Optical device

Abstract

Optical device The present description relates to an optical device, such as an imager, successively comprising a support (10) in which vias (12) are formed, a first electrode (32), an active layer (20) capable of absorbing photons and transforming them into electron-hole pairs, a second electrode (34), a conductive layer (35) connecting the second electrode (34) to one of the vias (12), a microlens matrix (70), the device further comprising an encapsulation layer (50, 55), arranged between the microlens matrix (70) and the active layer (20), the encapsulation layer (50, 55) comprising a first portion having a first density and a second portion having a second density, the first portion of the encapsulation layer (50, 55) being arranged between the active layer (20) and the second portion of the encapsulation layer (50, 55), the first density being lower than the second density. Figure for abstract: Fig. 1A

Description

Title of the invention: Optical device Technical field

[0001] The present description relates generally to optical devices, such as image acquisition devices or image sensors. More particularly, the present description relates to an optical device comprising pixels and microlenses. Prior art

[0002] An optical device, such as an imager, comprises microlenses that focus light radiation onto an active layer (also called a quantum film, or QF for 'quantum film') capable of transforming photons into electron-hole pairs. An antireflection layer is disposed between the microlenses and the active layer. The antireflection layer also acts as a protective layer for the active layer.

[0003] An upper electrode, for collecting holes, and a lower electrode, for collecting electrons, are arranged on either side of the active layer. The upper electrode makes contact on the upper part of the active layer and is offset to vias positioned in the substrate (BIP for 'Build in Pad') by means of a conductive layer, for example made of aluminum. To be able to make contact on the second electrode (CTPD or 'Contact on PhotoDiode'), an opening is formed in the anti-reflection layer.

[0004] Thus, when the image acquisition device is subjected to radiation of appropriate wavelength (for example at a short-wave infrared (SWIR) wavelength), the photons, after passing through the microlenses and the antireflection layer, penetrate into the active layer. Electrons and holes are then generated. The electrons are directed and guided by vias to the nodes of the device (i.e. to the active areas of the circuit). The holes are collected by the upper electrode and directed to the vias (BIP) via the conductive layer.

[0005] In order to guarantee the reliability of the electronic components and have a quantum efficiency (QE) of the active layer greater than 50%, the optical device must meet different criteria: temperature resistance, humidity resistance, etc.

[0006] However, in current optical devices, oxidation of the active layer is observed which may come from circuit manufacturing processes or reliability tests (exposure to humidity).

[0007] Indeed, it has been observed, on the one hand, that current anti-reflective layers are not sufficiently effective to prevent oxidation of the active layer.

[0008] On the other hand, the conductive aluminum layer allowing contact recovery may have defects, in particular pitting resulting from its manufacturing process. These defects are responsible for the creation of a certain permeability of the aluminum layer, which can cause problems during its structuring, in particular by wet etching. Indeed, the chemicals used for cleaning the photolithography resin can pass through the aluminum layer and contaminate the active layer, in particular at the level of the contact recovery region of the second electrode (CTPD or 'Contact on PhotoDiode').

[0009] Bringing the active layer into contact with air, water and / or other oxidizing agents leads to a reduction in its performance, and it may be difficult or even impossible to obtain a quantum efficiency of the active layer greater than 50%. Summary of the invention

[0010] There is a need to improve the performance of optical devices, and in particular to prevent oxidation of the active layer.

[0011] This object is achieved by an optical device, such as an imager, successively comprising a support in which vias are formed, a first electrode, an active layer capable of absorbing photons and transforming them into electron-hole pairs, a second electrode, a conductive layer connecting the second electrode to one of the vias, a microlens matrix, the device further comprising an encapsulation layer, arranged between the microlens matrix and the active layer, the encapsulation layer comprising a first portion having a first density and a second portion having a second density, the first portion of the encapsulation layer being arranged between the active layer and the second portion of the encapsulation layer, the first density being lower than the second density.

[0012] According to a particular embodiment, the encapsulation layer is based on silicon oxide.

[0013] According to a particular embodiment, the encapsulation layer is arranged between the active layer and the conductive layer.

[0014] According to a particular embodiment, the encapsulation layer covers the sides and a portion of an upper face of the active layer.

[0015] According to a particular embodiment, the encapsulation layer is arranged between and in contact with two layers of metal nitride, preferably of silicon nitride.

[0016] According to a particular embodiment, the encapsulation layer is arranged between the conductive layer and the microlens matrix.

[0017] According to a particular embodiment, the encapsulation layer is covered by a layer of metal nitride, preferably silicon nitride.

