U.S. patent application number 09/336546 was filed with the patent office on 2002-06-20 for a method for the production of a porous layer.
Invention is credited to ARENS-FISCHER, RUDIGER, BERGER, MICHAEL, HILBRICH, STEFAN, KRUGER, MICHAEL, LANG, WALTER, LUTH, HANS, THEISS, WOLFGANG, THONISSEN, MARKUS.
Application Number | 20020074239 09/336546 |
Document ID | / |
Family ID | 7815415 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020074239 |
Kind Code |
A1 |
BERGER, MICHAEL ; et
al. |
June 20, 2002 |
A METHOD FOR THE PRODUCTION OF A POROUS LAYER
Abstract
In an interference filter having a layer with an area consisting
of a porous material extending from the surface of the layer to the
interior, the dimensions of the porous layer area in a direction
normal to the layer surface have different values to provide for
varying reflection or, respectively, transmission
characteristics.
Inventors: |
BERGER, MICHAEL; (WACHTBERG,
DE) ; KRUGER, MICHAEL; (TUBINGEN, DE) ;
THONISSEN, MARKUS; (NETTETAL, DE) ; ARENS-FISCHER,
RUDIGER; (AACHEN, DE) ; LUTH, HANS; (AACHEN,
DE) ; LANG, WALTER; (VS-VILLINGEN, DE) ;
THEISS, WOLFGANG; (AACHEN, DE) ; HILBRICH,
STEFAN; (HERZOGENRATH, DE) |
Correspondence
Address: |
KLAUS J BACH & ASSOCIATES
4407 TWIN OAKS DRIVE
MURRYSVILLE
PA
15668
|
Family ID: |
7815415 |
Appl. No.: |
09/336546 |
Filed: |
June 19, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09336546 |
Jun 19, 1999 |
|
|
|
PCT/DE97/03006 |
Dec 20, 1997 |
|
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Current U.S.
Class: |
205/667 ;
205/640; 205/655; 205/656; 257/E21.219 |
Current CPC
Class: |
H01L 21/30604 20130101;
C25F 3/12 20130101; G02B 5/285 20130101 |
Class at
Publication: |
205/667 ;
205/640; 205/655; 205/656 |
International
Class: |
B23H 011/00; B23H
003/00; B23H 005/00; B23H 007/00; B23H 009/00; C25F 003/00; C25F
007/00; H01L 021/00; H05K 003/07 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 1996 |
DE |
196 53 097.0 |
Claims
1. An interference filter including at least one layer having a
porous area which extends from the surface of the layer to the
interior thereof, said porous area, having varying thicknesses in
the direction normal to the layer surface.
2. A process of manufacturing a layer with a porous layer area,
wherein said porous layer area is formed by etching, a physical
value is selected which correlates to one of the etching speed and
the porosity of said layer area, and means are employed which form
a gradient during etching.
3. A process for manufacturing a layer according to claim 2,
wherein electrodes provided as means for the electro-chemical
etching are utilized for establishing a field strength gradient
over said layer area.
4. A process according to claim 3, wherein, for the establishment
of the field strength gradient over said layer area, an electrode
disposed opposite the substrate surface area is arranged in a
tilted manner.
5. A process according to claim 3, wherein, for the establishment
of the field strength gradient over said layer area, a net-like
electrode having meshes is selected and the meshes contained in
said net-like electrode become narrower in the direction in which
the gradient is to be established.
6. A process according to claim 2, wherein one of a substrate and
an electrolyte provided for the etching are subjected over said
layer area to a temperature gradient, which is controlled from the
outside.
7. A process according to claim 6, wherein the substrate is heated
by an electric current at locally different rates.
8. A process according to claim 6, wherein a temperature gradient
is established in at least one of the substrate and the electrolyte
by local heating of the substrate.
9. A process according to claim 8, wherein said heating is
performed by one of laser and microwaves heating.
10. A process according to claim 6, wherein a modified substrate
material is selected.
11. A process according to claim 6, wherein said substrate id doped
and the doping of the substrate is selected as the physical
value.
12. A process according claim 1, wherein at least one area which is
structured by photolithography is subjected to the is gradient of
the physical value.
