PROCESS FOR MANUFACTURING A MICROELECTRONIC DEVICE HAVING A BLACK SURFACE, AND MICROELECTRONIC DEVICE

Abstract

A roughened silicon surface is formed by a process including repetitively performed roughening cycles. Each roughening cycles including a step for depositing a non-planar polymeric layer over an area of a silicon body and a step for plasma etching the polymeric layer and the area of the silicon body etch in a non-unidirectional way. As a result, a surface portion of the silicon body is removed, in a non-uniform way, to a depth not greater than 10 nm.

Claims

1. A process for forming a roughened silicon surface, comprising performing a plurality of roughening cycles, wherein each roughening cycle includes: depositing a polymeric layer having a non-planar top surface over an area of a silicon body; and plasma etching the polymeric layer and the area of the silicon body in a non-unidirectional way to remove, in a non-uniform way, a surface portion of the area of the silicon body.

2. The process according to claim 1, wherein plasma etching comprises removing said surface portion to a depth that is not greater than 10 nm at the area of the silicon body.

3. The process according to claim 1, wherein depositing is carried out with a C.sub.4F.sub.8 plasma and plasma etching is carried out with a C.sub.4F.sub.8 and SF.sub.6 plasma.

4. The process according claim 1, wherein plasma etching comprises supplying a first flow of SF.sub.6 and supplying a second flow of C.sub.4F.sub.8, wherein a ratio between the first flow and the second flow is approximately 1.520%.

5. The process according to claim 4, wherein plasma etching comprises also supplying at least one of C and O.sub.2.

6. The process according to claim 4, wherein depositing comprises supplying a third flow of C.sub.4F.sub.8, the third flow being greater than the second flow.

7. The process according to claim 6, wherein a ratio between the third flow and the second flow is less than 1.5.

8. The process according to claim 6, wherein the first flow is at a rate of 650 sccm, the second flow is at a rate of 400 sccm, and the third flow is at a rate of 425 sccm.

9. The process according to claim 1, wherein depositing is carried out for a deposition time and plasma etching is carried out for an etching time, and wherein a ratio between the deposition time and the etching time during at least one roughening cycle is between approximately 0.25 and 0.4.

10. The process according to claim 9, wherein said ratio is approximately 0.35.

11. The process according to claim 1, wherein depositing is carried out for a deposition time and plasma etching is carried out for an etching time, and wherein a ratio between the deposition time and the etching time during a first roughening cycle is different from a ratio between the deposition time and the etching time during roughening cycles subsequent to said first roughening cycle.

12. The process according to claim 11, wherein the etching time is substantially the same in all roughening cycles and wherein the deposition time is longer in the first roughening cycle than in roughening cycles subsequent to said first roughening cycle.

13. The process according to claim 12, wherein the deposition time in the first roughening cycle is approximately 1.5 s and the deposition time in roughening cycles subsequent to said first roughening cycle is approximately 1.4 s.

14. The process according to claim 13, wherein the etching time is approximately 4 s in all roughening cycles.

15. The process according to claim 1, wherein depositing is carried out at a first plasma pressure, and plasma etching is carried out at a second plasma pressure, and wherein a ratio between the first and second pressures is approximately 1.85% in at least one roughening cycle.

16. The process according to claim 15, wherein said ratio is the same in all roughening cycles except for a first roughening cycle.

17. The process according to claim 16, wherein the first plasma pressure is approximately 80 mtorr in the first roughening cycle and the first plasma pressure is approximately 90 mtorr in roughening cycles subsequent to said first roughening cycle, and wherein the second plasma pressure is approximately 50 mtorr.

18. The process according to claim 1, wherein the roughened silicon surface, after performing said plurality of roughening cycles, has a reflectivity of less than 10%.

19. A semiconductor device, comprising a silicon body having a roughened surface formed having a reflectivity of less than 10%, wherein said roughened surface is obtained by performing a plurality of roughening cycles, wherein each roughening cycle includes: depositing a polymeric layer having a non-planar top surface over an area of a silicon body; and plasma etching the polymeric layer and the area of the silicon body in a non-unidirectional way to remove, in a non-uniform way, a surface portion of the area of the silicon body.

