GB1594800A - Etching method for forming microstructures - Google Patents

Description

(54) ETCHING METHOD FOR FORMING MICROSTRUCTURES (71) We, N. V. PHILIPS' GLOEILAMPENFAB RIEKEN, a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of forming microstructures of different depths in a solid body, a surface to be structured being covered by a mask which leaves the areas to be removed exposed, and also to a product formed by such a method.

In order to form microstructures by material-removing etching methods, the surface to be structured is covered by a mask which is impermeable to the etching medium. The etching process is generally isotropic and continues underneath the etching mask. The degree of under-cutting determines how far etching can be continued, because the etching depths are usually not much larger than the natural distances between adjacent lines of the mask. The width of the under-cutting is approximately equal to the etching depth.

The object of the invention has is to improve the material-removing etching method, utilizing the mask etching technique, in respect of the mask conformity or image definition as well as in respect of the feasible profile depth, so that under-cutting is reduced and the etched edge profiles may be steeper.

The invention provides a method of forming microstructures of different depths in a solid body, in which method a surface to be structured is masked leaving areas to be etched exposed, characterised in that the surface to be structured is irradiated with a beam of energy-rich ions so as to form latent nuclear tracks in the areas to be etched, an apertured mask is arranged on the surface to be structured before irradiation, and after irradiation the surface bearing the apertured mask is chemically etched with an etching medium so as to form the said microstructures, the mask material being impermeable to the etching medium. Preferably, the irradiation dose is such that a resultant mean distance between the latent nuclear tracks is approximately equal to the diameter of the etched individual nuclear track.

Furthermore, the surface to structured is preferably irradiated in a direction which extends perpendicularly to the surface to be irradiated or which encloses an acute angle therewith.

Irradiation of the solid body may be effected by guiding an ion beam across separate areas of the surface to be structured by means of a device which modulates the intensity and the stay of the focussed beam over the individual areas of the surface to be structured.

The method according to the invention offers the following advantages: there is a substantial -improvement in respect of the edge sharpness and the mask conformity of the microstructures produced with the mask pattern, as well as in respect of a larger maximum structuring depth in the microstructures formed by a method according to the invention. A further advantage consists in the possibility of selecting the preferred etching direction by the presetting of the radiation direction, which also enables, for example, recesses to be formed which are obliquely inwardly directed. Because the degree of sensitizing can be selected by selection of the kind of ions or the total radiation dose or ion energy, the maximum depth of the sensitized area can be determined.

The method in accordance with the invention is particularly attractive for use in the manufacture of luminescent screens, the luminescent material being provided in elongate recesses in a substrate, which substrate is transparent to the light emitted by the luminescent material, the substrate having been provided with elongate recesses which extend transversely of the surface by a method according to the invention.

An embodiment of a method in accordance with the invention will be described hereinafter with reference to the accompanying diagrammatic drawings, in which: Figure 1 diagrammatically shows the etching procedure in the case of a mask etching process involving ion sensitizing by radiation, and Figures 2a, 2b and 2c show successive steps in performing a method according to the invention.

In a method according to the invention, a surface of the material to be structured is irradiated by energy-rich ions. Each individual ion produces a latent, i.e. initially invisible track in the form of a channel having a diameter of approximately 0.01 ssm. During etching, the etching medium penetrates into the channel thus formed at a rate which is substantially higher than that in the unirradiated material, and starts to dissolve the material from the inside. When the radiation density is sufficient, the increased surface are then appearing causes a substantially greater microscopic etching rate than that of unirradiated material.

Various possibilites exist for utilizing the increased etching effect in the direction of the ion traces in order to improve the conformity of the microstructures produced with the mask pattern.

1. Provision of a protective etching mask on the substrate to be structured in the form of a thin metal film which is impermeable both to ion beams and to the etching medium to be used, before irradiation by ions.

2. Provision before exposure to the ion beam of a radiation shielding mask on the substrate to be structured in the form of a sufficiently thick metal layer which is impermeable to the ion beams to be used, already prior to the exposure.

3. Local, selective irradiation of the substrate by means of a finely-focussed ion beam which is guided across the surface to be structured and which is at the same time modulated as regards intensity and duration. Modulation can be realized by an electronic or electro-magnetic device.

Thus, an increased macroscopic etching rate in the direction of the ion paths, i.e an anisotropic etching rate increase in the direction of the ion traces, is obtained by the use of ions.

