Planar process and structure for spectral filters in an optoelectronic device.

0001This invention is in the field of multilayer spectral filters. Specifically, it relates to the creation and structure of such devices within the limits of Silicon semiconductor technology and to the kinds of applications thereby allowed.

0002Generally, the prior art falls into two categories: the multi­layer approach to spectral filtering and the application of spatially miniture filters to imaging and display.

0003Multilayer spectral filtration is now within the domain of elementary textbooks, and is the basis for much capital within the optics community. However in their present form, multi­layer filters consist of macroscopic stacks of films residing on a glass or quartz substrate. Often these films are formed of relatively exotic materials using standard macroscopic thin film processes such as sputtering. Consequently, this technology has made little or no progress in the past years.

0004Recently however, with the increasing interest in compact image input devices, there has arisen a need for an inex­pensive method of producing large arrays of very small ad­jacent filters. Prior art in this area is centered around the neutral density type filter technology. Here a single layer of specialized organic or inorganic material is used to literally absorb the unwanted optical energy and to transmit the wanted light. The drawback to this approach is that it has little compatibility with the scanner/display technology that it is meant to augment. The materials used are not part of standard silicon, germanium or gallium arsenide technology, and the lithography is at best of a crude optical form. It is evident therefore, that these techniques will eventually fail to produce the increasingly fine line widths required by newer devices.

0005A number of patents are directed to neutral density type fil­ter technology. For example, Japanese patent 57-148367 to Fuji discloses a process which uses organic neutral density filters to achieve color imaging.

0006U.S. Patent 4,387,987 to Koike et al discloses an organic neutral density filter in which there is careful size control of the lateral dimensions of the filter.

0007U.S. Patent 4,418,284 to Ogawa et al discloses a process for fabricating an organic density filter, with the bonding of the organic filter to silicon, rather than incorporating the fil­ters as part of the silicon.

0008U.S. Patent 3,868,172 to Li et al discloses a multilayer fil­ter in which the materials used are electrically controllable, and the materials are not compatible with the semiconductor process. The resultant structure is too large to fit on an image, and the total transmission through a given band is too low for realistic imaging.

0009The cited art sets forth various methods of using neutral density approaches and organic dyes to form optoelectronic filters, however, there is no teaching in any of the cited art of utilizing semiconductor techniques to produce a multi­layer interference filter.

0010The invention as claimed solves the problem of how to devise spectral filters in an optoelectronic device. The term 'spectral filter' is thereby to include an array of spatially microscopic spectral filters as well as spectral filters which form a monolithic component of a solid state scanner and display.

0011Specifically, the processes and materials of semiconductor technology are combined to produce microminiature multilayer interference filters. The thickness of layers applied over an imaging area are modulated to create the filters. In a metal oxide semiconductor (MOS) process, a given pixel area consists of a substrate and an overlaying set of layers of silicon oxide, polysilicon and silicon nitride.

0012The invention is described in more detail below with refer­ence to drawings which illustrate only specific embodiments. <ul id="ul0001" list-style="none"><li>FIG. 1 is a diagram of a filter array of an optoelectronic scanner.</li><li>FIG. 2 is a representation of three adjacent pels of the optoelectronic scanner of FIG. 1.</li><li>Fig. 3 is a diagram of a single interface for incident light.</li><li>FIG. 4 is a diagram of a multilayer interface for incident light.</li><li>FIG. 5 is the spectral response calculated for the filters shown in FIG. 1.</li><li>Fig. 6 is a representation of a band reject filter con­structed from layers of standard silicon materials.</li><li>FIG. 7 is a diagram of the spectral response of the structure of FIG. 4.</li></ul>

0013In a semiconductor process alternating layers of materials are either deposited or grown on a bulk substrate according to required patterns and to specified thicknesses. The precision with which this can be accomplished is very great, and as the technology progresses, becoming greater. This technology to create spatially precise stacks of thin films - stacks which can include the films associated with the device structure itself. These multilayer stacks form a di­electric interference filter that can be designed to have almost any reasonable spectral transmission characteristic. As examples of this system, two possible applications are considered.

0014First, to allow a solid state scanner to provide color in­formation, pixel sized filters must be installed over the imaging area. There can be as many as 500,000 of these approximately 15 micron square filters on some scanners, so the technology for producing them must be fairly sophisticat­ed. It is found however, that even within the constraints im­posed by the device multilayers themselves, a small set of thin film thicknesses can be specified which together will provide most conceivable spectral filter profiles. These layers are generally applied in exactly the same manner as standard semiconductor device films are now applied. Specif­ically, if the layers are applied by Molecular beam epitaxy or chemical vapor deposition, then the filter to filter layer differences are easier to handle. Other process methods might require masking the selective back etching but even in that situation the technology is relatively simple.

