Micromechanical Sensor Fabrication Process

The present invention relates broadly to microscopic profilometry and more particulrly to a micromechanical sensor for an atomic force microscope ("AFM"), a scanning tunneling microscope ("STM"), or a magnetic force microscope ("MEM") and methods for fabricating such sensors. Micromechanical sensors for AFM/STM/MFM profilometry generally comprise a cantilever beam which bears a shaped tip at a free end.

The scanning tunneling microscope has stimulated the development of new techniques for microcharacterization of materials which are based on the use of a very fine tip. One of these techniques involves the atomic force microscope which has recently demonstrated the capability to profile and image conductors and insulators.

In an initial design of the AFM (Binnig G, Quate CF, Gerber Ch, (1986) "Atomic Force Microscope," Phys. Rev. Lett. vol. 56, pp. 930-933 and European patent application Serial No. EP-A-0 223 918) a sensor consisting of a spring-like cantilever which is rigidly mounted at one end and carries at its free end a dielectric tip profiles the surface of an object. The force between the object's surface and the tip deflects the cantilever, and this deflection can be accurately measured, for example, by a second tip which is part of an STM. A lateral spatial resolution of approximately 3 nm has initially been achieved.

Another version of the AFM includes optical detection instead of an STM detection. In this version, a tungsten tip at the end of a wire is mounted on a piezoelectric transducer. The transducer vibrates the tip at a resonance frequency of the wire, which acts as a cantilever, and a laser heterodyne interferometer accurately measures the amplitude of the a. c. vibration. The gradient of the force between the tip and sample modifies the compliance of the lever, hence incucing a change in vibration amplitude due to a shift of the cantilever resonance. Knowing the cantilever characteristics, one can measure the vibration amplitude as a function of the tip-to-sample spacing in order to deduce the gradient of the force, and thus, the force itself. See, Duerig UT, Gimzewski JK, Pohl DW (1986) "Experimental Observation of Forces Acting During Scanning Tunneling Microscopy," Phys. Rev. Lett. vol. 57, pp. 2403-2406; and Martin Y, Williams CC, Wickramasinghe HK (1987) "Atomic Force Microscope-Force Mapping and Profiling on a sub 100-A Scale," J. Appl. Phys. vol. 61(10), pp. 4723-4729.

A critical component in the AFM is the spring-like cantilever. For maximum sensitivity, a maximum deflection for a given force is needed, which calls for a cantilever that is as soft as possible. At the same time, in order to minimize the sensitivity to vibrational noise from the surroundings, a stiff cantilever with a high eigenfrequency is necessary. Ambient vibrations--which typically stem from building vibrations--ordinarily have a frequency spectrum essentially limited to less than roughly 100 Hertz.

If the cantilever is chosen such that it has an eigenfrequency f.sub.o .gtoreq.10 kHz, the ambient vibrations will be attenuated to a negligible value. These requirements can be met by reducing the geometrical dimensions of the cantilever as reflected by the two equations set forth below.

The eigenfrequency f.sub.o of the cantilever is given by ##EQU1## wherein E is Young's modulus of elasticity, .rho. is the density, and K is a correction factor close to unity, l is the length, and t is the thickness of the cantilever.

The spring constant of the cantilever on which its sensitivity depends is given by ##EQU2## wherein F is the force which causes the deflection y of the cantilever, E is Young's modulus of elasticity, w is the width, l is the length, and t is the thickness of the cantilever. In accordance with the spring constant term, the sensitivity of the cantilever is dependent on its dimensions and on the material of which it consists, with the highest sensitivity being obtained for long, thin and narrow cantilever beams. The width of the cantilever beam should be sufficiently large so that lateral vibrations are suppressed. Also, the width of the beam should permit the fabrication of additional structures, such as tips, thereon. Therefore, a minimum width w of around 10 .mu.m is generally required as a practical matter. In practice, the spring constant C has to be greater than about 1 N/m in order to avoid instabilities during sensing of attractive forces, to prevent excessive thermal vibrations of the cantilever beam, and to obtain a measurable response.

Approximates dimensions of a cantilever beam composed of single-crystal silicon compatible with C=1 N/m, and f.sub.o =10 kHz may be, for example: 1=800 .mu.m, w=75 .mu.m, and t=5.5 .mu.m.

In the normal deflection mode of the cantilever beam, forces in the order of 10.sup.-12 N can be detected. The sensitivity of the sensor head can be further enhanced by vibrating the object to be profiled at the resonance frequency f.sub.o of the cantilever beam, as described by G. Binning et al. in Phys. Rev. Lett. vol. 56 (1986), pp. 930-933.

