FR2779547A1 - LIGHT BEAM SCANNING DEVICE AND METHOD FOR MANUFACTURING SUCH A DEVICE - Google Patents

Abstract

The invention concerns a light beam scanning device, comprising a support (10), a light source (11) mounted on one face thereof, a mirror (3) arranged for receiving a light beam derived from said light source and a cantilever-type structure capable of elastic deformation which is also mounted on said face, bears said mirror and is capable of causing it to oscillate. Said device is characterised in that said source and said structure are mounted on coplanar portions of said face and said structure comprises at least one curved bimetal crossarm (22) forming an oscillator with the mirror.

Description

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  LIGHT BEAM SCANNING DEVICE AND

  METHOD FOR MANUFACTURING SUCH A DEVICE

  The present invention relates to a beam scanning device

  light as well as its manufacturing process.

  The invention relates more particularly to a scanning device comprising a support and, mounted on one face thereof, a light source and a movable mirror arranged so as to receive the beam of light coming from said source. Such devices are used in particular

for reading bar codes.

  Patent application EP 0 650 133 describes a device of this type, in which the support comprises a first planar surface and also a second planar surface, together defining an angle of 135. The first surface carries the light source while the mirror is pivotally mounted by means of two elastic arms on the second surface. The support is released under the mirror which can thus oscillate freely under the action of an electrostatic transducer connected to a source of electrical energy providing a periodic signal. Such a device allows the production of bar code readers having a

  integrated scanning system, low volume and low consumption.

  However, it has drawbacks.

  Indeed, the production of a support formed by two surfaces inclined relative to each other is expensive. In addition, the solution described only allows

very low scanning angle.

  The purpose of the present invention is to overcome these drawbacks and to allow the production of a device having an extremely simple structure, allowing scanning over a large angle, at a frequency

  relatively low and for low energy consumption.

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  This object is achieved thanks to the fact that the face of the support is planar and that the device further comprises a cantilever and bimetal type structure,

  elastically deformable and subjecting the mirror to the support.

  Such a device is advantageously produced by photo-

  lithographic. To optimize its implementation, the support is made of silicon and said structure comprises at least one beam formed by a first layer of a silicon compound and made of material with said support and a second electrically conductive layer and covering the first layer . io To ensure its scanning function, the mirror must be able to reflect the light beam from the light source. This is only possible if the surface of the mirror cuts the beam. This is why, in a particularly advantageous embodiment of the invention, the light beam is substantially parallel to the flat face of the support and the beam is curved so that the mirror makes an angle between 30 and 150 degrees with

said face.

  The present invention also aims to provide training of the

  mirror by simple and energy-efficient means.

  To this end, in a particular embodiment of the invention, the device includes means for connecting the conductive layer to a source of electrical energy capable of generating a periodic signal which has the effect of heating it and thus expanding it. , so as to cause the mirror to oscillate. Surprisingly, the use of thermal energy to generate the movement of the mirror requires only a small amount of energy. This is due to small

device dimensions.

  In order to obtain a mirror performing a movement of high amplitude, for as low energy consumption as possible, the first layer of the structure is made of silicon dioxide and the second layer of chromium. Advantageously, the thickness of the layer of silicon dioxide is

  substantially equal to twice the thickness of the chromium layer.

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  Another object of the present invention is to obtain good stability of

the orientation of the mirror.

  This object is achieved because the cantilever structure has two

  beams connecting the mirror to the support.

  To perform a scan under optimal conditions, it is necessary that the light beam is focused. The device according to the invention therefore further comprises a converging lens mounted on the support between the light source and the mirror and arranged to focus the beam on the object to be scanned. According to an advantageous embodiment, the device is completed by a photoelectric sensor, also mounted on the support and arranged to receive the light reflected by an object scanned by the beam coming from

source.

