Abstract:
A semiconductor strain sensor comprises a semiconductor cantilever probe having a free end and a surface portion for undergoing deformation due to a displacement of the free end. A Schottky junction is disposed on the surface portion of the semiconductor cantilever probe and is positioned to undergo a change in electrical characteristic in response to the deformation of the surface portion. The amount of displacement of the free end of the cantilever probe is detected on the basis of a change in the electrical characteristic of the Schottky junction.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor strain sensor, a method of processing the sensor, and a scanning probe microscope, particularly forming a Schottky-barrier by contacting metal to a beam portion of a probe made of a semiconductor substrate, detecting deflection of the probe as change of characteristic of the Schottky-barrier. 
     In the conventional scanning probe microscope (SPM), an exploring needle is attached at a free end of a probe, and deflection of the probe generated by movement up and down of the exploring needle response to raggedness of surface of a sample has been detected using optical interferometry of an optical polarization technique. However, there has been a problem that conventional microscope needs complex adjustment when using such an optical detecting method. On the other hand, recently a small-size, light-weight semiconductor strain sensor is widely used, which can output a deflection as an electric signal directly. The sensor is adopted for the probe of the SPM. 
     As shown in FIG. 20, for example, a probe-type semiconductor strain sensor comprises a cantilever arm portion (beam portion)  1  having a free end la formed by selectively etching a part of a semiconductor substrate  2  so as to have a U-shape and a gage portion  3  formed near a fixed end (root) of the cantilever arm portion  1 , and the gage portion  3  detects stress/strain at a portion of the cantilever in response to deflection of the free end  1   a , and the strain is converted to an electric signal and outputted. 
     In the conventional semiconductor strain sensor, as described in Japanese Opened Patent No. 5-196458 for example, the gage portion is constructed with piezo resistance. As the electric resistance of the piezo resistance varies by applying stress, deflection is detected by measuring a resistance change of the piezo resistance at the resistance bridge circuit, such as Wheatstone bridge or the like. 
     As above-mentioned, when deflection of the probe is detected as stress/strain applied to piezo resistance, as the resistance rate of change for strain of the piezo resistance, namely voltage or current rate of change, is little and sensitivity is low, not only is a complex bridge circuit needed for the detecting, but an extremely accurate adjustment of each resistance constructing the resistance bridge is also needed. 
     An object of the present invention is to provide a semiconductor strain sensor solving the conventional above-mentioned problem by outputting deflection of the probe with high response speed as a large signal change, a method of processing the sensor, and a scanning probe microscope adopting the semiconductor strain sensor for the probe. 
     SUMMARY OF THE INVENTION 
     To solve the above-mentioned problem, the present invention is characterized by the following means: 
     (1) A semiconductor strain sensor of the present invention comprises a probe having a semiconductor probe supported like a cantilever, a first metal electrode having a Schottky junction on at least a surface of a beam portion of a semiconductor probe, a high concentration contact domain formed at surface of said semiconductor probe, and a second metal electrode connected to the high concentration contact domain. 
     (2) A method of processing a semiconductor strain sensor of the present invention comprises forming a semiconductor probe by etching a semiconductor substrate, forming a high concentration contact domain selectively at a surface of said semiconductor probe, selectively Schottky-joining the first metal electrode at a surface of a beam portion of said semiconductor probe, and contacting a second metal electrode to said high concentration contact domain. 
     (3) In the semiconductor strain sensor of the present invention, a thin film is formed on at least one of main surfaces of said semiconductor probe so that stress strain always appears on at least the Schottky junction. 
     (4) A scanning probe microscope of the present invention uses a semiconductor strain sensor where a Schottky junction domain is formed at a surface of a beam portion as a scanning probe. 
     According to the above-mentioned configuration (1), as stress/strain appears at a Schottky junction and the electric characteristic (diode characteristic) of the Schottky junction is sharply changed when the free end of the probe bends, deflection of the free end can be measured by detecting with a proper detecting circuit. 
     According to the above-mentioned configuration (2), a cantilever-type semiconductor stress/strain sensor having a Schottky junction can be produced easily. 
     According to the above-mentioned configuration (3), as stress/strain always appears without relation with displacement of the free end at Schottky junction and larger stress strain appears at the Schottky junction when the free end of the probe bends, the electric characteristic (diode characteristic) of the Schottky junction can be sharply changed. When a direction of deflection is set opposite to a direction of an exploring needle, the strain is measured with high accuracy as the angle between the surface of the sample and the exploring needle is about 90 degrees. 