[0018] According to a particular embodiment, the device comprises two encapsulation layers, one of the encapsulation layers being arranged between the active layer and the conductive layer and the other of the encapsulation layers being arranged between the conductive layer and the microlens matrix.

[0019] According to a particular embodiment, the conductive layer is made of aluminum.

[0020] According to a particular embodiment, the first density is between 2.05 g / cm3 and 2.13 g / cm3, for example 2.09 g / cm3, and / or the second density is between 2.20 g / cm3 and 2.28 g / cm3, for example 2.24 g / cm3.

[0021] According to a particular embodiment, the first part of the encapsulation layer has a thickness of between 50 and 250 nm and / or the second part of the encapsulation layer has a thickness of between 3 and 50 nm.

[0022] This object is also achieved by a method of manufacturing an optical device according to any one of the preceding claims, the method comprising the formation of an encapsulation layer according to the following steps: - depositing a first precursor, at a first deposition rate, to form a first portion of the encapsulation layer, the first portion having a first density, - depositing a second precursor, at a second deposition rate, to form a second part of the encapsulation layer, the second part having a second density, the first deposition rate being greater than the second deposition rate, whereby the first density is less than the second density.

[0023] According to a particular embodiment, the first part of the encapsulation layer and the second part of the encapsulation layer are deposited by PECVD at a temperature less than or equal to 150°C.

[0024] According to a particular embodiment, the first precursor and the second precursor are silicon oxide precursors, the first precursor and the second precursor preferably being silicon alkoxides, even more preferably TEOS.

[0025] According to a particular embodiment, the first deposition speed is at least 5 times greater than the second deposition speed. Brief description of the drawings

[0026] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0027] [Fig.1A] schematically represents, in section, an optical device according to a particular embodiment;

[0028] [Fig. 1B] schematically represents, in section, an optical device according to a particular embodiment;

[0029] [Fig. 1C] schematically represents, in section, an optical device according to a particular embodiment;

[0030] [Fig.2] is a graph representing the variation of water (in mg) as a function of time of different layers: a SiON layer annealed at 300°C, a SiON layer annealed at 400°C, a 100 nm thick SiO2 layer deposited at low speed from TEOS ('TEOS LDR': 'TEOS Low Deposition Rate')) and annealed at 400°C, a 142 nm SiO2 layer and a 200 nm SiO2 layer deposited by PECVD;

[0031] [Fig.3] is a graph representing the tensile stress as a function of time for a layer of SiO2 according to a particular embodiment;

[0032] [Fig.4] is a graph representing the variation of the compressive stress as a function of time for a SiON layer;

[0033] [Fig. 5] is a photograph obtained using a scanning electron microscope (SEM) of a part of an active layer, one side of which is successively covered by a layer of SiN then by a layer of SiO2 according to a particular embodiment; and

[0034] [Fig.6] is a transmission electron microscope image (MET-EDX) of the BIP region of an optical device according to a particular embodiment. Description of the embodiments

[0035] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0036] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed.

[0037] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0038] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "on", "under", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures.

[0039] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean 10%, preferably 5%.

[0040] Unless otherwise specified, by between X and Y, we mean that the limits X and Y are included in the range.

[0041] In the remainder of the description, unless otherwise specified, a layer or film is said to be opaque to radiation when the transmittance of the radiation through the layer or film is less than 30%, and preferably less than 10%.

[0042] In the remainder of the description, a layer or film is said to be transparent to radiation when the transmittance of the radiation through the layer or film is greater than 50%, and preferably greater than 70%.

[0043] Embodiments of optical devices will now be described for optical devices comprising an array of micrometer-sized optical elements in the case where each micrometer-sized optical element corresponds to a micrometer-sized lens, or microlens, composed of two diopters. However, it is clear that these embodiments can also be implemented with other types of micrometer-sized optical elements, each micrometer-sized optical element being able to correspond, for example, to a micrometer-sized Fresnel lens, a micrometer-sized gradient index lens or a micrometer-sized diffraction grating.

[0044] The optical device may be an image acquisition device operating in the near infrared (NIR for 'Near InfraRed'), that is to say for electromagnetic radiation whose wavelength is between 800 nm and 2500 nm, and more particularly, in the short infrared (SWIR or 'Short-Wave InfraRed'), that is to say for electromagnetic radiation whose wavelength is between 800 nm and 2000 nm, preferably between 900 nm and 1700 nm, typically 1.4 pm.