  The device according to the invention is particularly well suited for the production of bar code readers, of small volume and easily industrializable. The present invention also relates to a method for manufacturing a device comprising a planar support and a mirror mounted on the support by means of a structure of cantilever bimetallic strip type. This structure is formed of a beam having a first layer of silicon compound and a second electrically conductive layer and covering the first layer. This process makes it possible to produce a beam which automatically takes a curved shape making it possible to orient the mirror adequately with respect to the light beam. To this end, the method according to the invention comprises the following steps: - oxidation of the faces of the support with generation of an internal compression tension, - deposition, with internal tension of traction, of the electrically conductive layer intended to form a part said beam as well as layers intended to form the mirror, and

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  - attack of the support to remove the silicon under the entire surface of the mirror and under the beam to its point of anchorage to the support, so

  to form a hollow and release the cantilever structure.

  Other advantages and characteristics of the invention will emerge from the

  description which follows, made with reference to the appended drawing, in which:

  In Figure 1 shows, in perspective, a device according to the invention; Figure 2 illustrates, in sectional views, the different stages of the device manufacturing process; FIG. 3 shows in plan (a) and in section (b) a device according to the invention; and ò Figure 4 is a diagram showing the angular sensitivity of the device

  suspension of the mirror according to the thickness of the layers.

  The device shown in FIG. 1 comprises a support 10 carrying a light source 11, a converging lens 12 and a mirror 13, arranged to receive the beam of light emitted by the source 11. Advantageously, the support 10 also carries a photoelectric sensor 14 for receiving the light beam after it has been returned by an object

  for example carrying a barcode.

  The support 10 is formed from a flat monocrystalline silicon plate. The light source 11 is a laser diode emitting in the visible, like that sold by Sony under the reference SLD 1101 VS and attached to the support 10. The lens 12 consists of a spherical microlens,

  advantageously fixed by gluing to the support.

  The reflecting surface of the mirror 13 is formed by a layer of chromium 20,

  carried by a substrate 21, made of nickel for example.

  The mirror 13 is secured to the support 10 by two beams 22 each formed of a bimetallic strip, with an electrically conductive layer and an insulating layer. The conductive layer is advantageously made of chromium and the layer

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  insulating silicon dioxide, as will be seen more precisely

with reference to Figure 3b.

  The support 10 is, moreover, coated with pairs of conductive tracks bearing the references 25 and 26, 27 and 28, 29 and 30, respectively connecting the light source 11, the beams 22 and the sensor 14 to a control device and

  to a source of electrical energy, not shown in the drawing.

  The method of manufacturing such a device uses micro-

  machining, widely using photolithographic processes. The process used is described below, with reference to FIG. 2. The various paragraphs describing this process are identified by a letter

  corresponding to the numbering of the elements in Figure 2.

  It will be noted that, in this figure, the scale is not respected, the thicknesses of the layers being considerably enlarged compared to the lengths,

  to make the drawing easier to read.

  A. The device is produced from a flat plate of monocrystalline silicon 30, the upper and lower faces of which each carry a layer of silicon dioxide, respectively referenced 31 and 32,

  obtained by heating the plate in an oxidizing medium.

  B. The upper face and the lower face are covered with a layer of photoresist 33. The upper layer is then exposed to light through a mask, then etched so as to release an opening 34

  through which appears the layer of silicon dioxide 31.

  C. The layer 31 of silicon dioxide is attacked chemically, forming

  thus an opening 35 which gives off the monocrystalline silicon 30.

  D. A layer 36 of aluminum is deposited on the entire upper face of the support. E. By eliminating the photoresist 33, the aluminum layer 36 is also

removed, except in opening 35.

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  F. We deposit a layer of chromium 37, then a layer of gold 38 on

the entire upper face.

  G. The layer of gold 38 is covered with a layer 39 of photoresist. This is exposed to light through a mask, then dissolved in the areas that have been lit, so as to define a clearance 40, located in the

right part of opening 35.

  H. The layers of gold 38 and of chromium 37 are attacked in a conventional manner, through the clearance 40, until reaching the layer 36 of underlying aluminum and covering the opening 35. An opening 40a is thus created

  1o between the layer of chromium 37 and the layer of silicon dioxide 31.

  I. The upper face is again covered with a layer of photoresist, bearing the reference 41, in which an opening 42 is made according to the same procedure as described above. The opening 42 extends over the left part of the opening 35 and beyond, above the space carrying the

  overlapping layers of silicon dioxide 31, chromium 37 and gold 38.

