BACKGROUND OF THE INVENTION Processing of Silicone wafers for integrated circuits relies on common alignment marks for each subsequent step.

However, in recent years, the addition of the process of chemical-mechanical planarization (CMP), has caused problems of significant smearing of the alignment marks on some layers.

When marks are smeared, their correct detection may be impossible or greatly affected (reduced) in terms of accuracy and precision.

All lithography processes rely upon the correct detection of the alignment marks for proper layer to layer overlay, and therefore these alignment marks are crucial for the overall yield and quality of the final integrated circuits products.

Metrology targets such as overlay for layer to layer measurements are also subject to the CMP smearing effect, and are greatly affected by that smearing.

There are various types of alignment marks, which are"printed", depending on the type of stepper or scanner used for the lithography process.

Shapes and line types of the marks depend on the detection system used, on the accuracy and precision required, and on the stepper/scanner equipment manufacturer, among which are ASML (Netherlands, U. S. A. ) Nikon and Canon (Japan).

However, in all methods, the notorious smearing of the CMP process is prevailing, and this invention provides the potential solution of burying the marks inside transparent media.

It is a purpose of the present invention to provide a novel method for marking alignment marks and other fiducials and targets inside transparent or partially transparent layers, which will not be affected by the subsequent CMP process.

It is a purpose of the present invention to employ ultra-short pulsed laser, such as femto-second laser. Marking by ultra-short pulsed femtosecond laser can be utilized by all

types of lithography detection systems: Coherent source types (lasers), white-light systems, or phase grating marks types.

SUMMARY OF THE INVENTION There is thus provided, in accordance with a preferred embodiment of the present invention, a method for improving alignment of a transparent or semi-transparent workpiece consisting of silicone wafer for integrated circuits undergoing multi-step processing during their manufacturing process, the method comprising: providing laser patterning system comprising an ultra-short pulsed laser source for generating an ultra-short pulsed beam, optical elements for directing and focusing the beam at predetermined target locations within the workpiece, and control unit for controlling the ultra-short pulsed laser source and the optical elements, so as to produce a predetermined pattern inscribed within the workpiece; inscribing a predetermined pattern within the workpiece.

Furthermore, in accordance with a preferred embodiment of the present invention, the optical elements comprise a beam expander, a scanner and focusing element.

Furthermore, in accordance with a preferred embodiment of the present invention, the laser patterning system further includes an X-Y-Z movable stage on which the workpiece is placed, and which is controlled by the control unit.

Furthermore, in accordance with a preferred embodiment of the present invention, the ultra-short pulsed beam has a pulse duration in the order of picoseconds.

Furthermore, in accordance with a preferred embodiment of the present invention, the ultra-short pulsed beam has a pulse duration in the order of femtoseconds.

Furthermore, in accordance with a preferred embodiment of the present invention, the ultra-short pulsed beam has a pulse duration of pulses of less then 100 picoseconds, at energy of less then 0.5 microjoules.

Furthermore, in accordance with a preferred embodiment of the present invention, the numerical aperture associated with the optical elements is at least 0.3.

Furthermore, in accordance with a preferred embodiment of the present invention, the numerical aperture is higher than 0.5.

Furthermore, in accordance with a preferred embodiment of the present invention, the ultra-short laser source generates a beam having a wavelength in the range of 200 to 1200 nm.

Furthermore, in accordance with a preferred embodiment of the present invention, the ultra-short laser source generates a beam having a wavelength of 775 nm.

Furthermore, in accordance with a preferred embodiment of the present invention, the ultra-short pulsed laser source has a repetition rate of 1000 Hz.

Furthermore, in accordance with a preferred embodiment of the present invention, the workpiece comprises a substrate made from silicon coated with a coating layer of silicon oxide, and wherein the predetermined pattern is inscribed at an interface of the substrate and the coating layer.

Furthermore, in accordance with a preferred embodiment of the present invention, the workpiece comprises a substrate made from silicon coated with a coating layer of silicon oxide, and wherein the predetermined pattern is inscribed within the coating layer.

Furthermore, in accordance with a preferred embodiment of the present invention, the workpiece comprises a substrate made from silicon coated with a coating layer of silicon oxide, and wherein the predetermined pattern is inscribed in a groove in the substrate filled with silica.

Furthermore, in accordance with a preferred embodiment of the present invention, the predetermined pattern consists of a series of substantially parallel straight lines.

Furthermore, in accordance with a preferred embodiment of the present invention, the series of substantially parallel straight lines are horizontal.

Finally, in accordance with a preferred embodiment of the present invention, the series of substantially parallel straight lines are vertical.

BRIEF DESCRIPTION OF THE FIGURES In order to better understand the present invention, and appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention.

Like components are denoted by like reference numerals.

Figure 1 illustrates a general scheme of a laser patterning system for marking alignment marks in various levels of silicon wafers, in accordance with a preferred embodiment of the present invention.

Figure 2a illustrates alignment marks inscribed in Si-Si02 interface of a wafer, in accordance with a preferred embodiment of the present invention.