[0045] The optical device may be an optical sensor, in particular a proximity sensor.

[0046] This is a device having photodiodes arranged above the integrated circuit ('Photodiode Above IC').

[0047] The optical device is interesting for many applications, and in particular for the automotive market (fog sensors, for example) or in the field of telephony (smartphones).

[0048] We will now describe the image acquisition device in more detail with reference to [Fig.lA], [Fig.lB] and [Fig.lC].

[0049] The device comprises a support 10 in which vias 12 are formed. The vias 12 are filled with a conductive material, such as copper. The support 10 may comprise a stack of insulating layers (not shown) as well as tracks conductive layers (not shown) between the insulating layers. The stack preferably has a thickness of the order of 2 pm. The vias 12 can be 3.2 pm deep.

[0050] The support 10 is covered by an active layer 20. The active layer 20 is a layer making it possible to transform the photons, coming from the upper face of the device, into electron / hole pairs.

[0051] The active layer 20 comprises a first face (lower face), partially in contact with the substrate 10, a second face (upper face) and sides.

[0052] The active layer 20 is preferably made of a semiconductor material, for example, silicon.

[0053] Electronic components are formed under the vias ('Viatop'). These may be insulated gate field effect transistors, or MOS ('Metal Oxide Semiconductor') transistors, notably manufactured in CMOS ('Complementary Metal Oxide Semiconductor') technology and / or photodetectors.

[0054] The active layer 20 has, for example, a thickness of the order of 0.4 μm to 1 μm.

[0055] The active layer 20 is arranged between a first electrode 32 and a second electrode 34.

[0056] The first electrode 32 makes it possible, for example, to collect electrons. The second electrode 34 makes it possible, for example, to collect holes.

[0057] The first electrode 32 (or lower electrode) is in contact with the first face of the active layer 20 and with the substrate 10.

[0058] The first electrode 32 is, for example, metallic.

[0059] The first electrode 32 is, for example, in the form of conductive pads.

[0060] The first electrode 32 is connected to the vias 12 of the substrate 10.

[0061] The second electrode 34 (or upper electrode) preferably covers, as completely the second face of the active layer 20.

[0062] The second electrode 34 is, for example, a bilayer or a trilayer (i.e. the second electrode comprises three layers). The second electrode 34 has, for example, a thickness of between 40 and 60 nm.

[0063] The second electrode 34 is connected to one of the vias 12 positioned in the substrate 10 (BIP for 'Built in Pad') by means of a conductive layer 35 which allows the contact to be moved.

[0064] The conductive layer makes contact with the second electrode 34 at the region called CTPD ('Contact on PhotoDiode').

[0065] In order to be able to make a contact on the second electrode (CTPD or 'Contact on PhotoDiode'), an opening is formed in an anti-reflective coating which covers the second electrode 34.

[0066] As we will see later, the anti-reflective coating can also play the role of protecting the active layer 20.

[0067] The device also comprises a planar layer 60 covering the anti-reflective coating. Preferably, it is a resin layer.

[0068] A lens array 70 (or microlens matrix), of micrometric size, covers the planarization layer 60. The lenses are, for example, formed of elements having a convex plane and a flat face. The flat face of the microlenses preferably rests on the upper face of the resin layer and in contact with the resin layer.

[0069] The microlens array 70 focuses the radiation onto the pixels.

[0070] The microlens array 70 is preferably arranged in a matrix form, the optical axis of each microlens being coincident with the center of a block of resin. The microlenses are, for example, composed of an organic resin. The microlenses are, preferably, transparent in the wavelengths considered.

[0071] According to one embodiment, the microlenses are all the same, that is, they have the same chemical composition and the same dimensions. Alternatively, the microlenses may not all have the same dimensions.

[0072] The device may comprise a protective layer 71 covering the microlenses 70. The protective layer 71 may be made of an inorganic material, for example silicon oxide (SiO2) or silicon oxynitride (SiON). According to one embodiment, the protective layer is substantially impervious to moisture. The protective layer has, for example, a thickness of between 100 nm and 600 nm.

[0073] The device may comprise a resin layer 80, called black resin, which strongly absorbs incident light in the wavelengths of interest. The black resin 80 is also called LOBO resin ('LOGICAL LIGHT BLOOKING'). The black resin 80 covers a portion of the microlenses, the other portion not being covered by the black resin. The resin layer 80 may be protected by a protective layer 81. The protective layer 81 may be made of an inorganic material, for example silicon oxide (SiO2) or silicon oxynitride (SiON). According to one embodiment, the protective layer is substantially impervious to moisture. The protective layer has, for example, a thickness of between 100 nm and 600 nm.