  J. A layer of nickel 43 is closed by electrolysis, which closes the opening 42. K. The layers of nickel 43 and of photoresist 41 are covered with a

layer of gold 44.

  L. The photoresist layer 41 is eliminated. The layer of gold 44 is simultaneously removed, except in the part where it covers the layer of

nickel 43.

  M. We attack the aluminum layer 36 as well as the silicon underlying the

  through the opening 40a. In this way, a recess 45 is made over

  below the areas comprising the nickel layer 43 and the layers of

  silicon dioxide 31, chromium 37 and gold 38.

  It will be better understood how the recess 45 can be practiced under the different layers by referring to FIG. 3, which a) shows in plan and in

  b) in section the device at the end of manufacture.

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  More precisely, we can see, on the plan part of this figure, the support 10, the mirror 13, the beams 22, the conductive tracks 27 and 28 as well as the recess 45. The sectional part makes it possible to recognize the monocrystalline silicon layer 30, silicon dioxide layers 31 and 32, chromium layer 37, gold layers 38 and 44, nickel layer 43 as well

than recess 45.

  As can be seen in FIG. 3a, the recess 45 completely surrounds the mirror 13 and extends below the beams 22. In this way, the mirror 13 and

  the beams 22 together form a cantilever type structure.

  o Figure 3b allows to understand the structure of the device at the end of manufacturing. It can be seen that the beams 22 have a bimetallic strip structure, formed by a layer of silicon dioxide 31 and a layer of chromium 37. They are oriented parallel to the plane of the plate 30. In reality, the beams 22 are curved, as shown in Figure 1, from

  that the lower surface of the mirror 13 is released.

  This particular shape of the beams is obtained by generating compression stresses in the silicon dioxide layer 31 and / or tensile stresses in the chromium layer 37 during their manufacture. When the underlying silicon layer is removed, that is to say at the time of the creation of the recess 45, the beams 22 are deformed until the equilibrium is reached between the metal layers 37 and 38 and the layer of silicon dioxide 31. The beams 22 thus curve until defining with the support an angle of up to 1350, as can be seen in FIG. 1. Thanks to this feature, it is possible to place the light source so that it emits in a direction parallel to the plane of the support, and machine the mirror in this same plane, it taking, without other, the position

  shown in Figure 1 at the end of manufacturing operations.

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  In the configuration described above, the mirror surface is defined by the chromium layer 37. The manufacturing procedures and the qualities of the

  chrome make this mirror particularly good.

  Alternatively, one could place the beams 22 on the side of the light source 11 and dimension them so that the mirror forms an angle of 45 with the support 10. One would obtain a completely comparable structure. In this case, it is the layer of gold 44 which would fulfill the function of mirror. The quality of the latter would however be lower, the flatness of the nickel layer 43, on which the gold layer 44 is deposited, being significantly worse than

  io that of the layer of silicon dioxide 31 carrying the layer of chromium 37.

  Generally and depending on the type of application, the mirror 13 advantageously forms an angle between 30 and 150 relative to the plane of the

support 10.

  The function of the nickel layer 43 is to strengthen the mirror and above all to adjust its mass of inertia, which defines the frequency of the oscillator that

  together form the mirror 13 and the beams 22.

  To ensure scanning by the light beam, a periodic electrical signal is applied to terminals 27 and 28 whose frequency is equal to the natural frequency of the oscillator formed by the beams 22 and the mirror 13. This signal generates heating of the layers of chromium and, consequently, their expansion, which causes the bending of the beams 22. The cooling taking place very quickly as soon as the current is interrupted, it follows that the oscillator thus formed oscillates at its natural frequency, with an amplitude which is a function of the heating current, and which can easily reach

30 .

  In this way, the total angle traversed by the mirror 13 is 60 and, consequently, the light ray sweeps an angle of 120 in a plane perpendicular to the surface of the mirror 13. This is more than sufficient for

  read a barcode.

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  This angle could be further increased, by increasing the intensity of the pulse current. Tests have shown that the amplitude of the oscillation can

  reach 90 without affecting the mechanical characteristics of the beams.