Figure 2b illustrate alignment marks inscribed in the Si02 layer of a wafer, in accordance with a preferred embodiment of the present invention.

Figure 3 illustrates alignment marks inscribed in an a groove etched in an oxide layer of a wafer, in accordance with a preferred embodiment of the present invention.

Figure 4 illustrates typical alignment marks on zero-level of a wafer, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS An aspect of the present invention is the provision of a method for aligning silicon wafers, using alignment marks inscribed within silicon wafers, yielding more accurate and precise alignment of the wafers.

Another aspect of the present invention is the use of ultra-short pulsed laser for the inscription of alignment marks within wafers. Ultra-short pulsed lasers can reach very refined resolution and employing them renders the resulting alignment marks very precise and accurate. By ultra short pulses is meant, in the context of the present invention, pulses at pulse duration of less than 500 fs with a Gaussian shape, where only the tip of the Gaussian reaches a breakdown threshold of the workpiece medium, therefore, exceeding the Rayleigh resolution limit, at controlled pulse energy.

Generally, the method of the present invention involves using a general purpose laser patterning station, equipped with an ultra-fast femtosecond or picosecond pulsed laser, capable of delivering short laser pulses of less then 100 picoseconds, at low energies of less then 0.5 microjoules, to create a local damage zone at the interface of the absorptive Silicone layer and transparent Si02 layer, or change in the index of refraction of the transparent or semitransparent layer on the Silicone substrate material, or scattering off a damaged zone at higher energies.

Laser beam is focused by an optical system with a high numerical aperture (NA) to achieve localized small pixels with a short enough depth of focus for marking under thin transparent layers.

For a fast operation, marking and laser beam steering, are performed by a high speed galvo-scanner, combined with a fast moving X, Y, stage.

At pulse duration of less then 500 femtoseconds, collateral damage is nearly absent, and therefore, patterned laser pixels sizes are below the Rayleigh resolution limit.

Such a method designed for laser writing of alignment marks and fiducial targets, for a photolithography process inside transparent or partially transparent layers, will not be affected by the subsequent CMP (chemical mechanical planarization) process.

Alignment marks detection systems are not affected by further semiconductor industrial processes, since patterned pixels are buried under or within, a transparent Si02 layer.

Reference is now made to Figure 1, illustrating a general scheme of a laser patterning system for marking alignment marks in various levels of silicon wafers, in accordance with the present invention. A general purpose laser patterning station is equipped with an ultra- fast femtosecond or picosecond pulsed laser 10, capable of delivering short laser pulses, preferably of less then 100 picoseconds, at energies high enough to create a local damage zone at the interface of the absorptive Silicone layer and the transparent Si02 layer of a wafer, or change in the index of refraction of the transparent or semitransparent layer on the Silicone substrate material, or scattering off a damaged zone at higher energies. The physical mechanism by which this change occurs, and the absence of collateral damage as happens with long duration pulsed lasers, is known and was described before (see, for example, U. S. patent 5,656, 186 (Mourou et al.), Thresholds for femtosecond laser-induced breakdown in bulk transparent solids and water. Chris B. Schaffer, Nozomi Nishimura, and Erick Mazur. Proc. of SPIE vol. 3451 (1998) Harvard University, Department of Physics, Cambridge, MA 02138, and Ultrafast laser induced microexplosions : explosive dynamics and sub-micrometer structures. Chris B. Schaffer, Eli N. Glezer, Nozomi Nishimura, and Erick Mazur. Proc. of SPIE vol. 3269 (1998) 36-45. Harvard University, Department of Physics, Cambridge, MA 02138.

The setup illustrated in Fig. 1 includes laser source 10, for generating ultra-short pulses. The laser source beam is preferably expanded by a beam expander 12 and then directed to a scanner, for fine step control of the desired displacement of the beam. The scanned beam is preferably focused by a focusing element 16 onto a workpiece 20, writing in the workpiece marks 18 in a predetermined manner. The workpiece is preferably placed on an X-Y-Z movable stage, for coarse step control. A control unit 24 controls the system, actuating and governing the laser source, the scanner and the X-Y-Z stage, according to a predetermined two or three dimensional pattern design 26 entered or programmed into the control unit.

A setup as depicted in figure 1 is not limited to the high energy regenerative amplified lasers types, since energy required for marking of Si/SiO2 interface is lower then a fraction of a microjoule. Therefore, an oscillator, such as SErF Fiber ring laser (Stretched-pulse Erbium-doped Fiber) can be used as the laser source (see, for example, US Pat. No.

5,617, 434 (Tamura et al.)).

Laser wavelengths of 200-1200 nanometers are preferred but the present invention is not limited to this range only, since the SiO2 layer is transparent and the Silicone substrate is absorptive.

SErF type laser, with a wavelength of 1550 nanometers (nm), followed by a second harmonic generator (SHG) emits eventually radiation at the wavelength of 775nm and therefore can be used for low-energy markings.

