deflector 22.

It can be written Vx = F.dx, where F is the writing frequency, that is to say the number of control bits of the deflector per second, and dx is the increment of distance addressable according to the direction x.

 The control circuits necessary to carry out this exposure can be of the type indicated in FIG. 3.

They comprise a frequency ramp generator driving the deflector transducer 22, constituted for example by a voltage ramp generator 48 and a voltage controlled frequency oscillator 50 which must have an accuracy of the order of + 0.2% in the example considered.

The ramp generator is of the type triggered by a synchronization signal applied to its output 52, supplied by a computer 54. At a frequency FD applied to the deflector 22 corresponds a deflection angle #D = xFD / v, where A is the length wave supplied by the laser and v the speed of sound propagation in the material.

The synchronization signals supplied by the computer are produced from information supplied by the means for measuring the displacement of the table in x and y, means of which only one, corresponding to the displacement in x, is represented diagrammatically at 56 in FIG. The modulator 20 is, for its part, controlled by all-or-nothing modulation of a carrier attacking its transducer. The circuit supplying this carrier comprises an amplifier 58 preceded by a switch member 60 which receives the successive bits coming from the caiculator 54 according to a program contained in an msslm (1i nr
62 (disc or magnetic strip) and an oscillator 64. 's dre
diffracted +1, which is used to attack the deflector-r, being rnodu
with a delay time otM (temporal delay introduced by
the modulator 20), the description signals F must be introduced with a delay st x of correction of #tM.

 We will see later that the modulator can also be used to provide an angular deflection of the beam in a direction orthogonal to that provided by the deflector 22. The result is achieved by controlling the driving frequency of the modulator 20. The diagram shown in FIG. 3 allows this modulation to be carried out by developing the control frequency using a voltage-controlled oscillator 64 which receives its control signal from the computer 54 through a digital / analog converter 66.

 Obviously, the response time of all the beam control elements must be such that its transfer function is wider than the transfer function associated with the exposure distribution according to x, represented by x ( x-Vx.t).

We see that the writing frequency F (that is to say the number of addressable bits per second) is related to the number
M of points addressable by line by means of the deflector 22 by the relation
F- = M. Vy / dx
This frequency F is practically limited by the response of the acousto-optic deflector. As a general rule, it will be necessary to adopt a frequency F not exceeding 10 MHz, which leads, in the case considered above to 256 addressing points , to the following two solutions

<tb><SEP> F <SEP> Vy <SEP> (speed <SEP> table) <SEP> TL <SEP> (period <SEP> of <SEP> scan <SEP> of <SEP> line)
<tb><SEP> 5MHz <SEP> 9.7 <SEP> mm / s <SEP> 50s <SEP>
<tb> 10MHz <SEP> 19.5 <SEP> mm / s <SEP> 25s <SEP>
<tb>
We see that the values required for Vy are moderate and perfectly compatible with the limiting speeds that can be obtained using a table moving on an air cushion. The calculation shows that, if we take into account of the time lost during displacements in x without writing, of the dead times necessary for the corrections in x and y and of the loading and unloading time possible, the writing times for a reticle of 50 mm described in steps of 0.5 ( or 100 mm described in steps of iiim) is approximately 20 min.

With an acousto-optic deflector used at the limit of its possibilities, currently corresponding to approximately 30 MHz, one could arrive at a direct inscription on the whole of a conventional 4 inch (100 mm) slice
in about an hour.

 We will now describe, with reference to FIGS. 3 and 4, the corrections that the computer 54 must introduce to take into account the characteristics of the acousto-optical means used and possible scanning errors, such as accidental or systematic linearity defects in the displacements. from table 16.

Corrections related to the characteristics of acousto-optical means
The time T necessary for the ultrasonic wave supplied by the oscillator 50 to fill the acoustooptical material represents a large fraction of the line scanning period TL at the high working frequencies which will generally be used. Several frequencies are therefore present simultaneously in the acoustooptical material and cause an average deflection and an optical distortion of the beam.

This drawback, which a priori seems to lead to discarding the electro-optical deflectors, can however be easily corrected in the case where the scanning is linear. Indeed, the distortion corresponds to a focusing in the scanning direction, at a distance
L given by the formula
L = v2 / (# .df / dt)
Such focusing is equivalent to that given by a cylindrical lens. When the acousto-optical material is flint and a scanning speed of 3.2 MHz / # s is used, the length L is of the order of 10 m. - -. . When the acousto-optic material is TE02 paratellurite,
L is around 1.10 m. The focusing must then be corrected. This correction can easily be carried out using a displacement of the cylindrical lens 40 of focal length -L.