Description
[0001] This is a continuation-in-part application of international
application PCT/DE97/03006 filed Dec. 20, 1997 and claiming the
priority of German application 196 53 097.0 filed Dec. 20,
1996.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a layer with a porous area for use,
for example, in an interference filter. The invention further
relates to a process of manufacturing such a layer.
[0003] Porous silicon (PS) is known which, because of its
compatibility with the highly developed Si-microelectronics and
because it is easy and inexpensive to manufacture, is a promising
material for use as sensors (M. Thust et al., Meas. Sci. Technol.
6, (1995), as well as Y. Duvault-Herkera et al., colloid. Surf.,
50,197, (1990) and, because of its electroluminescence, is suitable
for applications in the area of display technology (P. Steiner et
al., Appl. Phys. Lett., 62(21), 2700, (1993). Furthermore, porous
silicon is known to be used in connection with color-sensitive
photo detectors (M. Kruger et al., EMRS 96, Thin Solid films) or
passive reflection filters.
[0004] The manufacture of layer systems of PS has been demonstrated
(M. G. Berger et al., J. Phys. D: Appl. Phys. 27, 1333, (1994), DE
P 43 19 413.3 or M. G. Berger et al., Thin Solid Films, 255, 313,
(1995)). These layer systems exhibit for example, a color-selective
reflectivity depending on the manufacturing parameters.
Furthermore, the structured manufacturing of PS is known, whereby
areas with different spectral behavior can be made (M. Kruger et
al., EMRS 95, Thin Solid films).
[0005] Specifically, porous silicon consists of a foam-like
skeleton of silicon crystallites, which include pores. The size of
the crystallites and of the pores varies, depending on the doping
and the manufacturing parameters, between some nanometers and some
micrometers. If the wave length of the light is much greater than
the structures in the PS, the PS appears to the light to be a
homogeneous material ("effective medium") and its properties can
therefore be described by an effective refraction index (W. Theiss:
The Use of Effective Medium Theories in Optical Spectroscopy, in
Festkorperprobleme/Advances in Solid State Physics, Volume 33, page
149, Vieweg, Braunschweig/Wiesbaden), which depends on the
refraction indices of the silicon crystallite, an oxide possibly
present on the surface of the crystallite and of the material in
the pores. Consequently, interference filters can be constructed
from various porous layers, which extend parallel to one another
and have different optical properties. The various layers are
constructed parallel to one another and have, within the respective
layer, a constant layer thickness normal to the layer surface. It
is however disadvantageous that for each of the different spectral
characteristics, a separate filter must be made in separate
manufacturing steps.
[0006] It is also disadvantageous that, in the known methods, the
manufacture of adjacent filters with different characteristcs can
be achieved only by photolithographic steps, or respectively, by
separate etching of the filters with particular
characteristics.
[0007] It is therefore the object of the invention to provide a
layer including a porous layer and an interference filter including
such a layer as well as a process of manufacturing such an
interference filter wherein a simplified establishment of
interference filter functions of porous silicon with laterally
gradually variable reflection and transmission characteristics is
achieved.
SUMMARY OF THE INVENTION
[0008] In an interference filter having a layer with an area
consisting of a porous material extending from the surface of the
layer to the interior, the dimensions of the porous layer area in a
direction normal to the layer surface have different values to
provide for varying reflection or, respectively, transmission
characteristics.
[0009] It has been found that, based on the well known manufacture
of layer systems of porous silicon, a method could be provided
wherein a lateral change of the reflection and transmission
capabilities are achieved.
[0010] In particular, a layer system with a well-defined laterally
variable spectral characteristic is manufactured thereby in a
single process step. In this process, a porous layer area is so
formed that the porous layer thickness assumes different values
within this layer. In this way with such a porous area adjustable
characteristics within this single layer can be achieved depending
on desired boundary conditions. It is no longer necessary to
manufacture several components individually which have different
characteristics and which are then combined.
[0011] Furthermore, it may be advantageous to provide within the
porous area several different porosity values in order to provide
for individually desired characteristics in a controllable manner.
Also, the degree of porosity may be established laterally for
example in a continuous way. The area furthermore may have several
partial layer areas with different degrees of porosity.
[0012] For a simplified manufacture, it may be advantageous to form
the porous layer and/or the partial layer areas such that they are
wedge-shaped. Preferably Al, GaAs, or SiGe are be used as the
material, but most advantageous is the silicon which is used in
many ways in microelectronics.
