# Patent

## Title
Photodiode for multiple wavelength operation

## Abstract

A photodiode includes a substrate having a first semiconductor type surface region on at least a portion thereof, and a second semiconductor type surface layer formed in a portion of the surface region. A multi-layer anti-reflective coating (ARC) is on the second semiconductor type surface layer, wherein the multi-layer ARC comprises at least two different dielectric layers. A layer resistant to oxide etch is above a peripheral portion the multi-layer ARC. Further layers are above the layer resistant to oxide etch, and thereby above the peripheral portion the multi-layer ARC. A window extends down to the multi-layer ARC. A photodiode region is formed by a pn-junction of the first semiconductor type surface region and the second semiconductor type surface layer.

## Background
PRIORITY CLAIM
The present application is a Divisional of U.S. patent application Ser. No. 11/532,762, filed Sep. 18, 2006, now U.S. Pat. No. 7,485,486, issued Feb. 3, 2009. U.S. patent application Ser. No. 11/532,762 claims priority to Provisional Patent Application No. 60/813,910 entitled “PHOTODIODE FOR MULTIPLE WAVELENGTH OPERATION” filed on Jun. 15, 2006, and is a continuation-in-part (CIP) of U.S. patent application Ser. No. 11/378,843, entitled “PHOTODIODES WITH ANTI-REFLECTION COATING” filed on Mar. 17, 2006, which claims priority to Provisional Patent Application No. 60/663,239 filed on Mar. 18, 2005. The present application claims priority to each of the above applications. Further, each of the above applications is incorporated herein by reference.