20. The device according to claim 19, wherein said roughened surface is characterized by projections having a height-to-width ratio comprised between 1:1 and 1:5 and a high density.

21. The device according to claim 19, wherein the silicon body includes a cavity having side walls and a bottom wall, said sidewalls and bottom wall including said roughened surface.

22. The device according to claim 21, wherein the silicon body carries an oscillating element forming a MEMS mirror element.

23. The device according to claim 22, wherein the device is a component of an electrical apparatus comprising: an image-projection module that includes said MEMS mirror element; and a light source configured to generate a source light beam directed towards the MEMS mirror element.

24. The device according to claim 23, wherein the electrical apparatus further comprises an image-capture module operatively coupled to the image-projection module and configured to capture images associated to a light beam reflected by the MEMS mirror element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For a better understanding of the present invention, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:

[0029] FIG. 1 is a schematic perspective view of a picoprojector;

[0030] FIG. 2 is a schematic representation of an embodiment of the micromirror of FIG. 1;

[0031] FIG. 3 is a longitudinal section of an embodiment of the micromirror of FIG. 2;

[0032] FIGS. 4-7 are cross-sections through a semiconductor material wafer in successive steps of formation of a blacked cavity;

[0033] FIGS. 8A and 8B are cross-sections of a silicon body in two successive steps of the present process;

[0034] FIGS. 9-12 are cross-sections through a semiconductor material wafer having a cavity repeatedly subjected to the manufacturing steps of FIGS. 8A and 8B, during blackening according to the present process;

[0035] FIGS. 13A-13C are cross-sections through a semiconductor material wafer for forming a cavity, prior to blackening;

[0036] FIG. 14 is a block diagram of a picoprojector using the present micromirror device; and

[0037] FIGS. 15 and 16 show different solutions for coupling the picoprojector of FIG. 14 and a portable electronic apparatus.

DETAILED DESCRIPTION

[0038] The blackening process described herein is based upon formation of roughness on a silicon surface, by repeating cycles comprising two steps: a first step (referred to as a deposition step, see FIG. 8A) in which a polymeric layer 31 is deposited over a silicon region 30, where the polymeric layer 31 comprises an organic polymer having a non-planar surface structure; and a second step (referred to as an etching step, see FIG. 8B), so that, approximately, the non-planar surface structure of the polymeric layer 31 is reproduced in (i.e., transferred to) the silicon region 30. Typically, in the second step, the polymeric layer (designated by 31 in FIG. 8B) is not completely removed. However, the process functions also in the case of complete removal.

[0039] In fact, as may be noted from FIGS. 8A and 8B, during the etching step, by virtue of the non-planar surface structure of the polymeric layer 31, in the points where the layer 31 has a smaller thickness, also a surface part of the silicon region 30 is etched and removed. Thereby, at the end of the second step, the surface 30A of the silicon region 30, which is initially planar (FIG. 8A), becomes rough (surface 30A of FIG. 8B).

[0040] The described steps are carried out, for example, in an Inductive Coupled Plasma (ICP) reactor or in a Transformer Coupled Plasma (TCP) reactor. The polymeric layer 31 is deposited so that the layer 31, which is completely organic, may be easily removed and does not interact with the silicon of the silicon region 30, but deposits thereon. Etching of the polymeric layer 31 and of the surface part of the silicon region 30 is very soft and studied so as to cause a corrugation of the surface of the silicon region 30, without creating deep structures in the silicon (such as pillars or spikes, the so-called grass), which could detach during operation of the finished device with the blackened surface.

[0041] To obtain this, the time of residence t of the species in the plasma of the process is modulated. In fact, as is known, the time of residence t is proportional to the pressure P in the reactor and to its volume V and inversely proportional to the flow of gas Q,

t=PV/Q,

[0042] so that, by changing the time of residence (i.e., acting on the parameters pressure and flow), it is possible to modify the amount of deposited polymer and the rate of the second step in order to obtain selective surface removal of silicon that gives rise to the desired roughness.