 An error calculation shows that the tolerance on the linearity is of the order of f 1% if one wishes to maintain the astigmatism and the aberrant deviation at the wave surface within limits compatible with the precision sought in convenient.

Correction of table positioning errors
The means of displacement of the table do not make it possible to impose a position thereon at a given instant better than several tenths of a micron. The position of the table is affected by a position error which can be described as "slow", due to the difference between the theoretical speed. It is also affected by a "local" position error, due to. irregularities in advance of the table as a result of solid friction which cannot be completely eliminated.

 The position error along direction x, where the table is advanced in steps, can be corrected by providing the deflector and its control circuits so as to have a few additional addressing points.

 For operating speed issues, position errors can only be known once per line scan. The displacement measurement means therefore supply the computer 54, at the end of each scan, with two position errors Ô x- and Ôy.

 To correct the positioning error along x, it suffices to trigger the modulator at an instant function of ox relative to the start of the useful ramp of the sweep, which implies #, since the error can be in one direction or in the opposite direction, that this sweep must be planned with a delay which corresponds to the maximum terror to be compensated for when the table is in its exact theoretical position.

If, for example, a maximum position error of + 5 pm is accepted, the delay tx in opening the modulator for an error Ôx will be
tx = (5 + 6X) / VX with vx = -F.dx
It is also necessary to anticipate the command-e of the 6tM modulator, time necessary for the acoustic filling of the modulator.

 These delays appear on the lower line of FIG. 4. It can be seen there that the synchronization signal 68 triggering the opening of the modulator and the writing of a line is delayed, relative to the start of useful line scanning, by < ax / vx) - #tM. This start of useful line scanning does not begin until time 6tD (after the start of line scanning) which is necessary for the acoustic filling of the deflector 22.

 The following position error y can have two causes, as noted above.

.The speed error, due
to the fact that the speed vT of the table is not exactly equal to the theoretical speed vy can only be caught by actio-n on the line scan by a variable delay between the end of the line scan and the start the next line. In other words, one can only correct an insufficiency of the speed of movement of the table along y.

 For this reason, it will be necessary to systematically adopt for the speed vT of the table not the theoretical speed vy, but a lower speed, the difference of which with vy is at least equal to the maximum error.

 The upper line in Figure 4 shows that the delay to be achieved will be 6y / vT = tD.

 By itself, this delay in the application of the top 70 of the triggering of the ramp applied to the oscillator 50 makes it possible to correct the displacement errors following-y, but it can be seen that the time lost becomes considerable if one wishes to compensate for errors of important speed.

In the example given above for example, the line scanning time increases by 50 for a resolution of
0.5 vm when we take sy = - 0.25 zm.

In some cases the local error is overlooked
geable and error correction can be performed sim
variable delay delay on transmission of the top of
trigger 70. When you want to compensate
local errors and decrease the loss of time due to
correction of speed error, it is advantageous to
frequency modulate the carrier applied to the modulator
20, which involves choosing a carrier frequency
high, at least equal to 200 MHz. As indicated
higher, you can easily get a deflection
spanning two or three points apart
resolution while maintaining a rise time of 50 ns
for a frequency F of 10 MHz. The optical conjugation of
pupil remains carried in this direction.

We deduce in this case from the error Ôy the correction
6fM frequency to bring. to the carrier of the modulator.

If we denote by ey the deflection capacity of the
modulator, expressed in linear spacing, it will not
necessary to introduce a dead time tD on the ramp
line scan only when Ôy is greater than Ey
and this time will be reduced to:
tD = (êy - Ey) / vy
If a significant speed error correction
or local error proves necessary, it is possible to add to the modulator 20 an additional deflector
not shown, acting in direction y. This
additional deflector can be of the same type as the
deflector and be addressable on a large number of
points, for example on 400 points representing 200 mm.

But the optical system then becomes much more compact.
brant, due to the need to combine the # pupils.

Displacement measurement means and calculation
reader 54 must obviously be fast enough
to be compatible with scanning speeds
in practice, for F = 10 MHz, 6tx and 6ft1 must
be obtained in a time not exceeding approximately 5 # s.

 By way of example, the following characteristics can be given of a device using Flint as an acousto-optical material and using a helium-cadmium laser as a light source. The optical system is designed to maintain the distortion at a value lower than 0.1 vm, that is to say 0.1% for a field of 128 zm.