  FIG. 4 is a diagram showing the angular sensitivity of a structure as described, as a function of the thickness of the chromium layer, for

  different thicknesses of the silicon dioxide layer.

  As one can easily imagine, the angular sensitivity is lower the greater the thickness of the beams 22. For very weak

  thicknesses of the chromium layer, the sensitivity is also very low.

  The diagram in FIG. 4 shows that the sensitivity increases up to a thickness of the chromium layer equal to approximately 45% of the thickness of the silicon dioxide layer. Beyond this value, the

  sensitivity decreases, due to the increase in the stiffness of the beams 22.

  Consequently, to obtain a maximum amplitude for a minimum of energy consumption, the carbon dioxide layers will be sized.

  silicon and chromium in a ratio of approximately 2 to 1.

  For example, a device according to the invention, comprising a mirror of 500X300 Fm2, for a substrate thickness of 4.Lm and beams of 1.4 gm, oscillates at a frequency of 330 Hz and at an amplitude of 30, for a

consumption of 4mW.

  The device as described with reference to the drawing comprises two beams.

  In a variant not shown, it would also be possible to obtain a comparable result by means of a single beam. This then carries two parallel chrome tracks, linked together by the mirror layer

  and on the support side on tracks 26 and 27.

  In this configuration, the oscillating movement of the mirror is however

less well controlled.

  It would also be possible to provide a structure with two beams and, on each of them, two conductive tracks. Such a solution would

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  as so far to oscillate the mirror in a plane, but also to change its orientation or generate complex movements. In this way, it would be possible to define the surface swept by the light ray by applying the appropriate signals to the conductive tracks of each of the beams. nr l 2779547

Claims (11)

  1. Device for scanning by a light beam, comprising a support (10) and, mounted on one face thereof, a light source (11) and a movable mirror (13) arranged so as to receive a light beam coming from said source (11), characterized in that said face is planar and in that it further comprises a structure (22) of cantilever and bimetal type,   elastically deformable and subjecting the mirror (13) to the support (10).   2. Device according to claim 1, characterized in that the support 1o (10) is made of silicon and said structure comprises at least one beam (22) formed of a first layer (31) of a silicon compound and made of material with said support (10) and a second layer (37) electrically   conductive and covering the first layer (31).   3. Device according to claim 2, characterized in that said beam is substantially parallel to the plane of the support (10) and in that the beam (22) is curved so that the mirror (13) makes an angle between 30 and 150 degrees with said face.   4. Device according to claim 2 or 3, characterized in that it further comprises means (27, 28) for connecting said conductive layer (37) to a source of electrical energy capable of generating a periodic signal which has the effect of heating it and thus dilating it so as to make oscillate said mirror (13).   5. Device according to one of claims 2 to 4, characterized in that the   first layer (31) is made of silicon dioxide and the second layer (37) in chrome.   6. Device according to claim 5 characterized in that the thickness of the layer (31) of silicon dioxide is substantially equal to twice   the thickness of the chromium layer (37). 12 2779547   7. Device according to one of claims 1 to 6, characterized in that   said structure comprises two beams (22) securing the mirror (13) to the support (10).   8. Device according to one of claims 1 to 7, characterized in that it   further comprises a converging lens (12) mounted on the support (10)   and arranged to focus said beam on the mirror.   9. Device according to one of claims 1 to 8, characterized in that it   further comprises a photoelectric sensor (14) mounted on the support (10) and arranged to receive the light reflected by an object scanned by the 1st beam from said source (12) 10. Device according to claim 9, characterized in that it is arranged   to allow reading of bar codes.   11. Method for manufacturing a device according to claim 3 from a silicon support (10), characterized in that it comprises the following steps: - oxidation of the faces of the support (31) with generation of an internal compression tension, - deposition of the electrically conductive layer (37) intended to form a part of said beam (22), as well as layers (43) intended to form the mirror (13), and - etching of the support ( 10) to remove the silicon (30) under the entire surface of the mirror (13) and under the beam (22) up to its point of anchoring to the support (10), so as to form a recess (45) and clear the structure cantilever type.