Figure 1 shows a relevant etching procedure. In a surface 1 of a disc 2 of material to be structured, particle traces 3 which extend perpendicularly to the surface 1 are shown in Figure 1. Surface areas not be etched are masked by an apertured mask 4. The etching medium acts on the surface areas which are not covered by the mask 4, the rate of etching being graphically represented by the etching lines 6 as a function of the etching time t. The undercutting width is denoted by the letter B and the etching depth by the letter T. It has been found that the higher the etching rate is in the direction of the particle traces 3 with respect to the non-disturbed etching rate, the steeper the covered etching sides will be, which means the larger the ratio of T to B.In the exposed area of the material 2, intersecting etching cones are formed in an irradiated area I by superposition of all nuclear tracks 3 simultaneously exposed to the etching medium. As long as the mean lateral distance between the latent nuclear traces 3 is sufficiently large compared with their diameter, the rate of under-cutting remains practically the same, because these nuclear tracks are time sequentially exposed to the etching medium. A limitation of this kind does not occur in an unirradiated area II.

Figures 2a and 2b diagrammatically show a specimen pre-treatment. On a specimen 10, a glass cover mask 11 is provided, after which a chromium/gold mask 12 is vapour deposited (Figure 2a); subsequently, the mask 11 is removed. Then four further masks 13 are provided (in the manner shown) leaving an area 14 exposed (Figure.

2b) and the irradiation of the specimen 10, now area-wise protected against heavy ions, is then performed.

The ion beam is stopped to form a very thin beam by a first adjustable slit aperture which measures current. A radiation shield serves as an exposure shutter in combination with a further adjustable slit aperture. Behind this adjustable slit aperture, the beam is widened by a magnetic, double four-pole lens and is incident on the surface to be irradiated. A dispersion aperture serves to suppress parasitic particles of other energies of parasitic particles having a different angle of incidence. For the beam diagnosis, use can be made of a profile grid, an illumination- target or an image intensifier plate in accordance with the radiation intensity.

Subsequently, the masks 13 are removed and the chromium/gold mask 12 and the areas of the specimen 10 which were covered by the mask 11 are exposed to the etching medium, so that a microstructure 15 is produced. Recordings of the etching regions, for example, made by means of a scanning electron microscope, reveal that the etching method in accordance with the invention offers a substantial improvement over prior art methods and that the inclination angle of the sides of the nonexposed regions is substantially larger than the inclination of the sides of the exposed regions.

The method in accordance with the invention thus utilizes a beam of energy-rich heavy ions for sensitizing the subsequent wet-chemical etching, sensitization being anisotropic in the direction of the ion tracks, which means that the etching rate in this direction is increased in respect of the etching rate in the nontreated material. As a result of using an etching mask which shields the areas of the body which have not been irradiated the ratio of the etching depth to the undercutting width is increased (the rate of under-cutting does not change with respect to the non-irradiated material).

During etching, an acute angle side edge structure is obtained on the upper side as well as on the lower side of the non-affected intermediate piece as a result of the slowly widening etched nuclear track holes merging. The ion dose can be adapted to the required side edge definition, the depth of the layer to be removed in an accelerated manner being determined by the energy of the irradiating heavy ions. The method can be used for all electrically insulating materials such as crystalls, glasses, organic materials, semiconductors and the like which have a sufficiently high specific resistance (p > 2000Qcm).

A preferred etching method in accordance with the invention is used to provide a substrate of a luminescent screen with recesses, for example, having a depth of approximately 5 ym, a diameter of approximately 1 ,um, and a spacing of approximately 2 ssm. The luminescent material is depdsited in these recesses. As a result, a window having a comparatively high resolution can be realized. In order to counteract lateral dispersion of light, the recesses are preferably each provided with a reflective layer on the lateral wall.By a suitable choice of the refractive indices of the window material, for example, glass having a very low refractive index, such as an optical fibre cladding glass, and the luminescent material, for example, CsI, it can be achieved that the recesses act as conductors for the light, emitted by the luminescent material.

WHAT WE CLAIM IS: 1. A method of forming microstructures of different depths in a solid body, in which method a surface to be structured is masked leaving areas to be etched exposed, characterized in that the surface to be structured is irradiated with a beam of energy-rich ions so as to form latent nuclear tracks in the areas to be etched, an apertured mask is arranged on the surface to be structured before irradiation, and after irradiation the surface bearing the apertured mask is chemically etched with an etching medium so as to form the said microstructures, the mask material being impermeable to the etching medium.