     According to the above-mentioned configuration (4), as deflection of a probe is detected as a change of the electric characteristic of a Schottky junction, a surface shape of the sample can be observed with high sensitivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an outline of the present invention. 
     FIG. 2A is a plane view of a first embodiment of the present invention, 
     FIG. 2B is a sectional view taken by  2 B— 2 B line of FIG.  2 A, and 
     FIG. 2C is a sectional view taken by  2 C— 2 C line of FIG.  2 A. 
     FIG. 3A is a view showing a current-voltage characteristic of forward direction of a Schottky junction according to the present invention, and 
     FIG. 3B is a current-voltage characteristic of reverse direction. 
     FIG. 4 is a view showing current-voltage (I-V) characteristic of a Schottky junction and a piezo resistance. 
     FIG. 5 is a current-strain characteristic of a Schottky junction comparing with a piezo resistance. 
     FIG. 6 is a voltage-strain characteristic of a Schottky junction comparing with a piezo resistance. 
     FIGS. 7A to  7 F are sectional views showing a method of processing of the probe according to FIG.  2 . 
     FIG. 8A is a plane view of a second embodiment of the present invention and 
     FIG. 8B is a sectional view taken by  8 B— 8 B line of FIG.  8 A. 
     FIG. 9 is a plane view of a third embodiment of the present invention. 
     FIG. 10 is a plane view of a fourth embodiment of the present invention. 
     FIG. 11 is a plane view of a fifth embodiment of the present invention. 
     FIG. 12 is a plane view of a sixth embodiment of the present invention. 
     FIG. 13 is a plane view of a seventh embodiment of the present invention. 
     FIG. 14 is a sectional view of a space charge layer appearing at a Schottky junction part taken by  14 — 14  line of FIG.  9 . 
     FIG. 15A is a plane view of an eighth embodiment of the present invention, and 
     FIGS. 15B and 15C are a sectional views taken by  15 — 15  line of FIG.  15 A. 
     FIG. 16A is a plane view of a ninth embodiment of the present invention, and 
     FIGS. 16B and 16C are a sectional views taken by  16 B— 16 B line of FIG.  16 A. 
     FIG. 17A is a plane view of a tenth embodiment of the present invention, and 
     FIGS. 17B and 17C are a sectional views taken by  17 B— 17 B line of FIG.  17 A. 
     FIG. 18A is a plane view of the eleventh embodiment of the present invention, and 
     FIG. 18B is a sectional views taken by  18 B— 18 B line of FIG.  18 A. 
     FIG. 19 is a block diagram of main components of a scanning probe microscope according to the present invention. 
     FIG. 20 is a perspective view showing a conventional probe-type semiconductor strain sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the figures, an embodiment of the present invention will be described in detail. FIG. 1 is a perspective view showing a general construction of a main part of a semiconductor probe according to the present invention. A probe  10  comprises an arm portion  10   a  having a free end, an exploring needle  10   c  at a tip of the arm portion, a supporting portion  10   b  holding and fixing the arm portion, a sensor portion  9 , and a thin film  8  (insulation film  40 ) formed at a domain including at least the sensor portion  9 . As bending of the arm portion  10   a  is concentrated on a portion of a root thereof, the sensor portion  9  is desirably formed at a domain including the border of at least the arm portion  10   a  and the supporting portion  10   b . In the present invention, a Schottky junction is used for the sensor portion. 
     An electrode for detecting a signal is connected to the sensor portion with wiring  33 . Low resistance metal material such as mainly Al, W, Ti, Ta, Cr, and so on are used for the wiring material. Among the materials, Al is the most general material. 
     The thin film  8  functions as previously, applying strain (stress) to the sensor portion  9 . The case in which stress is compressive stress is desirable because the direction of the exploring needle  10   c  is almost vertical toward the sample so as to observe accurately during sample observation as the arm portion  10   a  bends to an under side of FIG.  1 . Layer insulation film  30  where stress is applied can replace the insulation film  40 . In this case too, the thin film  8  can be omitted. Stress can be applied to the sensor portion by using a material having stress for wiring  33  so as to replace the thin film  8 . Namely, any material except the insulation film can be used if the thin film  8  applies stress to the sensor portion  9  and does not exert a bad influence such as a short-circuit to the electric characteristic of the sensor portion  9 . 