FIELD OF THE INVENTION
The invention generally relates to photodiodes, and more particularly to semiconductor photodiodes that have at least one anti-reflective layer, and processes for fabrication to form integrated circuit devices that include photodiodes.
BACKGROUND OF THE INVENTION
The structure and function of semiconductor photodiodes are well known. Photodiodes convert photons into electrical energy. Conventional photodiodes operate in the visible and near-infrared range of the electromagnetic radiation spectrum. The particular semiconductor materials used determine the particular wavelength or wavelength range of the radiation to which the photodiode responds. Photodiodes can be fabricated from elemental semiconductors, such as silicon, as well as compound semiconductors, such as gallium-arsenide.
Photodiodes are typically either vertical P+N or N+P structures. Although a P+N diode is described in this paragraph, by reversing the diffusion types, an N+P diode will be formed. (This is also called a PIN structure, with “I” indicating the intrinsic layer). A conventional P+N photodiode includes a surface P-type anode region to which an anode contact is formed. An antireflective film generally overlies the P-type region to assure a high degree of transmission of radiation at the wavelength that the photodiode is designed to absorb. Beneath the P-type region is a very lightly-doped N-type drift region (also called the intrinsic region or space charge region in the literature) in which photons of the incident radiation are absorbed, generating hole-electron pairs. Adjoining the N-type drift region is a heavily-doped N+ cathode region, to which a cathode contact is formed at a surface of the device. In operation the P+N junction between the P-type anode region and the N-type drift region is reverse biased by an applied potential expanding the depletion layer on both sides of the junction. Because the N-type drift region is relatively lightly doped, the depletion layer is predominantly on the N-type side of the junction extending deeply into the drift region. Holes and elections generated in the depletion layer are swept in opposite directions in response to the applied potential, thus providing a current that is a function of the intensity of the incident radiation.
In many applications photodiodes are formed on integrated circuit die. As a result, when integrating photodiodes on the same semiconductor chip as other circuit elements such as transistors and resistors to perform complex functions in response in part to incident radiation signals, the constraints of the process for making such other elements must be considered in the design of the photodiode. It is desirable to minimize the complexity of a semiconductor fabrication process while maximizing the flexibility available to the designer to provide complex functionality in the device design. The inclusion of a photodiode on an integrated circuit chip made with state-of-the-art CMOS or BiCMOS process technology contributes to the foregoing design considerations.
Photodiodes in certain applications must be efficient, and/or provide substantially the same efficiency, over a relatively wide range of wavelengths, such as blue to red light, even in the face of changing process parameters. FIG. 1 shows the normalized optical response of an exemplary photodiode having a 3.sup.rd order anti-reflective coating (ARC) optimized for blue light, for red (650 nm), near infrared (IR; 780 nm) and blue (405 nm) light as a function of silicon dioxide thickness (in microns) above the photodiode. The oxide film stack over the photodiode comprising the inter-level dielectric (ILD) is a by-product of forming the multi-level metal interconnect. Because the ARC layer is generally optimized for blue light, the optical response at the three different wavelengths (such as used in optical storage products) can be seen to be a strong function of the oxide thickness over the photodiode, especially for red and IR.
Moreover, the thickness of respective oxide films forming the ILD are not well controlled due to thickness variability predominantly due to ILD layer deposition and planarization processing. Oxide (or other dielectric) thickness variability is generally both across a wafer or die, wafer to wafer, and lot-to-lot. Such variability in the thickness of layers disposed on top of the photodiode ARC can result in substantial loss and/or variation in efficiency. What is needed is a new photodiode architecture which provides multiple wavelength operation and a manufacturable process for forming the same.
SUMMARY OF THE INVENTION
A method of a fabricating multiple wavelength adapted photodiodes and resulting photodiodes includes the steps of providing a substrate having a first semiconductor type surface region on at least a portion thereof, implanting and forming a second semiconductor type shallow surface layer into the surface region, and forming a multi-layer anti-reflective coating (ARC) on the shallow surface layer. The forming step includes depositing or forming a thin oxide layer on the shallow surface layer and depositing a second dielectric layer different from the thin oxide layer on the thin oxide layer. An etch stop is formed on the second dielectric, wherein the etch stop includes at least one layer resistant to oxide etch. At least one oxide comprising layer (e.g. ILD) is then deposited on the etch stop. The oxide comprising layer and then the etch stop are removed to expose at least a portion of the ARC to the ambient.
The first semiconductor type can be p-type and the second semiconductor type can be n-type, such as As or Sb. In another embodiment, the first semiconductor type is n-type and the second semiconductor type is p-type.
A thickness of the thin oxide layer is preferably between 1.5 nm and 5 rum, such as between 2.0 to 4.0 nm. The second dielectric layer can comprise silicon nitride.
Following the removal of the etch stop to expose said ARC the etch stop layer can remain exclusively on a periphery of the ARC. The layer resistant to oxide etch can comprise silicon nitride or polysilicon.
A photodiode comprises a substrate having a first semiconductor type surface region on at least a portion thereof, a second semiconductor type shallow surface layer diffused into the surface region. A multi-layer anti-reflective coating (ARC) is disposed on the shallow surface layer, wherein said ARC comprises a thin oxide layer on the shallow surface layer and a second dielectric layer different from the thin oxide layer on the thin oxide layer. At least a portion of the ARC is exposed to the ambient. The photodiode can further comprise a layer resistant to oxide etch disposed on a periphery of the ARC, such as silicon nitride or polysilicon. The thin oxide layer is preferably between 1.5 nm and 5 nm. The second dielectric layer can comprise silicon nitride.