[0013] It has been realized that it is advantageous to establish
during the manufacture of such a layer, upon etching for making the
material porous, a gradient with respect to a physical value which
corresponds to the etching velocity of such an etching procedure.
Alternatively, or additionally, a value can be selected which
corresponds to the porosity of the material. As physical value,
preferably the electric field is employed and, the temperature is
used. The material of the substrate or the doping of the material
may be employed. The values may be employed individually or in
combination together.
[0014] It is advantageous to utilize the electrodes provided for
the electrochemical etching for the formation of the field
gradient. In this case, a first electrode may be arranged in the
electrolyte disposed above the surface to be etched, whereas a
second electrode is disposed on the side of the substrate remote
from this surface. Furthermore, such an electrode may be arranged
in a tilted fashion so as to form a field gradient. Furthermore,
the electrode or electrodes may be in the form of a net and may
include a mesh structure to form gradients wherein the mesh
openings are increasingly narrower in the direction of the
gradient.
[0015] The layer system according to the invention can be made for
example by current flow with a lateral gradual change of the
reflection- or respectively, transmission characteristic utilizing
the temperature dependency of the etching process. As a result of
the temperature dependency the etching rate or, respectively, the
porosity changes when the temperature of the electrolyte/substrate
changes. Consequently, temperature gradients in the electrolyte or
in the substrate can change locally the etching rate and, as a
result, also the optical properties of a filter.
[0016] Alternatively, the layer system according the invention can
be made with a laterally gradual change of the reflection and or,
respectively, transmission characteristics utilizing a changed
anode or, respectively, cathode arrangement. As a result of the
dependency of the etching process on the field strength between the
anode and the cathode, the etching rate or, respectively, porosity
is changed between the electrode. Consequently, field strength
gradients may change the etching rate between the electrodes and
consequently also the optical properties of a filter.
[0017] The invention however is not limited to such processes. In
accordance with the invention, alternative processes are possible
wherein another value, which affects the etching process, is used
to achieve a gradual change of the porosity. For example, the
doping of the substrate material before the etching may be so
selected that there is a lateral gradient in the substrate.
[0018] The invention is explained below in greater detail on the
basis of figures and embodiments.
BRIEF DESCRIPTION OF THE DRAWING
[0019] FIG. 1: A layer arrangement according to the invention,
[0020] FIG. 2: A layer arrangement according to the invention,
[0021] FIG. 3: A layer arrangement according to the invention,
[0022] FIG. 4: A structure according to the invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0023] FIG. 1 shows, in a cross-sectional view, an interference
filter formed on a wafer on which porous silicon was etched.
Because of the different temperatures T.sub.1 and T.sub.2 and the
changing etching rates resulting therefrom a wedge-shaped structure
is formed.
[0024] FIG. 2 also shows, in a cross-sectional view, a filter
according to the invention which however consists of a layer system
with differently porous areas. Because of the different thicknesses
of the individual layers, a laterally gradual change of the
interference characteristics is achieved.
[0025] Finally, FIG. 3 shows, similar to the previous figures, a
laterally gradual layer system with individual pixels made in a
single manufacturing step.
[0026] Particularly, with regard to the invention as described, the
following comments are made:
[0027] Within the frame of the object of the invention to make
laterally inhomogeneous layers or filters, particularly with
laterally varying reflection characteristics, it has been found
that the desired arrangement or formation or, respectively, the
change of the rear contact, influences advantageously the structure
of the subject of the invention, particularly of the filter.
[0028] In the structure according to the invention as shown in FIG.
4, the current flow from the local back-side contact to the porous
layer differs since the charge carriers have paths of different
length from the contact point to the porous layer. This behavior
depends, on one hand, on the size of the porous structure and, on
the other hand, on the distance of the back-side contact from the
porous layer, and consequently, finally on the substrate
resistance. In this manner, the etching rate is different at
various points of the structure, which results in the formation of
interference filters with laterally gradually variable
characteristics.
[0029] In this way, particularly with respect to the change of the
electrical field, a wide application may reside in suitably forming
the geometric pattern of the reflection characteristics of the
structure according to the invention in a lateral direction. For
example, as the contact geometry of the rear contact a plurality of
individual local contacts may be provided at different locations on
the backside. Furthermore, these local contacts may be formed while
being subjected to different currents.
* * * * *