[0043] In detail, the blackening process of the surface 30A shown in FIGS. 8A-8B is carried out using at least some of the following parameters: [0044] in the deposition step, the polymeric layer 31 is deposited using C.sub.4F.sub.8 plasma and, in the second step, etching is carried out using C.sub.4F.sub.8 and SF.sub.6 plasma; [0045] in the etching step, also C and/or O.sub.2 may be present; [0046] in the etching step, the ratio between the flow of SF.sub.6 and the flow of C.sub.4F.sub.8 is 1.520%; for example, the flow of SF.sub.6 may be 600-700 sccm (standard cubic centimeters per minute) and the flow of C.sub.4F.sub.8 may be 400-450 sccm; [0047] the flow of C.sub.4F.sub.8 during the deposition step is typically a little higher than the absolute value of the flow of the same gas during the etching step; for example, with the values indicated above, it is approximately 425 sccm; [0048] the ratio between the duration of the deposition step and the duration of the etching step is comprised between approximately 0.25 and 0.4, for example, approximately 0.35, except possibly in the first cycle, where, for activating the reactor, the duration of the deposition step may be longer; for example, the deposition step may last 1.4 s (1.5 for the first cycle), and the etching step may last 4 s; [0049] the ratio of gas pressure in the deposition step to the gas pressure in the etching step is approximately 1.85%, except possibly in the first cycle, where, for the same activation reasons, the ratio may be lower; for example, the deposition step may be carried out at a pressure of 90 mtorr (80 mtorr for the first cycle), and the etching step may be carried out at a pressure of 50 mtorr.

[0050] With the indicated parameters, performance of the etching step leads to removal of a layer of 10.5 nm of the polymeric layer 31 and of a surface portion of at the most 3 nm of the underlying silicon region 30, where the latter is completely exposed.

[0051] The roughness of the surface of the silicon region 30 may be regulated, repeating the two steps a certain number of times or cycles, as described hereinafter with reference to FIGS. 9-13.

[0052] In detail, in FIG. 9, a body 40 of monocrystalline or polycrystalline silicon, covered by a mask 41, for example of photoresist, is initially etched via a deep-etching process, in a per se known manner, for forming a cavity 42 having side walls 42A and bottom walls 42B. This step may be carried out as shown in FIGS. 13A-13C, via repeated cycles of: [0053] conformal deposition of a passivation layer 50 (for example, via deposition in C.sub.4F.sub.8 plasma, FIG. 13A); [0054] removal of the passivation layer 50 from the horizontal surfaces of the structure, via highly directional ion etching in SF.sub.6/O.sub.2 plasma (FIG. 13B), where the active ions of the plasma (which are positively charged) are designated by 55, and the free fluorine radicals are designated by 56; by virtue of the directionality of the etch, protective portions 50A remain only on the lateral surfaces 42A of the cavity 42 being formed (FIG. 13C); and [0055] subsequent etching of the silicon from the bottom of the cavity 42 as a result of the free fluorine radicals 56 for removing a thickness of a few hundred microns (FIG. 13C); the cavity 42 thus increases in depth, as represented in FIG. 13C with a dashed line and arrow.

[0056] Repeating the steps of FIGS. 13A-13C, the desired depth of the cavity 42 is obtained. For example, the depth of the cavity 42, at the end of the deep etching step, may be of 100-500 m.

[0057] After reaching the desired depth for the cavity 42, and maintaining the same mask 41, the surface of the cavity 42 is roughened or blackened a first time (i.e., a first cycle), using the two-step process described with reference to FIGS. 8A, 8B, with a result as shown in FIG. 10. In this way, the side walls 42A and the bottom 42B have a first roughness. It should be noted that, due to the absence of a polymer protective layer on the side walls 42A and on the bottom 42B of the cavity 42, this process is not directional and operates at a superficial level, removing a thickness of at the most 10 nm, thus without substantially modifying the depth of the cavity 42.

[0058] In FIG. 11, the surface of the cavity 42 is blackened, i.e., becomes rougher, by exploiting a second roughening cycle with the two-step process described with reference to FIGS. 8A and 8B. Since, at the start of this second cycle, the surface of the cavity 42 has the first roughness, at the end of the second cycle, the roughness of the side walls 42A and of the bottom 42B is increased, and these have a second roughness, greater than the first roughness.