BRIEF DESCRIPTION OF THE DRAWINGS
A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which:
FIG. 1 shows the normalized optical response of an exemplary photodiode having a 3rd order anti-reflective coating (ARC) optimized for blue light, for red, IR and blue light as a function of silicon dioxide thickness (in microns) over the photodiode which illustrates the problem largely solved by the present invention.
FIG. 2 shows a cross section of the resulting photodiode structure for multi-wavelength detection according to the invention following standard CMOS multi-level metal processing including contact etch, metallization, CVD ILD deposition and planarization.
FIG. 3(a) shows a cross sectional schematic of the in-process photodiode after forming the etch stop layer stack on the ARC.
FIG. 3(b) shows a cross sectional schematic of the in-process photodiode after resist patterning on top of the photodiode followed by an etch to remove the etch stop stack and ARC outside the photodiode, and resist removal.
FIG. 4 shows a cross sectional schematic of the in-process photodiode structure following ILD, metal, contact, via and passivation processing.
FIG. 5 shows a cross sectional schematic of the in-process photodiode structure following passivation layer deposition, photoresist patterning to open a window over the photodiode having an area less than the photodiode and the ARC layer pattern disposed over the photodiode.
FIG. 6 shows a cross sectional schematic of the in-process photodiode structure following etch of the passivation and oxide ILD layers, the etch is stopped on/in the etch stop layer with the photodiode.
FIG. 7 shows a cross sectional schematic of the in-process photodiode structure following etch of the upper etch stop layer, such as silicon nitride or polysilicon, followed by silicon dioxide etch, typically a wet etch, followed by resist removal. The ARC layer is seen to be open to the ambient except at its periphery.
FIG. 8 shows the normalized response of a PD according to the invention at three wavelengths with the 3rd order nitride ARC thickness (optimized around the 405 nm operation) as a function of the ARC nitride layer thickness. It is noted that there is no efficiency variation based on a SiO2 layer since there is no SiO2 layer on top of the ARC layer.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
A method of a fabricating multiple wavelength adapted photodiodes and resulting photodiodes includes the steps of providing a substrate having a first semiconductor type surface region on at least a portion thereof, implanting and forming a second semiconductor type shallow surface layer into the surface region, and forming a multi-layer anti-reflective coating (ARC) on the shallow surface layer. The forming step includes depositing or forming a thin oxide layer on the shallow surface layer and depositing a second dielectric layer different from the thin oxide layer on the thin silicon dioxide layer. As used herein, “thin silicon dioxide layer” refers to a silicon dioxide thickness of 1.5 to 20 nm, preferably being 1.5 to 8 mm. With the ARC as described herein, .ltoreq.+/−2% variation in efficiency have been obtained across the photodiode as compared to around 10% without the ARC.
An etch stop is formed on the second dielectric, wherein the etch stop includes at least one layer resistant to oxide etch. At least one oxide comprising layer (e.g. ILD layers) is then deposited on the etch stop. The oxide comprising layer and then the etch stop are removed to expose at least a portion of the ARC to the ambient.
A cross sectional view of a N+P photodiode 200 according to the invention is shown in FIG. 2. A P+ substrate 201 having a p-surface region 202 on at least a portion thereof has a shallow n-type surface layer 203 (labeled N+ diffusion) formed into the surface region 202. The n-type surface layer 203 preferably comprises As or Sb. Metallization 230 is coupled to a P+ contact 221 for low resistance contact to p-surface region 202. N+ diffusion contacts are not shown in FIG. 2.
A multilayer anti-reflective coating (ARC) 210 is disposed on the n-type surface layer 203. The ARC shown preferably comprises silicon nitride as the second dielectric 212 on thin silicon dioxide 211. As shown, the silicon nitride ARC layer 212 extends beyond the N+ diffusion 203 to ensure a good seal structure for the photodiode 200.
The thin silicon dioxide layer (e.g. 1.5 to 8 nm) disposed between the ARC nitride layer 212 (or other second dielectric) and the Si surface of the photodiode is used to reduce tension/stress between the Si and the silicon nitride or other second dielectric. For optical performance, a 1.5 nm to 4 nm thick, such as a 2 nm or 3 nm thick, silicon dioxide layer is preferred. Since the silicon dioxide layer has been found by the Inventors to begin to affect the reflected phase wave for thin oxide thicknesses of about 4 nm to 5 nm or greater, the nitride thickness is preferably thinned slightly to account for the thin silicon dioxide layer, and to restore the optical performance of the device to an optimum or near optimum value. For example, if the silicon dioxide layer is 5 nm thick, the optimum nitride thickness is reduced from 145 nm to about 140 nm for 405 nm operation. The thickness tolerance of the second dielectric layer should be within +/−10 nm of the calculated optimum value as described below, and is preferably within +/−5 nm.
In the case of the second dielectric being silicon nitride, the nitride thickness is preferably found using the following relation (where 2.1 is assumed to be the refractive index of the silicon nitride):

Thickness=n*(1⁄4 of the optical wavelength in nitride), where n=1, 3 or 5 . . . .

For 405 nm optimized operation, using n=1, the preferred nitride thickness is:

(1⁄4*405 nm)/2.1=48 nm, while n=3 results in a thickness of 144 nm.