[0074] The device further comprises at least one encapsulation layer 50, 55 disposed between the active layer 20 and the microlens array 70. The encapsulation layer 50, 55 may be a layer made of SiON or SiO2. Preferably, it is made of SiO2.

[0075] The encapsulation layer 50, 55 is formed of two parts: a first part (lower part) having a first density and a second part (upper part) having a second density. The first part is disposed between the active layer and the second part. In other words, the first part is arranged on the lower side of the device, facing the active layer 20, and the second part on the upper side of the device, facing the microlens matrix 70.

[0076] The first density is less than the second density.

[0077] For example, the first density (i.e. the density of the lower layer) is between 2.05 g / cm3 and 2.13 g / cm3 and / or the second density (i.e. the density of the upper layer) is between 2.20 g / cm3 and 2.28 g / cm3. For example, for an SiO2 encapsulation layer, the first density is 2.09 g / cm3 and / or the second density is 2.24 g / cm3.

[0078] The upper part forms a dense layer (or crust) encapsulating the lower part of the layer.

[0079] The second part has a thickness, for example, between 3 and 50 nm, preferably between 10 and 50 nm, and even more preferably between 10 and 30 nm, for example of the order of 20 nm.

[0080] The first part has a thickness greater than the thickness of the second part. The thickness of the first part is, for example, between 50 and 250 nm, preferably between 150 and 250 nm, even more preferably between 180 and 230 nm, for example of the order of 210 nm.

[0081] The first part has, for example, a refractive index of between 1.43 and 1.45 (for a wavelength of 633 nm). The second part has, for example, a refractive index of between 1.47 and 1.48 (for a wavelength of 633 nm).

[0082] According to a first embodiment, the encapsulation layer 50 is positioned between the active layer 20 and the conductive contact recovery layer 35 ([Fig.1A]).

[0083] The encapsulation layer 50 covers the upper electrode 34 and the active layer 20. It also protects the sides of the active layer 20. It can also cover a portion of the support 10.

[0084] In this first embodiment, the encapsulation layer 50 also acts as an anti-reflective layer.

[0085] According to a second embodiment, the encapsulation layer 55 is positioned between the conductive contact recovery layer 35 and the microlens matrix 70 ([Fig.1B]). More particularly, it is in contact with the conductive layer 35. Preferably, it completely covers the conductive layer 35. This makes it possible to protect not only the CTPD zone and the BIP zone.

[0086] According to a third embodiment ([Fig. 1C]), the device may comprise two encapsulation layers 50, 55: - a first encapsulation layer 50 is positioned between the active layer 20 and the conductive layer 35, and - a second encapsulation layer 55 is positioned between the conductive layer 35 and the microlens array 70.

[0087] The device may further comprise one or more metal nitride layers 51, 52, 56. For example, the encapsulation layer 50 may be interposed between two metal nitride layers 50, 55 (i.e., layers 51 and 52 as in FIGS. 1A and 1C) and the encapsulation layer 55 may be covered by a metal nitride layer (i.e., layer 56 as in FIGS. 1B and 1C). The encapsulation layer and the metal nitride layer(s) form an encapsulation element.

[0088] The nitride is preferably a silicon nitride. Each nitride layer 51, 52, 56 has, for example, a thickness of between 300 and 400 nm, preferably between 350 nm and 380 nm.

[0089] The encapsulation element may be, in particular, a bilayer (SiO2 / SiN for example) or a trilayer (SiN / SiO2 / SiN). A bilayer will advantageously be used when the encapsulation layer is between the conductive layer 35 and the microlens array 70. A trilayer will advantageously be used when the encapsulation layer is between the active layer 20 and the conductive layer 35. The trilayer can play not only the role of encapsulation element but also the role of antireflection element.

[0090] The encapsulation element has, for example, a total thickness of between 30 nm and 200 nm, preferably between 40 nm and 130 nm, more preferably equal to approximately 40 nm. The thickness of the encapsulation element obtained (bilayer or trilayer) will depend in particular on its position. The thickness of the encapsulation element is, advantageously, adapted so that the layer is transparent in the visible range.

[0091] We will describe more particularly the method of manufacturing the encapsulation layer 50, 55 of the device.

[0092] The encapsulation layer is formed according to the following steps: - deposition of the first portion of the encapsulation layer 50, 55, - depositing the second portion of the encapsulation layer 50, 55 on the first portion of the encapsulation layer 50, 55.