[0059] In FIG. 12, the surface of the cavity 42 is roughened and blackened a third time, performing a further roughening cycle with the two-step process described with reference to FIGS. 8A and 8B. Consequently, the roughness of the side walls 42A and of the bottom 42B increases, and they have a third roughness, greater than the second roughness.

[0060] The cycles described with reference to FIGS. 10-12 may be repeated several times, even 20-30 or more, until a desired level of roughness is reached, compatibly with an industrial process.

[0061] The process described has numerous advantages.

[0062] In particular, it is simple and inexpensive, since it does not require additional masks or additional layers on thin layers. It integrates the structure definition steps, in particular for forming a cavity, with the blackening step, since they may be carried out in a same reactor, and does not cause formation of structures and particles that could cause disturbance for operation of the finished device. The process is further completely compatible with the current processes of micromachining semiconductor materials.

[0063] Finally, the described process is particularly suitable for blackening the cavity underlying the oscillating element of a mirror element 5, 6 of the type shown in FIGS. 2-3, since the obtainable blackening may achieve very low indices of spurious reflectivity, depending upon the roughness obtained, for example indices lower than 2%.

[0064] The obtained rough surfaces have a morphology characterized by low projections at a high density and rather blunt. For example, the obtained projections may have a diameter comprised between 0.5 and 2 m and a height of approximately 0.5 m, according to the roughness to be obtained, without bonds, in particular SiO bonds, since the polymers deposited during the first step of FIG. 8A (similar to Teflon) are simply deposited on the silicon and do not bind thereto. In this way, in samples obtained by the applicant, it has been possible to achieve a reflectivity in the region of 1%.

[0065] The mirror element 5, 6 having the cavity 42 may be part of a micromirror such as the micromirror 8 of FIG. 1 and be used in a picoprojector 201 designed to be functionally coupled to a portable electronic apparatus 200, as illustrated hereinafter with reference to FIGS. 14-16.

[0066] In detail, the picoprojector 201 of FIG. 14 comprises a light source 202, for example of a laser type, intended to generate a light beam 203; the micromirror device 8, intended to receive the light beam 203 and to orient it towards a screen or an image-capture module 205 (external and arranged at a distance from the picoprojector 201); a first driving circuit 206, intended to supply appropriate control signals to the light source 202, for generating the light beam 203 according to an image to be projected; a second driving circuit 208, intended to supply driving signals to rotate the oscillating element 14 (FIGS. 2 and 3) of the micromirror element 5, 6; and a communication interface 209, intended to receive, from a control unit 210, internal or external, for example included in the portable apparatus 200, brightness information on the image to be generated, for example in the form of an array of pixels. The brightness information is sent to an input to drive the light source 202.

[0067] Further, the control unit 210 may comprise a unit for controlling the angular position of the mirror of the micromirror device 130. To this end, the control unit 210 may receive the signals generated by photodetectors (not represented in FIG. 14) through the interface 209 and control the second driving circuit 208 accordingly.

[0068] The picoprojector 201 may be formed as a separate and stand-alone accessory with respect to an associated portable electronic apparatus 200, for example a cellphone or smartphone, as shown in FIG. 15. In this case, the picoprojector 201 is coupled to the portable electronic apparatus 200 by appropriate electrical and mechanical connection elements (not illustrated in detail). Here, the picoprojector 201 is provided with an own casing 241, which has at least one portion 241 transparent to the light beam 203 coming from the micromirror device 130. The casing 241 of the picoprojector 201 is releasably coupled to a respective casing 242 of the portable electronic apparatus 200.

[0069] Alternatively, as illustrated in FIG. 16, the picoprojector 201 may be integrated within the portable electronic apparatus 200 and be arranged within the casing 242 of the portable electronic apparatus 200. In this case, the portable electronic apparatus 200 has a respective portion 242 transparent to the light beam 203 coming from the micromirror 8. The picoprojector 201 in this case is, for example, coupled to a printed-circuit board in the casing 242 of the portable electronic apparatus 200.

[0070] Finally, it is clear that modifications and variations may be made to the process and to the device described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the attached claims.

[0071] For example, the process described herein applies to any silicon surface that is to be rendered rough, even in the absence of cavities or trenches.