In the AFM realized in accordance with the aforementioned Binning et al. article and with European patent application EP-A-0 223 918, the requirements for cantilever and tip were met by a gold foil of about 25 .mu.m thickness, 800 .mu.m length, and 250 .mu.m width to which a diamond fragment was attached with a small amount of glue. Another proposal used microfabrication techniques to construct thin-film (1.5 .mu.m thick) SiO.sub.2 microcantilevers with very low mass on which miniature cones could be grown by evaporation of material through a very small hole (Albrecht ThR, Quate CF, (1988) "Atomic Resolution with the Atomic Force Microscope on Conductors and Nonconductors," J. Vac. Sci. Technol., pp. 271-274.

From the foregoing description of the state of the art it was known to construct, in a first process step, cantilevers, and, in a second process step, to attach tips thereto. However, the construction of a cantilever with tip of that type is extremely delicate and prone to low yield.

Most recently, several processes have been developed for producing micromechanical sensor heads for AFM/STM profilometry. See, for example, European patent applications EP-A-89 115 100.3, EP-A-89 115 099.7, and EP-A-89 115 097.1.

European patent application EP-A-89 115 100.3 describes a process for producing a cantilever beam with an integrated tip. In this process a mask is produced which contains all relevant information for the desired cantilever beam pattern and tip pattern. In a subsequent etching process the pattern of the mask is transferred step by step to a silicon substrate. The shape of the tip is determined by anisotropic wet and underetching of the tip mask. The tip terminates as a straight line in the form of a multisided pyramid. The cone angle of the tip made in accordance with this process is .gtoreq.35.degree..

However, for the present invention it is desirable to have a method by which conically shaped tips can be produced with cone angles <30.degree., preferably about 10.degree., with highest accuracy and high yield. Particularly in magnetic force microscopy, the domain resolution depends on the tip shape and cone angle.

It is an object of the invention to provide a method for the construction of microcantilevers with integrated tips, which method uses a suitable combination of deposition, lithography, etching, and oxidation steps. The invention also comprises structures made in accordance with the method of the invention.

Prior to starting with the description of the details of the invention, reference is made to the following publications relating to micromechanics:

Petersen, KE, "Dynamic Micromechanics on Silicon: Techniques and Devices," IEEE Tran. Electron Devices, Vol. ED-25, No. 10, October 1978, pp. 1241-1250;

Petersen, KE, "Silicon as a Mechanical Material," Proc. of the IEEE, Vol. 70, No. 5, May 1982, pp. 420-457; and

Jolly, RD, Muller, RS, "Miniature Cantilever Beams Fabricated by Anisotropic Etching of Silicon," J. Electrochem Soc.: Solid-State Science and Technology, December 1980, pp. 2750-2754.

With the method of the present invention, low-mass microcantilever beams can be made with an integrated tip for interaction with the surface of a sample to be profiled. The force and resonance frequency requirements of the cantilever beam and integrated tip are met by using microfabrication techniques. Due to the fact that cantilever and tip are made from one piece of material there are no adhesion problems between cantilever and tip.

The process of the present invention involves producing a mask which contains effectively all relevant information for the substrate etching of the cantilever beam and the tip in a manner which generally parallels the mask production process of European patent application EP-A-89 115 100.3 noted above, (see FIGS. 4a to 4c of the European application. In the present invention, an anisotropic ion etching process is used for etching the cantilever beam and the basic shape of the tip into the substrate. The process of the invention includes a thermal oxidation step to define the shape of the tip. The thermal oxidation step is followed by a stripping process which gently removes the thermally grown silicon dioxide.

The oxidation step in particular allows an extremely exact definition and variation of tip height and cone angle via oxidation time. The thermal oxidation is preferably carried out at a temperature of at least about 1000.degree. C. in the presence of water vapor. A temperature range of from about 1000 to about 1100.degree. C. is preferred for the thermal oxidation, with a temperature of about 1050.degree. C. being particularily preferred. The pressure of the water vapor is preferably about one atmosphere.

Micromechanical sensors with tips having cone angles of from about 50.degree. down to 10.degree. or so can readily be achieved in high yields with the present invention.

A first preferred embodiment of the process of the invention relates to the development of a structure, such as a tip, on an already 3-dimensionally structured substrate, such as a cantilever beam. Conventionally, in such processes problems can arise, for example, when the cantilever beams, which are flexible and fragile, have to be coated with photo-resist for subsequent exposure. These and other problems are solved in the first embodiment in that a multiple-step mask which is applied to a substrate is structured in such a manner that it contains effectively all relevant information for the subsequent substrate etching process. The structures of all lithography steps are etched one after the other into the multiple-step mask before the substrate is etched. Subsequently, the information is transferred step by step from the mask into the substrate. Between two successive substrate etching process steps, there is a mask etching step which does not require an additional lithography step. The multiple step mask can be fabricated in conventional planar technology. A further advantage of the preferred embodiment is that problems with respect to photoresist coverage and exposure are obviated as the multiple-step mask has a thickness of only a few .mu.m. The process benefits from a high selectivity of mask versus substrate in the substrate etching process, which comprises several dry etching steps.