High energy femtosecond lasers with a regenerative amplifier (such as CPA-2001 <BR> <BR> from Clark-MXR, Mi, U. S. A. ) radiates also at 775nm, and hence are suitable for marking once used with a variable energy attenuator.

The latter has the advantage of a much larger energy budget, which facilitates the insertion of various optical subsystems in the beam path.

Repetition rate of 1000Hz is a typical value for such lasers, and with a fast Galvo- scanner (14 in Fig. 1), it is a normal practice for those skilled in the field, to mark a line of 1000 microns of length at about one second.

Typical Si02 layer thickness of below 2 microns, requires a high numerical aperture (NA) system.

The laser beam is focused by an optical system with a high NA of at least 0.3, but preferably over 0.5, to achieve localized small pixels with a short enough depth of focus for marking under thin transparent layers.

For fast operation, marking and laser beam steering are performed by a high speed galvo-scanner (14), combined with a fast moving X, Y, stage (20).

By employing ultra-short laser pulses of Picosecond or Femtosecond duration, there are three major phenomena, which may occur: 1. Scattering centers around the damage zones 2. Absorption at the Silicone/Si02 interface 3. Phase modulation.

Picosecond duration pulses at the range of 1 to 500 picoseconds create damage zones on the order of 5 to 80 microns of lateral dimension and light is scattered and partially absorbed.

As a result, these marks can be detected by a white-light imaging system or a laser alignment system, but the line accuracy is limited. The limit is set to approximately 10 times less then the accuracy level of one pixel, for statistical averaging of 100 pixels. For a single pixel of 5 micron accuracy, the full line will result in accuracy of few hundreds nanometers.

Pulses at below 1 picosecond duration, when used at energies of about 0.5 microjoules, create smaller sized damage zones, of less then 10 microns size, and their detection may employ the same systems as for the higher duration pulses.

Absorption of femtosecond pulses, can reach accuracy of less then 0.5 micron, and once detected with a multi-pixel detection system, accuracy is enhanced more then ten folds -down to a few nanometers.

When using 150 femtosecond duration pulses, at threshold energy of less then 0.4 microjoules, clear cut images of <1.0 micron size lines, grabbed with a phase-contrast microscope, were acquired.

The third phenomena of phase modulation, is a result of femtosecond pulses, at energies below ~0. 5 microjoules, with high NA optics, the index of refraction of the transparent layer is modified.

At the focus, the laser pulses create a localized plasma by a nonlinear absorption of radiation.

As a result, density and structural changes inside the transparent media lead to modification of Si02 index of refraction, and therefore, to phase modulation of light reflected into the detection system.

Since layer thickness in wafers is typically very thin (less then 2 microns), the preferred method is to mark the Si/Si02 interface rather then marking the transparent layer itself.

Figure 2a illustrates alignment marks inscribed in Si-Si02 interface of a wafer, in accordance with a preferred embodiment of the present invention. A wafer combining silicon layer 30 as a substrate and a Si02 layer 32. In a preferred embodiment of the present invention the laser beam is focused 38 and alignment marks 36 (usually a series of substantially parallel straight lines) inscribed on the Si/SiO2 interface. Figure 2b illustrate alignment marks inscribed in the Si02 layer of a wafer, in accordance with a preferred embodiment of the present invention. Alternatively, or in conjunction with the interface marking, the laser beam is focused 39 and alignment marks 37 may be inscribed within the Si layer.

Figure 3 illustrates focusing of the laser beam 44 and inscribing alignment marks 42 in a groove 40 etched in an oxide layer 32 of a wafer, in accordance with a preferred embodiment of the present invention. This is done when the oxide layer is too thin to write in.

The etched groove is filled with transparent (or semi-transparent) silica so that the alignment marks may be located and detected.

Figure 4 illustrates typical alignment marks (vertical 46 and/or horizontal 48) on zero- level of a wafer, in accordance with a preferred embodiment of the present invention.

The method of the present invention may be characterized by the following advantages:

1. White-light detection with a separate optical system ("off-axis") is made possible, using a high resolution CCD camera system to align the wafer marks to the reticle marks (Canon) 2. Enhanced global alignment-using laser scanning alignment system to acquire the position of 5 to 10 alignment marks across the wafer (Nikon).

3. Zero-level alignment-The wafer has alignment marks etched directly into the bare silicone before any other processing commences. Subsequent masks are aligned to the zero-level marks.

This concept, which is used by ASML, was further developed by using a laser interferometric grating alignment system. A major disadvantage of the existing marking systems is their vulnerability to smearing by the CMP (Chemical Mechanical Polishing) process, which is carried out after certain steps of lithography. The method of the present invention overcomes this vulnerability by placing the marks beneath the surface thereby rendering it immune to CMP smearing.

The detection of the alignment marks inscribed within the wafer in accordance with the present invention may be accomplished using in-situ vision/alignment/inspection systems or off-line inspection systems, both of which are known.

It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope.

It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the present invention.