Thin film obscurants for microelectronics.

Abstract

A method and apparatus for an assembly having a first wafer including a layer having bulk material and microelectronics, and a wafer having a deposited thin film bonded to the first wafer such that the reflective film is embedded within the assembled assembly. The reflector wafer can include a handle wafer and a thin film having reflective properties that prevent imaging of the microelectronics by light passing through the bulk material.

Description

[Background technology]

[0001] As is known in the art, electronic circuits can be formed on substrates such as silicon substrates. Silicon is transparent in the short-wave infrared (SWIR) spectrum starting at about 900 nm. Short-wave infrared energy can be used to image microelectronic circuits through the backside of the silicon substrate on which the electronic devices are fabricated. SWIR imaging can be for inspection, failure analysis of fabricated microelectronic circuits, etc. However, SWIR imaging can also be used for unauthorized imaging of integrated circuits, which may enable reverse engineering of electronic devices. Summary of the Invention

[0002] Exemplary embodiments of the present disclosure may provide methods and apparatus for microelectronic circuit assembly having a film, such as a multilayer material, on a substrate, such as crystalline silicon, to reflect incident radiation. In embodiments, the film includes layers with contrasting optical properties due to constructive / destructive interference from the surfaces of alternating material layers of contrasting refractive index. Such multilayer reflectors are known as Bragg reflectors. It should be noted that the goal is to reduce the transmission of light; reflected light is not necessarily transmitted. In some embodiments, other thin films capable of attenuating transmission, such as light-absorbing films, total internal reflection coatings, or other reflective films, may also be used. Thin films may be applied by various techniques known in the art, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD), including techniques such as sputtering, cathodic arc, and atomic layer deposition (ALD).

[0003] In an embodiment, the SWIR thin-film reflector is configured to block light from about 900 nm to about 1600 nm, which may correspond to the wavelengths of commonly available lasers. The film can be deposited on a blank silicon substrate wafer, for example. Direct bonding of the wafer to a similar silicon wafer to produce an engineered wafer with a buried reflective layer (see, e.g., SH Christiansen, R. Singh, and U. Gosele, "Wafer Direct Bonding: From Advanced Substrate Engineering to Future Applications in Micro / Nanoelectronics," in Proceedings of the IEEE, vol. 94, no. 12, pp. 2060-2106, December 2006, doi:10.1109 / JPROC.2006.886026) is also possible. The engineered wafer can reflect and scatter SWIR radiation. In an embodiment, microelectronics are fabricated at the wafer level. Thinning these wafers and bonding them to a thin film would prevent inspection via SWIR from the backside of the assembly. The film obscures any passive visual inspection of the microelectronic device from the backside.

[0004] In one aspect, the assembly comprises a first wafer including a layer having bulk material and microelectronics, and a reflector wafer bonded to the first wafer, the reflector wafer comprising a handle wafer and a thin film having reflective properties that prevent imaging of the microelectronics by light passing through the bulk material.

[0005] The assembly may further include one or more of the following features: the first wafer includes a readout integrated circuit, the first wafer includes an oxide layer, the assembly includes, in order, a handle wafer, a thin film, a bulk material, and a microelectronic layer, includes an oxide layer between the thin film and the bulk material, the light includes light in the infrared spectrum, the bulk material includes silicon, the thin film includes layers of first and second materials, the thin film includes layers of polysilicon and silicon oxide, the thin film includes a layer of aluminum, the thin film includes titanium oxide, the thin film includes a Bragg reflector, the first wafer includes sapphire and / or SiC, and / or the thin film is deposited.

[0006] In another aspect, a method includes forming microelectronics in a layer of a first wafer including a bulk material; bonding a reflector wafer to the first wafer, the reflector wafer including a handle wafer and a thin film having reflective properties to prevent imaging of the microelectronics by light passing through the bulk material.

[0007] The method may further include one or more of the following features: the first wafer includes a readout integrated circuit, the first wafer includes an oxide layer, the assembly includes, in order, a handle wafer, a thin film, a bulk material, and a microelectronic layer, includes an oxide layer between the thin film and the bulk material, the light includes light in the infrared spectrum, the bulk material includes silicon, the thin film includes layers of first and second materials, the thin film includes layers of polysilicon and silicon oxide, the thin film includes a layer of aluminum, the thin film includes titanium oxide, the thin film includes a Bragg reflector, the first wafer includes sapphire and / or SiC, and / or the thin film is deposited.