By the process of a second embodiment of the present invention, a tip on a pedestal is produced. As mentioned in the introduction to the description, the sensitivity of cantilever beam and tip depends on their dimensions. Where a very small diameter of the tip is desired for investigating submicron structures, the tip length is limited to a certain value to prevent the tip from sticking to the structures to be investigated. In practice, the ratio of tip diameter to tip length should be about 1:10 or less to avoid such sticking. Therefore, in order to obtain a distance from the tip end to the cantilever beam of about 6 to 10 .mu.m the tip has to be arranged on a pedestal.

The process of the second embodiment is a completely self-aligned process for producing a tip with a diameter of about 0.2 .mu.m and a length of about 2 .mu.m arranged on a pedestal with a diameter of about 1 .mu.m in an absolutely concentrical manner. This process is extremely advantageous over the prior art process (EP-A-89 115 100.3, third example, claim 19) where a tip with a base diameter of 30 .mu.m arranged on a pedestal with a diameter of about 100 to 200 .mu.m is produced, with the associated etching masks being adjusted against each other.

Details of several embodiments of the present invention will be described by way of example with respect to the accompanying drawings in which:

FIGS. 1A through 1I shows a sequence of process steps for making a cantilever beam with an integrated tip which has been worked out of a silicon wafer substrate, using a mask with two levels of information, and photolithographic, etching and oxidation steps; and

FIGS. 2A through 2E shows a sequence of process steps for making a cantilever beam with integrated tip on a pedestal which has been worked out of a silicon wafer substrate, using a mask with two levels of information, and photolithographic, etching and oxidation steps.

A first example of preferred embodiment of the process of the invention is described in accordance with FIGS. 1(a) to 1(i). In this example a cantilever beam approximately 100 .mu.m long, approximately 15 .mu.m wide and approximately 3 .mu.m thick is produced having an integrated tip projecting approximately 5 .mu.m above the beam surface.

Turning now to FIG. 1a, a (100) silicon wafer (1) about 250 .mu.m thick is bilaterally coated with silicon dioxide. The oxide layers (3) and (2) on a top side and on a bottom side of the wafer are thermally grown to a layer thickness of about 3 .mu.m. Alternatively, the oxide layers can be applied by sputter deposition or by (PE)CVD. In a first photolithographic step, the patterns of cantilever beam (4) and of rectangular openings (5) which are subsequently transferred into the upper part of silicon dioxide layers (3) and (2) are defined. For this purpose a positive photoresist commercially available from the Shipley Company under the trade designation "AZ1350" is applied sequentially to the two sides of the oxide coated wafer (1) (not shown). Specifically, a first layer of photoresist is first spun on one side of the wafer and the wafer baked to cure the layer, and then a second layer of photoresist is spun on the opposite side of the wafer and the wafer baked to cure the second layer. The photoresist layers on both sides are exposed at the same time and developed, with the respetive masks on the bottom side of the wafer being in alignment with those on the top side. The two-sided exposure may be carried out using a mask alignment and exposure tool commercially available from the Carl Suess company of Munich, Germany under the trade designation "Type MA-25." The oxide on both sides is etched in 5:1 buffered hydrofluoric acid or by reactive ion etching under the following process conditions:

etch gas: CF.sub.4

pressure range: from about 1 to about 10 .mu.bar.

The etching of the oxide is carried out for a time to etch the oxide on the top side to a depth of about 1.5 .mu.m to transfer the cantilever beam pattern to the oxide. Next, the top side is protected by a baked photoresist layer, and the oxide residue in the exposed area (5) on the bottom side of the wafer is removed by etching in 5:1 buffered hydrofluoric acid. Next, the remaining photoresist is removed. The resulting structure is shown in FIG. 1a.

In a second photolithographic step the pattern (6) of the tip is defined in a newly applied photoresist layer (not shown) over silicon dioxide cantilever pattern (4). The photoresist tip pattern is transferred into the silicon dioxide by etching in 5:1 buffered hydrofluoric acid or by reactive ion etching, as described above, to a depth of about 1.3 .mu.m. During this etching step the cantilever beam pattern (4) is transferred to a deeper level of layer (3), and the thickness of the remaining silicon dioxide layer (3) is reduced correspondingly to about 0.2 .mu.m or so. The bottom side of the wafer is protected by a baked photoresist layer during this step. The resulting multiple-step silicon dioxide mask structure (4, 6) which will then be transferred step by step into the silicon substrate (1) is shown in FIG. 1b.

