METHOD OF FORMING IDENTICALLY POSITIONED ALIGNMENT MARKS ON OPPOSITE SIDES OF A SEMICONDUCTOR WAFER

Abstract: Method of automatically forming identically positioned alignment marks on the front side and the back side of a silicon wafer especially prepared for use in silicon mi­ cromechanical technology. The front side and the back side of the silicon wafer (1) are coated with an insulating layer (2, 3). High-energy heavy ions are directed onto the front side insulating layer (2). The heavy ions penetrate the front side insulating (2), the wafer (1), and the back side insulating layer (3), thus forming single disturbed crystal lattice nuclear tracks (4, 4′) in both insulating layers (2, 3), with the wafer (1) remaining undisturbed. The nuclear tracks (4, 4′) in both insualting layers (2, 3) are etched so that corresponding identically positioned pores (5, 5′) are opened. These pores (5, 5′) are used as alignment marks for individual further method steps.

Description: This invention relates to a method of forming identically positioned alignment marks on opposite sides of a semiconductor wafer. Single-crystal silicon is being increasingly used in a variety of new commercial products both for its well-established electronic and excellent mechanical properties. Silicon is in the process of altering conventional perceptions of miniature mechanical devices and components. Increasing sophistication, decreasing size, and proliferating applications of integrated electronics have been coupled with a growing demand for higher precision, higher reliability, and lower cost miniaturized mechanical components that are compatible with the electronics. Examples of such components can be found in mechanical sensors, printing technologies including ink jet print heads and thermal print heads, magnetic and optical recording heads for data storage and retrieval, solid-state laser and fiber-optic mounting, coupling, and alignment, and high-precision mechanically resonant crystals. The rapid expansion of the relatively new field of silicon micromechanics has made it possible to fabricate many such complex mechanical devices on silicon, using conventional silicon IC manufacturing processes. Numerous advantages are apparent in this approach: interfacing or integrating mechanical and electronic devices is greatly facilitated; the key factor in the cost-effectiveness of the IC industry - batch fabrication procedures - is extended to miniature mechanical components; extremely well-known and well-characterized materials and processing techniques are utilized; the rigid control of material properties, which is essential for electronic devices, can be readily applied to mechanical components to increase performance, reliability and repeatibility in device characteristics; and, finally, the small dimensions and high accuracies which are commonly achieved in integrated electronics can be exploited to realize mechanical devices and structures which are not possible by any other methods. Processing techniques, structural features, and areas of application of silicon micromechanics are described in "Proceedings of the IEEE", Vol. 70, No. 5, May 1982, pp. 420 to 456, by K.E. Petersen. The problem with silicon micromechanics, when used in the manufacture of micromechanical devices, is to align both the front and the back side of a silicon wafer with high accuracy to a photolithographic mask. This problem has existed for quite a while and many proposals have been made for its solution. US-A 3,752,589 discloses an optical-mechanical means and method for aligning the pattern of a photographic mask on one side of a wafer to a pattern on the opposite side, wherein individual optical images of the patterns on both sides of the wafer are obtained, and the position of the mask is mechanically adjusted until the two optical images are aligned. US-A 4,046,985 discloses an optical-mechanical semiconductor wafer alignment apparatus for repeatedly aligning the back sides of wafers to the axis of a kerf-cutting laser. First, a transparent mask, having on its front side a duplicate of the device pattern and of the alignment marks on the opaque wafers, is visually aligned to the laser, and the apparatus is set to maintain this alignment. Thus, subsequently inserted opaque semiconductor wafers, having the same alignment marks, will travel through the apparatus along the same path as the transparent mask, including inversion of the wafers, so that their back sides are aligned to the laser whose beam automatically impinges on the kerf areas between the devices on the front side of the opaque wafer. This prior art alignment apparatus requires expensive and unwieldy mechanical alignment fixtures or means. The main problem of these approaches is that they are extremely labor intensive, require a significant amount of