2. A method as claimed in Claim 1, characterised in that such a radiation dose is used that the mean distance between the latent nuclear tracks is at least equal to the diameter of the etched nuclear tracks.

3. A method as claimed in Claim 2, characterized in that the surface is irradiated by a beam of energy-rich ions is a preferred direction.

4. A method of forming microstructures of different depths in a solid body, substantially as herein described with reference to the accompanying drawing.

5. A solid body having microstructures of different depths formed by a method as claimed in any preceding claim.

6. A luminescent screen comprising luminescent material and a solid body as claim in Claim 5, the luminescent material being disposed in the recesses of the microstructures.

7. A luminescent screen as claimed in Claim 6, characterized in that the recesses have a depth of approximately 5,um and a diameter of approximately lym and are provided in a structure with a spacing of approximately 2,us.

8. A luminescent screen as claimed in Claim 6 or Claim 7, characterized in that the sidewalls of the recesses are strongly reflective for light emerging from the luminescent material.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. over prior art methods and that the inclination angle of the sides of the nonexposed regions is substantially larger than the inclination of the sides of the exposed regions. The method in accordance with the invention thus utilizes a beam of energy-rich heavy ions for sensitizing the subsequent wet-chemical etching, sensitization being anisotropic in the direction of the ion tracks, which means that the etching rate in this direction is increased in respect of the etching rate in the nontreated material. As a result of using an etching mask which shields the areas of the body which have not been irradiated the ratio of the etching depth to the undercutting width is increased (the rate of under-cutting does not change with respect to the non-irradiated material). During etching, an acute angle side edge structure is obtained on the upper side as well as on the lower side of the non-affected intermediate piece as a result of the slowly widening etched nuclear track holes merging. The ion dose can be adapted to the required side edge definition, the depth of the layer to be removed in an accelerated manner being determined by the energy of the irradiating heavy ions. The method can be used for all electrically insulating materials such as crystalls, glasses, organic materials, semiconductors and the like which have a sufficiently high specific resistance (p > 2000Qcm). A preferred etching method in accordance with the invention is used to provide a substrate of a luminescent screen with recesses, for example, having a depth of approximately 5 ym, a diameter of approximately 1 ,um, and a spacing of approximately 2 ssm. The luminescent material is depdsited in these recesses. As a result, a window having a comparatively high resolution can be realized. In order to counteract lateral dispersion of light, the recesses are preferably each provided with a reflective layer on the lateral wall.By a suitable choice of the refractive indices of the window material, for example, glass having a very low refractive index, such as an optical fibre cladding glass, and the luminescent material, for example, CsI, it can be achieved that the recesses act as conductors for the light, emitted by the luminescent material. WHAT WE CLAIM IS:

1. A method of forming microstructures of different depths in a solid body, in which method a surface to be structured is masked leaving areas to be etched exposed, characterized in that the surface to be structured is irradiated with a beam of energy-rich ions so as to form latent nuclear tracks in the areas to be etched, an apertured mask is arranged on the surface to be structured before irradiation, and after irradiation the surface bearing the apertured mask is chemically etched with an etching medium so as to form the said microstructures, the mask material being impermeable to the etching medium.

2. A method as claimed in Claim 1, characterised in that such a radiation dose is used that the mean distance between the latent nuclear tracks is at least equal to the diameter of the etched nuclear tracks.

3. A method as claimed in Claim 2, characterized in that the surface is irradiated by a beam of energy-rich ions is a preferred direction.

4. A method of forming microstructures of different depths in a solid body, substantially as herein described with reference to the accompanying drawing.

5. A solid body having microstructures of different depths formed by a method as claimed in any preceding claim.

6. A luminescent screen comprising luminescent material and a solid body as claim in Claim 5, the luminescent material being disposed in the recesses of the microstructures.

7. A luminescent screen as claimed in Claim 6, characterized in that the recesses have a depth of approximately 5,um and a diameter of approximately lym and are provided in a structure with a spacing of approximately 2,us.

8. A luminescent screen as claimed in Claim 6 or Claim 7, characterized in that the sidewalls of the recesses are strongly reflective for light emerging from the luminescent material.