Patent ID: 12248160

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

The present invention relates to an optical scanner provided with a mirror which has an essentially planar front face, this front face thus extends according to a given plane P called “main plane” and a structured rear face (opposite to the front face). In particular, the structuration of the rear face of the mirror is adapted so that a radiation crossing the mirror from the front face towards the rear face undergoes a deflection with respect to an angle of incidence of said radiation on the front face. In this respect, the structuration may comprise at least one facet essentially planar and inclined with respect to the front face and therefore to the main plane P. Thus, the rotation of the mirror allows performing an angular sweeping with the transmitted radiation.

FIG.3Ais a schematic representation of an optical scanner100according to an embodiment of the present invention.

In particular, the optical scanner100comprises a mirror200pivotally mounted about a first pivot axis XX′.

In this respect, the mirror200comprises a front face210, essentially planar, and a rear face220opposite to the front face210.

The optical scanner100further comprises a light source300intended to emit an incident light radiation on the front face210of the mirror200. Advantageously, the light source300may be a monochromatic source, for example a laser source or a light-emitting diode. For example, the wavelength of the radiation emitted by the light source may be equal to 1,550 nm.

In this example, the mirror200is partially transparent to the light radiation emitted by the light source300. In other words, an incident light radiation (called incident radiation, and denoted “RI”), according to an angle of incidence θi, on the front face210of the mirror200crosses the latter from the front face210and comes therefrom, in the form of a transmitted radiation (denoted “RT”), by the rear face220.

Moreover, the front face210may be partially reflective so that the reflection of the incident radiation also produces a reflected radiation RR (FIG.3A).

In this respect, the mirror200may comprise a partially reflective layer230resting on a main face of a support substrate240. A face of the partially reflective layer230then forms the front face210of the mirror. More particularly, the mirror200may comprise from its front face210towards its rear face220, the partially reflective layer230and a support substrate240. Each of the partially reflective layer230and the support substrate240may have an absorption coefficient that is negligible, and even zero, in the wavelength range covered by the incident radiation.

According to an advantageous embodiment, the partially reflective layer230could, in turn, comprise a Bragg stack (or Bragg mirror) formed by at least one elementary Bragg stack. By “Bragg stack”, it should be understood a periodic series of transparent, or partially transparent, layers and with different refractive indices. An elementary Bragg stack comprises a stack of two dielectric and/or semiconductor layers.

More particularly, the mirror200may be formed, from its front face210towards its rear face220, by the partially reflective layer230and the support substrate240, each of the partially reflective layer230and the support substrate240may have an absorption coefficient that is negligible, and even zero, in the wavelength range covered by the incident radiation. The device still functions even when a quite considerable portion of the light is absorbed in the substrate240, for example about 50%. For a thinned substrate with a thickness from 50 μm to 100 μm, this corresponds to an extinction coefficient k=10{circumflex over ( )}−3 or to an absorption coefficient in the range of 100 cm-1.

In particular, when the considered incident light radiation is in the infrared range, and more particularly has a wavelength equal to 1,550 nm, the elementary Bragg stack may comprise a silicon dioxide layer with a thickness of 268 nm (whose refractive index at 1,550 nm amounts to 1.45) covered by an amorphous silicon layer with a thickness of 113 nm (whose refractive index at 1,550 nm amounts to 3.42). According to this configuration a Bragg stack comprising a unique elementary Bragg stack has, for an incidence comprised between 0° and 20°, a reflection coefficient equal to 88.8% and a transmission coefficient equal to 11% for a main light beam with a wavelength equal to 1,550 nm. Moreover, this stack is barely absorbing, and even not at all, and will consequently have an almost zero heat-up.

The limitation of the number of elementary Bragg stacks allows reducing the mechanical stresses imparted to the mirror, and thus prevent any deformation (for example incurvation) of said mirror.

Moreover, the sizing of the elementary Bragg stack allows adjusting the reflectivity in the wavelength range covered by the incident light radiation.

According to one variant, the mirror200may comprise an apertured reflective layer250resting on a portion of an upper face of the support substrate240made of a material transparent to the incident wavelength. Thus, the reflective layer250includes one or more openings revealing the support substrate240which absorption coefficient is negligible, in the wavelength range covered by the incident radiation.

In the embodiment illustrated inFIG.3B, a central opening254crosses the reflective layer250and extends up to the upper face of the support substrate240. According to one variant (not represented), several openings cross the reflective layer250and reveal the support substrate240.