handling, are slow and not well adaptable to automation, and cannot take advantage of modern projection printing tools. A method of forming alignment marks on both the front and the back side of a semiconductor wafer is suggested by prior art known from US-A 4,318,752 and 4,131,487. US-A 4,318,752 discloses a method of forming a P- doped area in an N-doped layer of a plural-layer semiconductor body by passing a laser beam through a first P-doped layer into an adjacent N-layer which absorbs the laser beam energy and generates heat to cause diffusion of the P-doped layer into the N-doped layer. The wavelength of the laser beam is chosen such that it passes through the P-doped layer without absorption. US-A 4,131,487 discloses a process for gettering a semiconductor wafer with a laser beam, wherein a laser beam is directed upon the back side of the wafer to create lattice damage and strain in the back side. Subsequent heating of the wafer releases the strain to attract mobile defects from the front side of the wafer to the back side. Using these prior art laser techniques, US-A 4,534,804 discloses a process for aligning the front and the back side of a semiconductor wafer without the need for an expensive and unwieldy alignment fixture or apparatus. Identically positioned alignment marks on both the front and the back side of a semiconductor wafer are created by using a laser beam to scribe an alignment mark on the back side of the wafer. The wavelength of the laser beam is chosen such that it passes through the lightly doped wafer without absorption but is absorbed by a following heavily doped semiconductor layer to generate therein heat and resulting defects. The semiconductor wafer is then heated to cause the defects to migrate through the wafer to the front side where there is formed an identically positioned image of the mark scribed on the back side. This alignment process eliminates the need for a photoresist blocking mask during an etch step and makes it possible to form alignment marks on sandwich or other structures where one side is covered or otherwise inaccessible for alignment procedures. However, with regard to the number of layers and the process steps required, this method is relatively elaborate and expensive and can only be applied to a limited extent, namely, for silicon wafers with a particular maximum doping and a particular layer sequence. The accuracy of alignment depends on the directional accuracy of the migration path of the defects to the front side. The invention as claimed is intended to remedy these drawbacks. It solves the problem of how to form automatically identically positioned alignment marks on the front and the back side of a semiconductor wafer. The method according to the invention requires no special previous preparation of the wafers, the wafers' crystal structures are not changed, and the alignment of the alignment marks to be formed is better than 0.1 µm. Ways of carrying out the invention are described below with reference to drawings in which Figs. 1A and 1B are sectional views of a semiconductor wafer processed according to a first method of the present invention, wherein blanket low-dose heavy ion exposure is used to form single randomly distributed alignment marks on the front and the back side of the wafer; Fig. 1C is a plan view of the wafer processed according to the first method of the present invention; Figs. 2A and 2B are sectional views of a semiconductor wafer processed according to a second method of the present invention, wherein blanket high-dose heavy ion exposure is used to form distinct alignment marks defined by a metal mask, and Fig. 2C is a plan view of the wafer processed according to the second method of the present invention. The method according to the invention utilizes a phenomenon which has been used for a number of years to detect the existence of fast heavy ions. For that purpose, a solid is introduced into a solid "bubble chamber" where it is bombarded with high-energy heavy ions. Along their path, the heavy ions penetrating the solid disturb the crystal lattice to such an extent that fine channels can subsequently be etched into the crystal. Each channel corresponds to a single heavy ion. The channels are marked by a dense distribution of the diameters which are substantially identical. The number of channels is effectively controlled as a function of the ion dose and independently of the diameter. The diameters of the channels produced by etching are effectively adjustable in a range from 10 nm to 100 µm. In addition, the length/diameter ratio is adjustable, and a ratio of 70 has already been realized. This phenomenon is also covered by the German Offenlegungsschrift 29 51 376 describing a method of generating microholes resulting from nuclear tracks of a single ion or a countable number of ions of