[0008] The foregoing features of the present disclosure, as well as the disclosure itself, can be more fully understood from the following description of the drawings. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of imaging the backside of a circuit of a substrate blocked by an opaque layer, according to an exemplary embodiment of the present disclosure. [Figure 2] FIG. 1 illustrates unauthorized backside imaging of a circuit through a substrate according to the prior art. [Figure 3] 1 is a cross-sectional view of an exemplary film stack for an anti-imaging layer. [Figure 4] 1 is a cross-sectional view illustrating constructive and destructive interference for a thin film. FIG. [Figure 5] 1 is a graph illustrating reflectance versus wavelength for an exemplary opaque layer. [Figure 6] FIG. 1 is a flow diagram illustrating an exemplary step sequence for providing an assembly with an opaque layer to prevent imaging of the backside of the circuitry. [Figure 7] 1 is a tabular representation of an exemplary thin film stack for an opaque layer. [Figure 8] 1 is a tabular representation of an exemplary thin film stack for an opaque layer. [Figure 9] 1 is a tabular representation of an exemplary thin film stack for an opaque layer. [Figure 10] 1 is a tabular representation of an exemplary thin film stack for an opaque layer. [Figure 11] 1 is a schematic diagram of an opaque layer that reflects IR light to prevent imaging of the backside of the circuit. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1 shows an exemplary assembly 100 having microelectronics 102 formed on a substrate 104 with an opaque layer 106 that blocks energy 108 from reaching the microelectronics 102 to prevent imaging of the circuitry. In an embodiment, the substrate 104 comprises silicon, through which SWIR is transparent. The opaque layer 106 prevents imaging of the circuitry 102 by non-destructive observation from the backside of the assembly 100.

[0011] 2 illustrates unauthorized inspection of a microelectronic circuit 200 on a substrate 202 by direct imaging according to the prior art. Backside infrared (IR) energy 204 passing through the substrate 302 can be used to generate an image 206 of the circuit 200.

[0012] 3 shows an exemplary opaque layer 300 that blocks energy from a selected bandwidth, such as IR. In the illustrated embodiment, a series of high reflectivity layers are provided by a first material 302 and a series of low reflectivity layers are provided by a second material 304, which couple and block energy from reaching a substrate 306, which may include silicon. The layers of the first and second materials 302, 304 are alternated to provide desired reflective properties.

[0013] The first and second materials 302, 304 may include any suitable materials combined to provide reflective properties that meet the needs of a particular application, for example, the reflective properties may include blocking energy within a particular bandwidth that may correspond to a particular type of laser.

[0014] Additionally, the thicknesses of the first and second materials 302, 304 can be selected to achieve the desired constructive and destructive interference characteristics. The thicknesses of the first and second materials can be the same or different. Furthermore, any practical number of different materials can be varied to achieve the desired reflectivity.

[0015] Any suitable technique, such as sputtering, can be used to form the film on the substrate. Furthermore, the substrate can comprise any suitable material, such as silicon, sapphire, carbide, etc.

[0016] 4 shows directed light 400, such as a laser beam, at an angle of incidence α on a surface 402 of a film / layer 404 having a thickness d and a refractive index n. A portion of the light is reflected from the surface 402, and a portion of the light enters the film at a refraction angle β. The bottom surface 406 of the film / layer may reflect light back to the surface 402. As is well understood by those skilled in the art, the reflected light waves can combine constructively or destructively. The difference in the light path through the film layer results in: Δ=n(AB+BC)-AD=2ndcosβ where points A, B, C, and D define distances AB, BC, and AD.

[0017] For a phase change = π, the destructive phase change can be defined as 2dn cos β = mλ, where m is an integer and λ is the wavelength, and the constructive phase change can be defined as 2dn cos β = (m-1 / 2)λ. For a phase change of 0, the constructive phase change is 2dn cos β = mλ and the destructive phase change is 2dn cos β = (m-1 / 2)λ. It is understood that reflected light experiences a 180 degree phase change when it reflects from a medium of higher refractive index and no phase change when it reflects from a medium of lower refractive index.

[0018] Commercially available software, such as that sold by a company called ESSENTIAL MACLEOD SOFTWARE, can be used to design thin films with different reflective properties based on the refractive indices of the various material layers.

[0019] 5 shows the reflectance characteristics 500 of an exemplary opaque film for blocking specific optical frequencies. As can be seen, the film is transparent in the visible light spectrum, i.e., has low reflectance over specific bandwidths 502, 504, 506. At common laser frequencies shown at 1060 nm, 1310 nm, and 1550 nm, the film approaches 100% reflectance. Since laser light or any wavelength is in the SWIR band, the circuit cannot be imaged by the laser in the bandwidth that is reflected.

[0020] 6 shows an exemplary sequence of steps for fabricating a wafer having a blocking layer. In step 600, an oxide layer 650 is formed or deposited on a wafer 652 having integrated circuits 654, such as readout integrated circuits (ROICs), formed in the wafer's active layer. In step 602, a handle wafer 656 is bonded to the ROIC wafer 652 to form an assembly 658. In step 604, the assembly 658 is flipped, and the ROIC wafer 652 is thinned, polished, and edge-ground. In a particular embodiment, the ROIC wafer 652 is thinned to a thickness on the order of 100 micrometers.