[0093] The deposition of the first part and the second part of the encapsulation layer 50, 55 is carried out by a plasma-enhanced chemical vapor deposition (PECVD) technique.

[0094] To form each of the encapsulation layers 50, 55 having different densities, the first part and the second part of the layers are formed at different deposition rates.

[0095] The first part is formed at a first deposition rate and the second part is formed at a second deposition rate. The first deposition rate is higher than the second deposition rate. The first deposition rate is at least 5 times higher than the second deposition rate. It is, for example, about 10 times higher than the second deposition rate.

[0096] In a PECVD deposition, it is, for example, possible to modify the deposition speed by adjusting one or more of the following parameters: the power of the high frequency, the use of a low frequency (in addition to the high frequency) and / or the heating temperature.

[0097] A first precursor may be used to form the first portion of the encapsulation layer(s) and a second precursor may be used to form the second portion of the encapsulation layer(s). Preferably, the same precursor is used to form the first portion and the second portion of the encapsulation layer(s).

[0098] The deposition of the encapsulation layer 50, 55 is preferably carried out at a temperature less than or equal to 150°C. It is advantageously carried out at a temperature greater than 100°C. Even more preferably, it is carried out at a temperature between 120°C and 150°C.

[0099] The layer 50, 55 thus deposited has very good conformability, which ensures better protection of the underlying pixel, particularly in terms of sealing. Oxidation phenomena are thus avoided.

[0100] Preferably, the encapsulation layer is made of silicon oxide 50, 55 and the precursors are silicon oxide precursors, in particular silicon alkoxides.

[0101] The first portion of the silicon oxide encapsulation layer 50, 55 may be deposited from a plasma comprising a first silicon alkoxide. Preferably, it is deposited from a plasma formed from the first silicon alkoxide, oxygen and helium.

[0102] The second portion of the silicon oxide encapsulation layer 50, 55 may be deposited from a plasma comprising a second silicon alkoxide. Preferably, it is deposited from a plasma formed from the second silicon alkoxide, oxygen and helium.

[0103] The first and second silicon alkoxides are preferably a silicon alkoxide all of whose functions are hydrolyzable. The use of a compound with hydrolyzable functions leads to the formation of a silicon oxide layer. The silicon alkoxide has in particular the formula (RiO)(R2O)Si(OR3)(OR4) with Rb R2, Ri and R4 linear alkyl chains, preferably linear alkyl chains of C1 to C5. Preferably, Rb R2, R3 and ILj are identical.

[0104] The first and second silicon alkoxides are preferably identical. The first and second silicon alkoxides are preferably tetraethyl orthosilicate (TEOS).

[0105] The other layers of the encapsulation element may be deposited by a chemical vapor deposition (CVD) technique, such as low-pressure chemical vapor deposition (LPCVD), or by a physical vapor deposition (PVD) technique, by an atomic layer deposition (ALD) technique, or by PECVD.

[0106] The deposition of the different layers of the encapsulation element is preferably carried out at a temperature less than or equal to 150°C.

[0107] The layers are formed in full plate, that is to say over the entire structure.

[0108] Illustrative and non-limiting examples of embodiments

[0109] In order to highlight the improvement in performance resulting from the presence of the encapsulation layer 50, 55 within an optical device, comparative tests of humidity and / or temperature resistance of different devices or structures were carried out.

[0110] In a first test, the water hermeticity of different layers was tested: SiON layers annealed at 300 °C and 400 °C, a SiO2 layer obtained from TEOS deposited at low deposition rate (TEOS LDR) and having been subjected to annealing at 400 °C, two SiO2 layers of 142 nm and 200 nm, obtained from TEOS deposited by PECVD. In each case, the layer is grown on a silicon substrate. The curves clearly show that the SiON layers are permeable to water while the SiO2 layers are hermetic ([Fig.3]). There is no significant water uptake for the SiO2 layers, even after 600 hours, unlike the SiON layers.

[0111] Mechanical tests were also carried out on SiO2 layers, obtained by deposition of TEOS by PECVD ([Fig.3]), and on SiON layers annealed at 150 °C ([Fig.4]).

[0112] The values ​​were obtained from the Stoney formula which establishes a relationship between the radius of curvature, the stress and the layer and substrate thicknesses.

[0113] As the sealing of the SiO2 layers is total, there is no water absorption and therefore no significant variation in stress after 600 hours, unlike the SiON layer.