The ARC second dielectric 212 generally has a refractive index significantly greater than the refractive index of silicon dioxide. Although the second dielectric is generally described herein as being silicon nitride, the invention is not limited to silicon nitride as the second dielectric. For example, the second dielectric can be silicon rich SiO2, silicon rich SiON or silicon rich Si3N4. One known method for depositing such silicon rich layers is using plasma enhanced deposition systems.
A thickness of the second dielectric layer 212 is preferably equal to an odd integer multiplied by 1⁄4 of the optical wavelength in the second dielectric for optimized response. In a preferred embodiment, the refractive index of the second dielectric layer is found by taking the square root of the product of the refractive index of the shallow surface layer and the silicon dioxide layer interlayer dielectric (ILD) generally disposed on the dense dielectric layer (3.75*1.46).sup.1⁄2=2.33 (optimized for blue light; 405 mm).
The thicknesses of thin oxide layer and second dielectric layer 212 comprising the ARC are selected depending on the wavelength of the light to which the photodiode is designed to best respond in its end-use application. The thicknesses are selected to achieve at least 99% transmission of the incoming light through the second dielectric and oxide layers down into the underlying silicon using the formula given above. By way of example, for optimization for a 405 nm (blue) optical wavelength, the ARC preferably comprises a 130 to 150 nm thick silicon nitride layer on top of a silicon dioxide layer having a thickness of less than 6 nm. It has been found that the reflection for 405 nm light for a N+P structure according to the invention when an As surface layer is provided can be reduced to less than 1% (>99% transmission).
Anti-reflection coatings (ARC) are known to increase photodetector responsivity. ARC coatings according to the invention strongly reduce undesired effects from optical interference thus largely eliminating optical interference. As noted above, reflections can be reduced to less than 1%, thus increasing photodiode efficiency.
The remnants of etch stop 215 based on the preferred method according to the invention is disposed on a periphery of photodiode 200. Etch stop 215 comprises a layer resistant to oxide etch 216, such as silicon nitride or polysilicon, disposed on an oxide or other layer 217.
The P-substrate 201 can be a silicon substrate having a resistivity of about 10 to 20 ohm-cm. Alternatively, the p-surface region can be a p-well diffused into an n-substrate, or a p-surface epitaxial film on a P+ substrate as shown in FIG. 2.
The phrase “shallow surface layer”, as used herein, generally refers to a 100 nm to 500 nm deep junction depth. In a preferred embodiment, for N+P photodiodes, the surface layer is As formed using an As implant dosage of 1.times.10.sup.14 to 5.times.10.sup.14/cm.sup.2 at about 10 to 25 keV. Alternatively, other heavy/slow diffusing n-type Si dopants, including antimony (Sb) or even bismuth (Bi), can be used. A second N-type implant (e.g. Phosphorus) of 10-100 times lower dose than the first N-type implant (and enclosing the first implant) may beneficial if contacts are formed directly to the detector cathode.
For P+N photodiodes, the shallow surface layer generally utilizes B, but can also include other Group III elements, such as In. For B, the implant energy is generally 3-10 keV, or if the molecular ion BF2 is used, the energy is generally 30 to 70 keV. For In, the energy is generally 10 keV to 35 keV. Doses are similar to As doses described above. The P+ region can be surrounded by a P− (lighter implant, e.g. B) similar to the N+P diode construction described in the paragraph above.
The process flow including ARCs according to the invention is compatible with most standard CMOS OR BiMOS processes. Only minor process changes to standard processes are generally needed to implement both the ARC and the etch stop.
Although various conventional semiconductor device fabrication techniques can be used to make either N+P or P+N photodiodes according to the invention, an exemplary process sequence for forming N+P photodiodes is described below. The exemplary process described below starts with the step of the shallow As photodiode (PD) implant, which is preferably before the source/drain implant in the process sequence to eliminate adverse effects of high temperature associated with the ARC/etch stop layer processing on CMOS devices.