Prior to this mask structure transfer, the silicon wafer (1) is thinned down by etching from the bottom side to a thickness which corresponds to about the thickness of the cantilever beam plus about twice the height of the tip plus a residual wafer thickness which is consumed during thermal oxidation. In this example, the wafer is thinned to a thickness of approximately 14 .mu.m. This etching step, which uses an about 37.5 wt percent aqueous KOH solution at about 80.degree. C., is anisotropic. The resulting structure is shown in FIG. 1 c.

Next, as shown in FIGS. 1c and d, silicon dioxide layer (3) with structure (4) and (6) is etched in 5:1 buffered hydrofluoric acid or by reactive ion etching, as described above, to such a depth that the silicon dioxide areas (7) abutting cantilever beam mask (4) are removed.

An anisotropic reactive ion etching step follows for the transfer of the cantilever beam pattern (4) into the silicon wafer (1). The reactive ion etching performed for that purpose is carried out under the following conditions:

etch gas: approximately 10 flow percent chlorine, remainder Ar or He;

pressure range: from about 2 to about 20 .mu.bar, preferably approximately 10 .mu.bar.

Alternative methods for etching silicon can use gas mixtures with a content of SF.sub.6, Cl.sub.2 and helium; or CCl.sub.2 F.sub.2 and O.sub.2 ; or SiF.sub.4, Cl.sub.2 and helium or argon. In a system with approximately 10 flow percent Cl.sub.2 in argon, etching rate ratios of silicon to silicon dioxide of up to 20:1 are obtained. The anisotropic etching step removes the areas of the silicon wafer (1) originally underlying silicon dioxide areas (7) in a depth which corresponds to the desired silicon cantilever beam thickness (FIG. 1e).

Now, silicon dioxide cantilever beam mask (4) and the silicon dioxide masks (2) on the bottom side of the wafer are removed by etching in 5:1 buffered hydro-fluoric acid or by reactive ion etching, as described above. The resulting structure with cantilever beam (41) and tip mask (6) on top is shown in FIG. 1f.

A particular highly anisotropic etching step follows for etching and shaping tip (61, FIG. 1g). First, the basic shape of the tip (61) is produced. At the same time the sidewalls of the resulting `blank` (61) are etched such that a concave shape is obtained. The concave tip sidewalls are a result of ion scattering in the plasma sheath according to an appropriate ratio of pressure to self bias voltage. This is achieved by an Ar/Cl.sub.2 or He/Cl.sub.2 reactive ion etching process. The reactive ion etching process uses a relatively high pressure of about 100 .mu.bar, and a comparatively low self bias voltage of about 300 V DC in a parallel plate reactor having a reactor chamber volume of about 50 liters, a cathode diameter of about 520 .mu.m and an electrode spacing of about 150 .mu.m. A radio frequency power signal applied to the electrodes has a frequency of about 13.56 MHz and a power level to provide a power density of approximately 0.15 watts/cm.sup.2. The etching time is determined by the desired ratio of tip thickness to tip height. Concurrently with the etching and shaping of the tip (61), the cantilever beam is released from the wafer body (1).

The silicon dioxide etching mask (6) is removed by etching with 5:1 or concentrated buffered hydrofluoric acid.

The radius of curvature of the resulting concave sidewalls essentially determines the smallest obtainable cone angle of the future conically shaped tip.

The reactive ion etching step is followed by a highly controlled process step, i.e., thermal oxidation. For the present example, this oxidation is carried out at a temperature of about 1050.degree. C. in water vapor at approximately atmospheric pressure. FIG. 1h shows a sectional view of a phase where the oxidation fronts (8) touch each other at the thinnest point of the blank. As the volume grows during the thermal oxidation, the thickness and height of the blank (61) are increased, which tends to increase the strength of the blank against mechanical damage. The thermal oxidation process is followed by a stripping process which gently removes the thermally grown silicon dioxide (8). The stripping process uses a buffered hydrofluoric acid solution prepared by mixing about one part of an approximately 50 percent by weight aqueous HF solution with about five parts of an approximately 40 percent by weight aqueous NH.sub.4 F solution. The stripping process was carried out by immersing the blank in the buffered acid solution at room temperature without agitation. The resulting structure which comprises cantilever beam (41) and tip (61) is shown in FIG. 1i.