[0114] [Fig.5] is a SEM image of a device on which the first layers of the antireflection element have been deposited: a layer of SiN and a layer of SiO2 obtained by deposition of TEOS by PECVD at 150°C. The deposition is compliant: the thickness on the sides and on the electrode varies by less than 10%. Thus, the layer of SiO2 provides uniform protection at all points: not only the top of the structure is protected but also the sides.

[0115] In order to test their resistance to heat and humidity, two devices underwent the highly accelerated stress test (HAST). One of the devices has a SiN / SiON / SiN antireflection element and the other device has a SiN / SiO2 / SiN antireflection element according to a particular embodiment. The HAST test consisted of exposing the devices to a temperature of the order of 110 °C and a relative humidity percentage of 85% for 60 hours and 144 hours in a pressurized chamber at 2 atm. The devices were then subjected to several characterizations. The device having a SiN / SiO2 / SiN antireflection element has better BIP resistance, better definition of the BIP post-lithography step, better dark current and better mechanical stability.

[0116] Finally, the BIP region of a device obtained according to a particular embodiment was observed by MET-EDX. The aluminum metal layer 35 is successively covered by an encapsulation layer 55 of SiO2, as described previously, and by a layer 56 of SiN. The SiO2 layer is clearly visible and conformal ([Fig.6]).

[0117] An advantage of the embodiments and implementation methods described is that they make it possible to improve the temperature and humidity resistance of the image acquisition devices.

[0118] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art.

[0119] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

Claims

1. An optical device successively comprising a support (10) in which vias (12) are formed, a first electrode (32), an active layer (20) capable of absorbing photons and transforming them into electron-hole pairs, a second electrode (34), a conductive layer (35) connecting the second electrode (34) to one of the vias (12), a microlens array (70), the device further comprising an encapsulation layer (50, 55), disposed between the microlens array (70) and the active layer (20), the encapsulation layer (50, 55) comprising a first portion having a first density and a second portion having a second density, the first portion of the encapsulation layer (50, 55) being disposed between the active layer (20) and the second portion of the encapsulation layer (50, 55), the first density being lower than the second density.

2. Device according to claim 1, wherein the encapsulation layer (50, 55) is based on SiO2.

3. Device according to claim 1 or 2, wherein the encapsulation layer (50) is disposed between the active layer (20) and the conductive layer (35).

4. Device according to claim 3, in which the encapsulation layer (50) covers the sides and a portion of an upper face of the active layer (20).

5. Device according to one of claims 3 or 4, in which the encapsulation layer (50) is arranged between and in contact with two metal nitride layers (51, 52).

6. A device according to claim 1 or 2, wherein the encapsulation layer (55) is disposed between the conductive layer (35) and the microlens array (70).

7. Device according to the preceding claim, in which the encapsulation layer (55) is covered by a metal nitride layer (56).

8. A device according to any preceding claim, comprising two encapsulation layers (50, 55), one of the encapsulation layers (50) being disposed between the active layer (20) and the conductive layer (35) and the other of the encapsulation layers (55) being disposed between the conductive layer (35) and the microlens array (70).

9. A device according to any preceding claim, wherein the conductive layer (35) is made of aluminum.

10. A device according to any preceding claim, wherein the first density is between 2.05 g / cm3 and 2.13 g / cm3 and / or the second density is between 2.20 g / cm3 and 2.28 g / cm3.

11. A device according to any preceding claim, wherein the first portion of the encapsulation layer (50, 55) has a thickness of between 50 and 250 nm and / or wherein the second portion of the encapsulation layer (50, 55) has a thickness of between 3 and 50 nm.

12. A method of manufacturing an optical device according to any one of the preceding claims, the method comprising forming an encapsulation layer (50, 55) according to the following steps: - depositing a first precursor, at a first deposition rate, to form a first portion of the encapsulation layer (50, 55), the first portion having a first density, - depositing a second precursor, at a second deposition rate, to form a second portion of the encapsulation layer (50, 55), the second portion having a second density, the first deposition rate being greater than the second deposition rate, whereby the first density is less than the second density.

13. The method of claim 12, wherein the first portion of the encapsulation layer (50, 55) and the second portion of the encapsulation layer are deposited by PECVD at a temperature less than or equal to 150°C.

14. A method according to either of claims 12 and 13, wherein the first precursor and the second precursor are silicon oxide precursors, the first precursor and the second precursor preferably being TEOS.

15. A method according to any one of claims 12 to 14, wherein the first deposition rate is at least 5 times greater than the second deposition rate.

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