Optical calculations only involve simple components. The modulator is framed by an upstream lens for coupling the beam to the modulator and two downstream lenses constituting an afocal with magnification of 20.

 The deflector has a pupil of 4 x 20 mm so that the beam leaving the afocal associated with the modulator must be coupled to it in the form of a line parallel to the direction of propagation of the acoustic beam. We are therefore led to frame the deflector by two cylindrical lenses constituting an afocal of magnification -1. The beam emerging from the second cylindrical lens is taken up by a new afocal which has several roles: it combines the pupil of the deflector and that of the scanning objective. It achieves the necessary pupillary enlargement. It gives access to an intermediate focal plane in which a spatial filtering is carried out intended to rule out diffracted orders other than the +1 order using a diaphragm.

 The scanning objective (46 in FIG # e 2) has characteristics close to those of a standard 0.65 aperture microscope objective. The depth of field of such a lens being small, of the order of 0.3 μm, an automatic focusing device must be provided: it may be of the type described in patent application FR 81 07709, to which one can report to.

 When looking for a high writing speed it is possible to use a multibeam modulator.

The writing speed, at constant scanning speed of the deflector, is multiplied by the number ttl of beams.

For this, it is necessary to control the modulator by
N carrier frequencies, supplied by N different circuits.

We then write N lines in parallel, which leads to adopting a table speed multiplied by N.

The different beams diffracted at the level of the modulator must obviously be confused at the level of the deflector, which requires to combine the pupils of the deflector and of the modulator
We can then use the optical system shown diagrammatically in FIG. 5. The beam supplied by the laser 10a is taken up by an afocal made up of two lenses 74 and 76 which adapt the diameter of the laser beam to a dimension compatible with the desired rise time. for modulator 20a, which uses paralellurite and has a circular entrance pupil. The modulator 20a is followed by a second afocal consisting of two lenses 78 and 80 and a cylindrical correction lens 82.

 A third afocal made up of two lenses or doublets 84 and 86 returns the light in the form of N parallel beams towards the pupil. Of entry of the scanning objective 46a. We can thus ensure the double conjugation of necessary pupils.

 The invention is not limited to the particular embodiments which have been represented and described by way of examples - and it should be understood that the scope of the present patent extends to all variants remaining within the framework of equivalences.

Claims (8)

 1. Device for impressing on the fly a layer of photosensitive resin along a determined path, comprising a support table (16) for the layer, provided with means (18) making it possible to move it in a first direction at constant speed and in a second direction perpendicular to the first and means (56) for measuring displacement along said directions, a laser (10) providing actinic radiation for the resin, and an optical system (12) for forming the point image of the laser beam on the surface, as well as means for synchronizing the emission of light, characterized in that the optical system successively comprises, on the path of the beam emitted by the laser, an acousto-optical modulator (20) making it possible to block the beam t an acousto-optic deflector (22) making it possible to deflect the beam in the second direction and associated with a frequency ramp generator (48, 50), the synchronization means being controlled to starting from the information provided by the displacement measurement means (56) and controlling the frequency ramp generator (48, 50) and the modulator in response to the movement of the table in the first direction, so as to impress the portion of the trace contained in a column of width equal to the sweep amplitude of the deflector,  2. Device according to claim 1, characterized in that the synchronization means are provided for moving the table in the second direction by a distance equal to the width of a column after the column has been scanned in view of a new movement in the first direction in the opposite direction to the first movement.  3. Device according to claim 1 and 2, characterized in that the synchronization means are provided for triggering the generator control ramp (48, 50) with a variable delay chosen to correct the positioning errors of the table # following the first direction and following the second direction.  4. Device according to claim 3, characterized in that the synchronization means are provided for controlling the modulator (20) with a determined delay to correct the positioning errors of the table in the second direction.  5. Device according to any one of the preceding claims, characterized in that the modulator (20) is arranged so as to deflect the beam in a direction orthogonal to the direction of deflection by the deflector (22) and in that the modulator (20) is associated with a carrier frequency modulation circuit applied to the modulator in response to errors in positioning the table in the first direction.  6. Device according to claim 5, characterized in that the carrier frequency applied to the modulator is at least equal to 200 MHz.  7. Device according to une.quelconque of the preceding claims, characterized in that it comprises a second deflector of beam deflection in a direction orthogonal to the direction of deflection of the first deflector (22).  8. Device according to any one of the preceding claims, characterized in that the modulator is associated with several control circuits supplying different frequencies, so as to supply several simultaneous output beams, the optical system being provided for forming the images of the different beams on successive scan lines.