FIG. 3(a) shows a cross sectional schematic of the photodiode after forming of the etch stop layer stack 215 on the ARC 210. On top of the ARC is a SiO2 layer 215 (e.g. 300 A to 2,000 A) and a non-oxide stop layer 216 (e.g. Si3N4.about.1,500 A to 2,500 A or a polysilicon layer .about.1,500 A to 2,000 A) on the SiO2 layer. Following resist patterning on top of the photodiode followed by an etch to remove the etch stop and ARC outside the photodiode, and resist removal, the cross sectional schematic of the photodiode shown in FIG. 3(b) results.
The etch stop layer etch must generally also clear the etch stop material from the CMOS gate topography (not shown). In case of the Si etch stop a variety of plasma etches stopping on the underlying oxide can be used. In case of a Si3N4 etch stop layer, plasma or wet etches can be employed, the latter generally including an oxide hard mask material (.about.30 nm or thicker) deposited over the Si3N4 etch stop layer. The oxide layer between the ARC and the etch stop layer is preferably removed using wet chemistry prior to the photo resist mask removal.
The ARC Si3N4 layer is preferably removed using wet chemistry due to difficulties with stopping the nitride plasma etch with the large overetch generally required to clear the CMOS topography on a very thin underlying oxide. In this case the Si3N4 etch stop layer is generally protected by the overlying oxide layer.
FIG. 4 shows a cross sectional schematic of the photodiode structure following ILD, metal, contact, via and passivation processing. ILD layers are shown as 401-405. Passivation layer is shown as reference 406. Arrows point to interfaces between different dielectric layers which adversely affect photodiode performance for photodiodes before the present invention. Following passivation layer deposition, such as an oxide layer capped with a nitride layer, photoresist 510 is patterned to open a window over the photodiode having an area less than the photodiode resulting in the structure shown in FIG. 5. Following etch of the passivation and oxide ILD layers, the etch is stopped on/in the etch stop layer 216 with the photodiode resulting in the structure shown in FIG. 6. FIG. 7 shows the photodiode resulting structure following etch of the upper etch stop layer, such as silicon nitride or polysilicon, followed by silicon dioxide etch, typically a wet etch, followed by resist removal. The ARC layer is seen to be open to the ambient except at its periphery. In FIG. 7, layers 401-406 are examples of layers that can be collectively referred to as further layers 407 above the layer resistant to oxide etch 216. As can be seen in FIG. 7, a window extends through the further layers 407 and through the layer resistant to oxide etch 216 down to the multi-layer ARC 210 such that an opening through the further layers 407 and through the layer resistant to oxide etch 216 is created by the window with the further layers 407 and the layer resistant to oxide etch 216 surrounding the opening.
A plurality of PMOS and NMOS transistors and photodiodes according to the invention can be elements of an integrated circuit that is useful in various different applications in which light signals are used as inputs to electronic circuitry within a larger apparatus or system. Photodiodes can be structured to respond to blue light, other desired wavelengths, or a broad range of wavelengths.
For example, photodiodes according to the invention integrated monolithically with CMOS circuits with a high sensitivity are needed for optical storage systems, such as digital versatile disk (DVD) or digital video recorders (DVR). Photodiodes according to the invention are also well suited for photodetectors for optical storage applications.
EXAMPLES
It should be understood that the Examples described below are provided for illustrative purposes only and do not in any way define the scope of the invention.
A simulation was performed to access photodiode response as a function of ARC nitride thickness. The ARC thickness was 3.sup.rd order optimized for blue light (405 nm) being about 0.144 um. Overlaying oxide (ILD) and passivation were removed as described above.
FIG. 8 shows the normalized response of the PD with the 3rd order ARC (for 405 nm) thickness as a function of the ARC layer thickness (in tens of microns). The rate of change in responsivity can be seen to be low as a function of nitride thickness as compared to the change in responsivity as a function of oxide thickness as shown in FIG. 1. The nitride thickness can be changed to optimize a particular color, or obtain a desired ratio in color responsivitites.
It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