an accelerator, e.g., heavy ions of a heavy-ion accelerator in solids. After exposure, the solid is subjected to an etch process wherein the said microholes or channels are formed. Applications of this phenomenon are described in "J. Vac. Sci. Technol.", Vol. 19, No. 4, November/December 1981 under the title "Ion Interaction with Solids", on pp. 864 and 865. The heavy ion bombardment method is used in particular to generate single holes in membranes and to produce microfilters. By bombarding Hostaphan (polyethylene terephthalate) with 4.7 MeV Xe ions at a dose of 1.6 × 10⁷ cm⁻², channels of about 50 µm length and 0.7 µm diameter, which produce a filter effect, are formed during etching in NaOH. A further application of this phenomenon is described in German Offenlegungsschrift 29 51 287 which discloses a method of producing planar surfaces with minimum peaks in the µm range or smaller, for instance, of flat field emission cathodes of conductive or semiconductive material by filling hollow spaces in matrices and subsequently removing the matrix containing the hollow spaces. The flat matrix accommodating the hollow spaces is exposed to high-energy ions, e.g., of a heavy ion accelerator. In a subsequent etch process, the nuclear tracks formed are bared. Finally, the holelike nuclear particle tracks and/or hollow spaces are filled with a conductive or a semiconductive material, thus yielding a needle structure of conductive or semiconductive material, which is used as a field emission cathode. German Offenlegungsschrift 33 28 839 discloses a method, wherein the described phenomenon is used to produce a layer structure for vertical magnetic data storage. For that purpose, a carrier of a non-magnetic material, such as resin, is applied to a suitable substrate. A pattern of nuclear tracks is produced in this carrier by high-energy heavy ion bombardment. By subsequent etching, these nuclear tracks are converted into channels. In a suitable plating process, the channels are filled with the magnetic material that permits vertical magnetic data storage. Thus, a microneedle structure is obtained that comprises a plurality of needles of anisotropic material which are arranged perpendicularly to the substrate surface. According to the invention, this known phenomenon is used in a method of forming identically positioned alignment marks on opposite sides of a semiconductor wafer. In a first example illustrated in Figs. 1A to 1C, a, for example, 100 µm thick wafer of silicon, of which one of the micromechanical devices described in the preamble of the description is to be produced and which because of the shape of its front and its back side is to be provided with identical alignment marks, is coated in an oxidation step known from integrated semiconductor technology with a, say, 100 nm thick SiO₂ layer. Fig. 1A is a sectional view of the wafer 1 with the two SiO₂ layers 2 and 3 on the front and the back side. In the subsequent method step, as is marked by the arrows in Fig. 1A, the front side of the wafer 1 and the SiO₂ layer 2, respectively, is blanket-exposed, using heavy ions at a low dose of about 10⁶ - 10⁷ions cm⁻². The heavy ions employed may be, for example, Xe²⁴⁺ ions with 10 MeV per nucleon. Realizing such low exposure doses is unproblematic in heavy ion technology. During eposure, each heavy ion leaves in the SiO₂ layer 2 in random distribution a latently disturbed crystal region and/or track 4 of about 10 nm diameter. The heavy ions also penetrate the wafer 1 which is not damaged in consequence. Each heavy ion finally penetrates the SiO₂ layer 3 on the back side of the wafer, where it leaves a disturbed nuclear track 4′ which is accurately aligned to the disturbed track 4 in the SiO₂ layer 2 on the front side. At the low dose chosen, a mirror image of individual disturbed nuclear tracks 4, 4′ is formed on the front and the back side of the wafer, with the spacing of adjacent tracks being of the order of 10 µm, for example. In the next method step, SiO₂ layers 2 and 3 are etched with, say, buffered hydrofluoric acid. The etchant used may also be KOH. The etch rate in the region of the nuclear tracks 4, 4′ is much higher than in the regions with undisturbed SiO₂, so that, as shown in the sectional view of Fig. 1B, pores 5, 5′ corresponding to the disturbed tracks 4, 4′ are opened in the two SiO₂ layers 2, 3. The diameter of the pores 5, 5′ corresponds to that of the tracks 4, 4′, thus being about 10 nm. During the etching of the pores 5, 5′, the silicon of the wafer 1 serves as an etch stop. It is essential that the pores 5, 5′ not be expanded with regard to their depth, as is known from disturbances caused by an ion collective. The accuracy of the pore alignment between the front and the back side exceeds the pore diameter. As shown in the plan view of Fig. 1C, each