[0021] In step 606, an oxide layer 660 is formed on a reflector wafer 662, for example, with a silicon wafer 664, and a thin-film reflector layer 666 is bonded. The reflector wafer 662 is bonded to an ROIC assembly 658. In step 608, after flipping the assembly, the reflector wafer 662 is thinned to a desired thickness and the handle wafer 656 is removed. In step 610, the top and bottom surfaces of the assembly can be polished. In step 612, a photopattern 670 can be placed on the assembly, and in step 614, the silicon can be etched to reveal connection pads. The assembly is then ready for, for example, ball grid array processing.

[0022] It is understood that the location / depth of the reflector and circuitry can be selected to suit the needs of a particular application. While example dimensions may be used herein, including in the drawings, it is understood that any practical dimensions may be used.

[0023] 7 shows an exemplary thin film stack that can provide the desired reflective properties. As can be seen, alternating layers of HfO2, SiO2, and SiPoly, each with a respective refractive index and selected thickness, can be used to form a blocking film that prevents imaging of the circuit layer.

[0024] FIG. 8 shows an exemplary film stack including layers of aluminum and silicon oxide.

[0025] Figure 9 shows an exemplary film stack including layers of SiO, SiPoly and SiO. Figure 10 shows an exemplary titanium oxide-based stack.

[0026] As seen in FIG. 11, a reflector film 1100 of a reflector wafer 1102 can reflect light to block imaging of the backside of an integrated circuit 1106, such as a circuit 1104 in an ROIC.

[0027] In some embodiments, a scattering layer may also be used.

[0028] As used herein, a thin film has at least 50% reflectivity to prevent imaging of microelectronics by light passing through the bulk material. A 50% reduction in reflectivity is a 50% reduction in contrast (MTF=0.5) and a 3× reduction in super-resolution due to increased transmission and scattering from the SWIR obscurant compared to no obscurant.

[0029] It is understood that the reflectivity of the thin film can be greater than 50%. For example, in some embodiments, the reflectivity of the thin film can be greater than 90%.

[0030] Having described exemplary embodiments of the present disclosure, it will now become apparent to those skilled in the art that other embodiments incorporating these concepts may be used. The embodiments contained herein should not be limited to the disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0031] Elements of different embodiments described herein can be combined to form other embodiments not specifically described above. Various elements that are described in the context of a single embodiment may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.

Claims

1. 1. An assembly comprising: a first wafer including a bulk material and a layer having microelectronics; an oxide layer, and a reflector wafer bonded to the first wafer, the reflector wafer including a thin film having reflective properties that prevent imaging of the microelectronics by light passing through the bulk material; and the assembly comprising, in order, the thin film, the oxide layer, the bulk material, and the microelectronic layer; the thin film having a first layer of hafnium dioxide on a side of the thin film opposite the oxide layer; The assembly wherein the thin film further comprises at least one layer of polysilicon and at least one layer of silicon dioxide.

2. The assembly of claim 1 , wherein the first wafer comprises a readout integrated circuit.

3. The assembly of claim 1 , wherein the first wafer further comprises a second oxide layer.

4. The assembly of claim 1 , wherein the light comprises light in the infrared spectrum.

5. The assembly of claim 1 , wherein the bulk material comprises silicon.

6. The assembly of claim 1 , wherein the thin film comprises a Bragg reflector.

7. The assembly of claim 1 , wherein the first wafer comprises at least one of sapphire or SiC.

8. The assembly of claim 1 , wherein the thin film is deposited.

9. 1. A method comprising: forming a microelectronic layer on a layer of a first wafer comprising bulk material; forming a first oxide layer over the microelectronic layer; bonding a reflector wafer to the first wafer, the reflector wafer having a thin film having reflective properties that prevent imaging of the microelectronics by light passing through the bulk material, and a second oxide layer disposed on the thin film; and the assembly having, in order, the layers of the thin film, the second oxide layer, the bulk material, the microelectronics, and the first oxide layer; the thin film having a first layer of hafnium dioxide on a side of the thin film opposite the second oxide layer; The method wherein the thin film further comprises at least one layer of polysilicon and at least one layer of silicon dioxide.

10. The method of claim 9 , wherein the first wafer comprises a readout integrated circuit.

11. The method of claim 9 , wherein the light comprises light in the infrared spectrum.

12. 10. The assembly of claim 1, wherein the thin film further comprises a layer of silicon oxide disposed between the at least one layer of polysilicon and the at least one layer of silicon dioxide.

13. The method of claim 9 , wherein the bulk material comprises silicon.

14. The method of claim 9 , wherein the thin film comprises a Bragg reflector.

15. 10. The assembly of claim 1, further comprising a second layer of hafnium dioxide disposed between one of said at least one layer of polysilicon and one of said at least one layer of silicon dioxide.

16. The assembly of claim 5 , wherein the silicon comprises crystalline silicon.

17. The method of claim 13 , wherein the silicon comprises crystalline silicon.

18. 10. The method of claim 9, further comprising forming the thin film using physical vapor deposition (PVD), chemical vapor deposition (CVD), sputtering, or atomic layer deposition (ALD).

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