## Claims

1. A photodiode, comprising:
a substrate having a first semiconductor type surface region on at least a portion thereof;
a second semiconductor type surface layer formed in a portion of said surface region, wherein a photodiode region is formed by a pn-junction of said first semiconductor type surface region and said second semiconductor type surface layer;
a multi-layer anti-reflective coating (ARC) on said second semiconductor surface layer, wherein said multi-layer ARC comprises an oxide layer on said second semiconductor type surface layer and a second dielectric layer different from said oxide layer on said oxide layer;
an etch stop layer stack on a first portion of said multi-layer ARC, said etch stop layer stack including a layer resistant to oxide etch on an oxide or other layer;
at least one dielectric layer on said etch stop layer stack; and
a window over at least a portion of said photodiode region, said window extending through said etch stop layer stack down to said multi-layer ARC such that an opening through said etch stop layer stack is created by said window with said etch stop layer stack surrounding said opening.

2. The photodiode of claim 1, wherein:
said window exposes a second portion of said multi-layer ARC to ambient; and
said first portion of said multi-layer ARC, on which is said etch stop layer stack, is not exposed to ambient.

3. The photodiode of claim 1, wherein:
said window allows at least a portion of light incident on an exposed portion of said multi-layer ARC to be received by at least a portion of said photodiode region.

4. The photodiode of claim 1, wherein:
said window allows at least a portion of light incident on a portion of said multi-layer ARC, which is not covered by said etch stop layer stack, to be received by at least a portion of said photodiode region.

5. The photodiode of claim 1, wherein said first portion of said multi-layer ARC, on which is said etch stop layer stack, comprises a peripheral portion of said multi-layer ARC.

6. The photodiode of claim 1, wherein said etch stop layer stack comprises silicon nitride or polysilicon, and overlies and extends beyond a peripheral portion of the second semiconductor layer.

7. The photodiode of claim 1, wherein a thickness of said oxide layer, of said multi-layer ARC, is between 1.5 nm and 5 nm.

8. The photodiode of claim 1, wherein said second dielectric layer, of said multi-layer ARC, extends beyond said second semiconductor type surface layer.

9. The photodiode of claim 1, wherein:
said first semiconductor type is one of p-type and n-type; and
said second semiconductor type is the other one of p-type and n-type.

10. A photodiode, comprising:
a substrate having a first semiconductor type surface region on at least a portion thereof;
a second semiconductor type surface layer formed in a portion of said surface region;
a multi-layer anti-reflective coating (ARC) on said second semiconductor type surface layer, wherein said multi-layer ARC comprises at least two different dielectric layers;
a layer resistant to oxide etch above a peripheral portion said multi-layer ARC;
further layers above said layer resistant to oxide etch, and thereby above said peripheral portion said multi-layer ARC; and
a window extending through said further layers and through said layer resistant to oxide etch down to said multi-layer ARC such that an opening through said further layers and through said layer resistant to oxide etch is created by said window with said further layers and said layer resistant to oxide etch surrounding said opening.

11. The photodiode of claim 10, wherein:
said window exposes a portion of said multi-layer ARC to ambient; and
said peripheral portion of said multi-layer ARC is covered by at least said layer resistant to oxide etch, and thus, is not exposed to ambient.

12. The photodiode of claim 10, wherein:
said window allows light to be incident on an exposed portion of said multi-layer ARC.

13. The photodiode of claim 10, wherein:
said window allows light to be incident on a portion of said multi-layer ARC that is not covered by said layer resistant to oxide etch.

14. The photodiode of claim 10, wherein a photodiode region is formed by a pn-junction of said first semiconductor type surface region and said second semiconductor type surface layer.

15. A photodiode, comprising:
a substrate having a first semiconductor type surface region on at least a portion thereof;
a second semiconductor type surface layer formed in a portion of said surface region;
a multi-layer anti-reflective coating (ARC) on said second semiconductor type surface layer, wherein said multi-layer ARC comprises at least two different dielectric layers;
a layer resistant to oxide etch on a first portion said multi-layer ARC; and
further layers above said layer resistant to oxide etch, and thereby above said first portion of said multi-layer ARC;
wherein a second portion of said multi-layer ARC is not covered by any other layers;
wherein said first portion of said multi-layer ARC surrounds said second portion of said multi-layer ARC;
wherein said second portion of said multi-layer ARC, that is not covered by any other layers, is exposed to ambient; and
wherein said first portion of said multi-layer ARC is covered by at least said layer resistant to oxide etch, and thus, is not exposed to ambient.

16. The photodiode of claim 15, wherein a photodiode region is formed by a pn-junction of said first semiconductor type surface region and said second semiconductor type surface layer.

17. The photodiode of claim 16, wherein at least a portion of light incident on said second portion of said multi-layer ARC can be received by at least a portion of said photodiode region.

18. The photodiode of claim 15, wherein said at least two dielectric layers of said multi-layer ARC comprise an oxide layer, and a silicon nitride layer on said oxide layer.