characteristically positioned randomly generated partial group of pores 5, 5′ in the two SiO₂ layers 2 and 3 may be used as an alignment mark for further masks or photoresist layers to be applied to the front and the back side, which will be required in the course of the further method steps for producing a micromechanical device. The alignment marks may also be directly applied to the surface of the wafer 1 by slightly etching the surface, using SiO₂ layers 2 and 3 as an etch mask. The two SiO₂ layers 2 and 3 are then removed and the front and the back side of the wafer 1 with the alignment marks are then directly available for further method steps to be implemented. Fig. 2A to 2C show a second example of the method according to the invention, the second example differing from the first only with regard to the initial method steps, so that the same reference numerals as in Figs. 1A to 1C are used for identical parts. As shown in the sectional view of Fig. 1A, initially the front and the back side of the silicon wafer 1 is coated with an insulating layer 2, 3 of SiO₂. Then, a mask, say, a metal mask 7, that is impervious to heavy ions, is applied to the SiO₂ layer 2 on the front side of the wafer 1. This metal mask has a pattern of openings 8, whose diameter and distribution reflect the alignment marks to be formed. Using this metal mask 7, the front side of the structure is exposed to high-energy heavy ions, for example, Xe²⁴⁺. As a heavy ion must be capable of impinging on the region of the individual mask windows 8, exposure must be be effected at a relatively high dose of about 10⁹ ions/cm⁻². The energy may again be 10 MeV per nucleon. The mask windows 8 define the points at which the heavy ions penetrate the two SiO₂ layers 2 and 3 and the wafer 1, again leaving disturbed nuclear tracks 4, 4′ only in the SiO₂ layers 2 and 3. After removal of the metal mask 8, etching is effected by, say, buffered hydrofluoric acid (Fig. 2B), causing pores 5, 5′ having a diameter of about 10 nm to be opened in the SiO₂ layers 2 and 3 on the front and the back side of the wafer 1. The plan view of Fig. 2C shows the masked alignment mark pattern thus formed which is used in the subsequent differentiated method steps. It is pointed out that in lieu of silicon also other semiconductor materials may be used as a wafer. As the disturbance by heavy ion exposure occurs in insulating layers, polymer or photoresist layers may, if necessary, be used in lieu of the SiO₂ layers on the front and the back side of the wafer.

Claims: A method of forming identically positioned alignment marks on opposite sides of a semiconductor wafer, characterized by coating the front and the back side of said semiconductor wafer (1) with an insulating layer (2, 3); directing high-energy heavy ions onto the front side insulating layer (2) so that said heavy ions penetrate the front side insulating layer (2), the semiconductor wafer (1), and the back side insulating layer (3) and form single nuclear tracks (4, 4') in both insulating layers (2, 3) by disturbing their crystal lattice without disturbing the crystal lattice of the semiconductor wafer (1); and etching said nuclear tracks (4, 4') in both insulating layers (2, 3) so that corresponding, identically positioned pores (5, 5') are opened in said insulating layers (2, 3) forming said alignment marks. The method as claimed in claim 1, characterized by using an unmasked blanket high-energy heavy-ion exposure for said front side insulating layer (2) at such a low dose so that randomly distributed single nuclear tracks (4, 4') are produced. The method as claimed in claim 2, characterized by said heavy ion exposure dose being about 10⁶ - 10⁷ ions/cm². The method as claimed in claim 1, characterized by coating the front side insulating layer (2) with a metal mask (7) having a pattern of openings (8) whose diameter and distribution reflect the alignment marks to be formed, and using blanket high-energy heavy ion exposure for said metal mask (7) at such a high dose that nuclear tracks (4, 4') are produced according to the pattern of openings (8). The method as claimed in claim 4, characterized by said heavy ion exposure dose being about 10⁹ ions/cm². The method as claimed in claims 1 to 5, characterized by employing Xe²⁴⁺ ions for heavy ion exposure, with the energy amounting to about 10 MeV per nucleon. The method as claimed in claims 1 to 6, characterized by using a semiconductor wafer (1) consisting of silicon with a thickness of about 100 to 300 µm, and by providing insulating layers (2, 3) consisting of SiO₂ with a thickness of about 100 nm.

Inventors: ELSNER, GERHARD, DR. DIPL.-PHYS., GRESCHNER, JOHANN, DR. DIPL.-PHYS., HINKEL, HOLGER, DR. DIPL.-PHYS.
Applicants: IBM DEUTSCHLAND GMBH, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L23
IPC codes: H01L21, H01L23