The reflective layer250may be made of a metallic material such as gold or aluminium. According to a variant illustrated inFIG.3C, the mirror200comprises a reflective layer250resting on only one portion of the upper face of the support substrate240made of a material transparent to the incident wavelength (FIG.3C).

In turn, the support substrate240may comprise a semiconductor material and more particularly silicon.

In either one of the examples ofFIGS.3B,3C, the front face210of the mirror200is formed by the reflective layer250.

In either one of the previously-described examples, the rear face220of the mirror may be formed by the lower face of the support substrate240opposite to said upper face of the latter.

In either one of the previously-described examples, the mirror200is fitted with at least one area transparent or partially transparent to the light radiation arranged between the front face and the rear face of the mirror. In the embodiment ofFIG.3A, this transparent or partially transparent area is formed by the layer230and the substrate240and extends from the front face210up to the rear face230of the mirror. Alternatively, the transparent or partially transparent area may extend from a region located between the front face and the rear face, up to the rear face. In the particular embodiment ofFIGS.3B and3C, this area is thus formed by the substrate240. Moreover, the rear face220is structured so as to impart a deflection of a transmitted radiation RT with respect to the incident radiation RI. As illustrated inFIG.4, the structuration may comprise at least one facet220iessentially planar and inclined by an angle β with respect to the front face210.

Thus, according to this configuration, the incident radiation RI, incident on the front face210according to an angle of incidence θi with respect to the normal N1of the front face200, undergoes a first refraction when it crosses the front face210so as to form a refracted radiation RR1. The refracted radiation RR1then forms an angle of refraction θ1with respect to the normal N1which meets the following relationship (1):
sin(θi)=nsin(θ1)  (1)
n being the refractive index of the mirror.

This same refracted radiation RR1forms an angle θ2 with respect to the normal N2of the rear face220, and an angle θ2+β with the direction N1′ parallel to the normal N1. Consequently, the angle θ2 meets the following relationship (2):
θ2=θ1−β  (2)

In turn, the refracted radiation RR1is refracted by the rear face220so as to form the transmitted radiation RT. This transmitted radiation RT forms an angle θ3 with the normal N2, and an angle θ4 with the direction N1′. Moreover, the angle formed between the transmitted radiation RT and the incident radiation RI is denoted et.

Thus, Snell-Descartes relationship allows writing:
sin(θ3)=nsin(θ2)  (3)

Moreover, the angles θ4 and θt meet the following relationships:
θ4=θ3+β  (4)
And
θt=−(θ1−θ4)  (5)

Thus, given the relationships (1) to (5), we could deduce a relationship between θt and θi:

θ⁢t=-θ⁢i+(β+sin-1(n⁢sin⁡(-β+sin-1(sin⁢(θ⁢i)n))))(6)

Thus, the implementation of a facet220iinclined by an angle β with respect to the front face210thus allows imparting a deflection of the transmitted radiation RT with respect to the incident radiation. In particular, this deflection varies as a function of the angle of incidence θi of the incident radiation RI.

In this respect,FIGS.5and6are graphical representations of the angles θt and θr as a function of the angle of incidence θi.

In particular,FIG.5represents the variation of the angles θt and θr as a function of the angle of incidence θi of a mirror200made of silicon (with a refractive index n equal to 3.48 at the wavelength 1.55 μm) and with an angle β=11°. According to this graphical representation, when the angle of incidence θi varies between 50° and 70°, the deflection θt of the transmitted radiation RT varies by 10°, whereas the angle θr varies by 40°.

Likewise,FIG.6represents the variation of the angles θt and θr as a function of the angle of incidence θi of a mirror200made of glass (with a refractive index n equal to 1.55 at the wavelength 1.55 μm) and with an angle β=26°. According to this graphical representation, when the angle of incidence θi varies between 50° and 70°, the deflection θt of the transmitted radiation RT varies by 10°, whereas the angle θr varies by 40°.

Thus, the rotation of the mirror200about the first pivot axis XX′ enables the reflected RR and transmitted RT radiations to angularly sweep two distinct areas, which could possibly overlap.

According to an advantageous aspect (FIG.7), the mirror200could also be pivotally mounted about a second pivot axis YY′ perpendicular to the first pivot axis XX′ and parallel to a second direction of the plane formed by the front face210. According to this aspect, each of the reflected RR and transmitted RT radiations could sweep a surface by rotation of the mirror about either one of the first and second pivot axes.