0015Another possible application for these filters is in display devices which selectively pass white light through some kind of shutter. Examples are liquid crystal or thin film trans­istor (TFT) displays. The multilayer structure of the device itself presents no contraint because the device is likely built upon a transmitting glass substrate hence the filters can be placed on the empty side.

0016FIG. 1 illustrates an array 2 of spectral filters formed by a semiconductor process on a light receiving source, such as a photosensor 4. Alternatively, the photosensor 4 may be replaced by a light source such as an array of light emitting diodes. This is due to the bidirectional light transmission characteristics of the array 2.

0017FIG. 2, three adjacent pels 11 in the filter array 2 of FIG. 1 are illustrated. The pels are comprised of a red sec­tion 6, a green section 8 and a blue section 10. These array sections 11 are formed on a substrate 12 utilizing a semi­conductor planar process technology, which is set forth in detail at pages 64-73 of "Physics of Semiconductor Device" by S. M. Sze; John Wiley &amp; Sons (1981). The described planar process is modified in the sense that layer thicknesses have been changed to provide the proper interference spectra in accordance with the dielectric constants of the various materials used.

0018The substrate 12 may be formed of silicon or glass, with the following description being relative to a silicon sub­strate 12. The planar process set forth above, is utilized to form a silicon oxide (SiO) layer 14 on the substrate 12, with a polysilicon (Si) layer 16 formed on top of layer 14. A SiO layer 18 is then formed on layer 16, with a top layer of silicon nitride 20 being formed on top of layer 18. The red 6, green 8 and blue 10 sections each have different layer thicknesses, as set forth in the table below, to provide the required spectral responses. The layer thick­nesses may vary on the order of ± 3% from the values shown in the table. <tables id="tabl0001" num="0001"><img file="EP0223136A2_D0001.tif" /></tables>

0019The following is a brief description of the steps taken to calculate the transmission efficiency of a multilayer structure such as shown in FIG. 2, considering a normal incidence of light.

0020An algorithm for computing the transmission of a multilayer stack starts with the simple Fresnel coefficients (FIG. 3):<maths id="math0001"><img file="EP0223136A2_D0002.tif" /></maths> Here <img file="EP0223136A2_D0003.tif" />_i = n_i + jk_i and k_i = α_iλ/4π where α_i is the absorbtion coefficient and n_i is the index of refraction of layer i. Now if we label these as the zeroth order coeffficients then the standard etalon equations look like (FIG. 4):<maths id="math0002"><img file="EP0223136A2_D0004.tif" /></maths>

0021Delta is defined as δ_i = 2 π<img file="EP0223136A2_D0005.tif" />_id_i/λ and d_i is the thickness of a given layer i. L and R indicate the light waves travelled to the left and right resepctively. Now treating this etalon as a single effective interface and adding more layers iteratively, the relations become:<maths id="math0003"><img file="EP0223136A2_D0006.tif" /></maths>

0022The final intensity transmission is then: (9) T = (n_exit/n₀) |t|

0023FIG. 5 is a spectral response curve for the different colors of light to be passed by the different sections of the bandpass filters of the array as depicted in the table above. The blue response is illustrated by curve 22, the green response by curve 24 and the red response by curve 26.

0024Band reject filters may likewise be constructed utilizing the planar process techniques described above. One such band reject filter 28 is illustrated in FIG 6. Band reject filters are also known in the art as band suppression filters or notch filters.

0025A substrate 30 is formed of glass or silicon, with the follow­ing description being relative to a glass substrate 30 having a thickness on the order of 2 millimeters. The planar process above is utilized to form alternating layers of silicon nitride and silicon oxide on the glass substrate 30, with the bottom and top layers each being formed of silicon nitride. The bottom layer 32 and layers 34, 36, 38 and 40 are formed of silicon nitride having a layer thickness on the order of 200 microns. The layers 42, 44, 46 and 48 are formed of silicon oxide having a layer thickness on the order of 125 microns. The respective layers may vary on the order of ± 3% from the thickness levels given above. The transmission efficiency is once again calculated aa shown in the equation above.

0026A spectral response curve 50 for the band reject filter 28 is shown in FIG. 7. The curve 52 illustrates the wavelength of light that is blocked by the filter 28.