Scanning-electron-microscope micrographs showed that the tip height and the cone angle of the tip could be modified in an extremely controllable manner by adjusting the oxidation time. The radii of the tips made in accordance with the process of this invention were 10 nm or less. Furthermore the tips were characterized by an excellent rotational symmetry. Compared with the prior art processes for making micromechanical sensors, the process of the present invention is characterized by a high flexibility with respect to the tip shape and an extraordinary high yield.

A second example of the process of the invention is described below in accordance with FIG. 2(a to e).

The process steps for producing the multiple-step cantilever beam and tip mask, and the mask on the bottom side of the wafer, as well as the step for thinning down the silicon wafer by etching from the bottom side are identical to those described for the first example above in accordance with FIGS. 1a to 1d.

The silicon dioxide areas abutting the cantilever beam mask were removed by etching, as described above (not shown). Next, the cantilever beam pattern was transferred into the silicon wafer by reactive ion etching in an Ar/Cl.sub.2 or He/Cl.sub.2 ambient at a pressure of about 10 .mu.bar. Subsequently, the silicon-dioxide cantilever beam mask, and the silicon dioxide mask on the bottom side of the wafer were removed by etching, as described in Example 1 (not shown).

Now follows an anisotropic etching step for shaping the tip. As noted below, the tip sidewalls may be either straight or concave. In order to obtain a tip shaft with straight sidewalls, the etching is carried out in an Ar/Cl.sub.2 or He/Cl.sub.2 ambient at a pressure of about 10 .mu.bar, and for a time depending on the desired etch depth, i.e., length of the tip shaft. In order to obtain a tip shaft with concave sidewalls the etching is carried out in an Ar/Cl.sub.2 or He/Cl.sub.2 ambient at a relatively high pressure and a comparatively low self bias voltage, as described in example 1. The resulting structure, comprising silicon substrate (21), cantilever beam (241), tip (261), and tip mask (26) is shown in FIG. 2a.

The reactive ion etching step is followed by a highly controlled process step, i.e. thermal oxidation. As described in example 1, the oxidation is carried out at a temperature of about 1050.degree. C. in water vapors at approximately atmospheric pressure. The resulting structure, with silicon dioxide (28) covering all horizontal and vertical surfaces of structure (261, 241) and substrate (21), is shown in FIG. 2b.

Next, the silicon dioxide (28) is removed from all horizontal surfaces of the structure by reactive ion etching in CF.sub.4 a pressure of about 10 .mu.bar. This etching is interrupted when all silicon dioxide has been removed from the horizontal surfaces. The resulting structure is shown in FIG. 2c.

Next, a pedestal (29) is etched into the cantilever beam (241) to a desired depth, with the layers of silicon dioxide (28) on the vertical surfaces of structure (261, 241) and tip mask (26) acting as masks. The reactive ion etching is carried out in an Ar/Cl.sub.2 or He/Cl.sub.2 ambient at a pressure of about 10 .mu.bar. The pedestal has a height of about 3 to 4 .mu.m measured from the surface of the cantilever beam and a diameter of roughly 1 to 2 .mu.m. Concurrently with the pedestal etching, the cantilever beam is released from the wafer body (21). The resulting structure is shown in FIG. 2d. The reactive ion etching process step is followed by a stripping process which uses 5:1 or concentrated buffered hydrofluoric acid, and gently removes the remaining silicon dioxide masks. The final structure, comprising cantilever beam (241), pedestal (29), and tip (261) with a rectangular cross-section, is shown in FIG. 2e.

The tip (261) may be pointed in a maskless argon ion etching step, utilizing the effect that for the combination of argon gas and silicon substrate, the etch rate is angle-dependent. The process parameters for the pointing of the tip are:--flow: approximately 20 sccm Ar,--pressure: approximately 3 .mu.bar,--RF: approximately 1000 V, and DC: approximately 630 V. The argon ion etching may be carried out in a parallel plate reactor having a cathode diameter of about 520 .mu.m, an electrode spacing of about 150 .mu.m and a reactor chamber volume of about 15 liters.

Alternatively and as mentioned above, the tip shaping can be performed at an earlier process stage, i.e. after the transfer of the cantilever beam pattern into the silicon wafer substrate, the removal of the silicon dioxide cantilever beam mask, and concurrently with the etching of the tip (see FIG. 2a). To obtain a tip shaft with concave sidewalls, the etching is preferably carried out in an Ar/Cl.sub.2 ambient at a high pressure of about 100 .mu.bar. In this case, the original height of the tip shaft (261) has to be about twice the height of the final tip, as the conically shaped tip shaft is divided in practically two equal parts by thermal oxidation and oxide stripping.