For the rotation about the axis XX′, the reflected and transmitted beams sweep the plane YZ, whereas for the rotation about the axis YY′, the reflected beam sweeps a plane XRR0, the transmitted beam sweeps a plane XRT0, RR0 and RT0 being directions of the reflected RR and transmitted RT beams when the angle of rotation about the axis YY′ is zero, i.e. when the incident ray is in the plane OYZ. Consequently, if a screen is initially positioned perpendicularly to each of the reflected RR and transmitted RT beams, the rotations about the axes XX′ and YY′ make the points of impact of these two beams describe a cross on the two screens.

In the particular embodiment illustrated inFIG.7, the prism has a cylinder-like shape, in particular a straight cylinder with faces701,702which are parallel to one another. Alternatively, a prism fitted with non-parallel faces701,702could be provided.

In a particularly advantageous manner, the optical scanner100may comprise first and/or second actuators intended to control the rotation of the mirror200about the first pivot axis XX′ and the second pivot axis YY′, respectively. The first and second actuators may comprise at least one of the elements selected from among: an electrostatic actuator, a magnetic actuator, a piezoelectric actuator, a thermal actuator.

Alternatively to either one of the described examples, it is possible to provide for making the mirror(s) pivot about a number of pivot axes greater than two. Alternatively to either one of the described examples, it is also possible to provide for a pivoting of the mirror(s) according to one or more axes forming a non-zero angle with a parallel to the main plane of the mirror and to its front face, in other words according to one or more axes non-parallel to the front face of the mirror.

According to another aspect illustrated inFIG.8, the structuration of the rear face220comprises a plurality of facets220iarranged according to a row. In particular, the facets220imay be arranged so as to form a sawteeth-like periodic profile. Advantageously, the interval Ii between two teeth of the sawteeth-like profile is comprised between 50 μm and 100 μm, and the depth pi of the teeth comprised between 5 μm and 10 μm. Complementarily (FIG.11), the rear face220may have a concave shape which thus allows increasing the sweep angle θt.

Thus, the angle between the input and output faces varies according to the position on the axis OY′ of the orthogonal reference frame [O; X′; Y′; Z′] given inFIG.11. When the angle of incidence of the beam on the input face is small for example less than 10° and the point of impact of the beam on the input face is close to the point where the two faces are parallel, the deviation of the transmitted beam is very small, and possibly zero if the angle of incidence is zero and the beam falls on the top of the concave shape. When the mirror is rotated about the axis OX′, the angle of incidence of the beam on the front face increases and the beam transmitted by the front face reaches the rear face at a position which goes away from the point where the two faces are parallel. Thus, the angle of incidence on the rear face increases and the deviation increases. This allows widening the swept angular range in transmission.

The invention also relates to a method for manufacturing the optical scanner and more particularly the mirror200.

The method comprises the supply of a substrate800provided with a front face810and with a rear face820(FIG.9a).

Afterwards, a structuration in the form of a triangular or sawteeth-like signal is formed starting from the front face810. The formation of this structuration may involve the implementation of a grayscale mask900(FIG.10a).

The insolation of a resin layer910with such a mask, followed by the development thereof confers a triangular profile on said resin (FIG.10b).

The triangular profile830of the front face810results from a dry etching followed by a step of annealing the resin. The triangular profile may have a period I comprised between 50 μm and 100 μm, and a depth p comprised between 5 μm and 10 μm.

The formation of the structuration is followed by a step of forming a SiO2layer by PECVD and by a planarisation of said layer840(FIGS.9band9c). In particular, upon completion of the planarisation, this layer has a thickness larger than the depth p.

Afterwards, the substrate800is assembled with a receiver substrate850by contacting the SiO2layer840with a main face of the receiver substrate850(FIG.9d).

This assembly may comprise a molecular bonding followed by a heat treatment intended to reinforce the bonding interface.

The assembly is then followed by a step of thinning, in particular mechanical thinning, of the substrate800(FIG.9d).

The second manufacturing method also comprises steps of forming electrodes710and wafers700delimiting in particular the mirror200(FIG.9e).

Finally, the second manufacturing method comprises a step of etching through an exposed main face of the receiver substrate850intended to clear the mirror200(FIG.9f). This etching step may be carried out by DRIE (standing for “Deep Reactive Ion Etching”) so as to preserve the rear face structuration of the mirror200. The SiO2layer may also be removed off the mirror200.

REFERENCES

[1] Sven Holmstrom et al., “MEMS laser scanners: a review”, Journal of Microelectromechanical Systems April 2014, DOI: 10.1109/JMEMS.2013.2295470.