     Although a shape of the sample can be observed at a tip part of the arm portion  10   a  without the exploring needle  10   c  in the sample observation, it is desirable that the exploring needle  10   c  is formed in order to obtain high accuracy and high resolution. In order to obtain high accuracy and high resolution, it is desirable that the exploring needle  10   c  is longer than raggedness of the observed sample and that the diameter of the tip is small. There are various kinds of shapes such as substantially rectangle-shaped as FIG. 1, U-shaped, or the like so as to design in accordance with use of characteristic in the arm portion  10   a.    
     FIG. 2A is a plane view of a probe-type semiconductor strain sensor of a first embodiment of the present invention, particularly for describing the arm portion  10   b  and the sensor portion  9  of FIG.  1 . FIG. 2B is a cross sectional view taken by  2 B— 2 B line of FIG.  2 A. FIG. 2C is a cross sectional view taken by  2 C— 2 C line of FIG.  2 A. In FIG. 2A, the exploring needle  10   c  and the insulation  40  (thin film  8 ) described later are not shown. 
     The probe  10  of the present invention comprises a U-shaped cantilever arm portion  10   a  and a supporting portion  10   b , and at a tip of the cantilever arm portion  10   a , an exploring needle  10   c  (not shown) for SPM is formed. In the present embodiment, the probe  10  comprises an N-type substrate  31 , and at a surface thereof, an electrode  32  is Schottky-joined in a generally U-shape along an inside of the arm portion  10   a . At a part of the surface of the substrate  31  which does not have the electrode, a layer insulation film  30  is formed. On the other hand, at the supporting portion  10   b , an N +  contact domain  21  is formed on a surface of the N-type substrate  31 , at a surface thereof, electrodes  22  are ohmic-contacted. Electrodes  22  are connected to the wiring  33  of FIG. 1, supplies bias voltage (current), and detects signal. The electrode  22  can be produced with the same material as the wiring  33 , and both can be formed at the same time. 
     An insulation film  40  such as, for example, SiO 2  film, Si 3 N 4  film, or the like is layered at a surface of the N-type substrate  31  as the thin film  8  for always applying stress to a Schottky junction  50 . It is desirable to form the insulation film  40  so that the stress/strain is about 1×10 9  Pa. However, the value is not determined depending on the structure of the junction portion and processing condition. 
     It is not required that the insulation film  40  be formed on the entire surface of the N-type substrate  31 , and the film may be formed, for example, only at surface of the cantilever arm portion  10   a  or only at a border part between the arm portion  10   a  and the supporting portion  10   b  if stress/strain can appear at the Schottky junction  50  of the substrate  31  and the electrodes  32 . 
     In this construction, as the cantilever arm portion  10   a  of the probe  10  bends at the supporting portion  10   b  as a fulcrum when the exploring needle  10   c  displaces toward a vertical direction to the plane of the paper, strain/stress appears at the Schottky junction  50  formed at the arm portion  10   a , especially at a surface of the beam portion. 
     FIGS. 3A and 3B are views showing an example of a diode characteristic of a Schottky junction changed by stress/strain. FIG. 3A shows the characteristic at forward bias and FIG. 3B shows the one at reverse bias. It is noticed that forward direction current to forward bias changes when stress/strain appears at the Schottky junction  50  during forward bias. During reverse bias too, break voltage changes and leakage current too changes when stress/strain appears. 
     FIGS. 4 to  6  are views of current-voltage (I-V) characteristics of the Schottky junction, current I-strain characteristic, and voltage V-strain characteristic comparing with piezo resistance. Changing rate of current I and voltage V to stress are larger in the Schottky junction than in the piezo resistance at stress more than some value as shown in FIGS. 5 and 6. Therefore, by forming the Schottky junction  50  at the beam portion of the probe  10  and by detecting a change of the diode characteristic at large domain in the changing rate like the present embodiment, the detecting sensitivity of strain improves so as to correctly measure deflection of the probe  10  without using a bridge circuit such as Wheatstone bridge and the like. 
     The change rate of the diode characteristic to deflection of the probe is larger at larger domain in deflection, and sufficient detecting sensitivity can not be obtained when deflection is small at smaller domain in deflection. In the smaller domain in stress/strain, a signal process becomes rather complex because current I changes like secondary function to stress/strain as shown in FIG.  5 . 
     Compared with that, stress/strain is always applied to the Schottky junction  50  by applying the insulation film  40  (the thin film  8 ) in the present invention. Therefore, strain appearing transiently depending on displacement during observation and stress/strain always appearing by the insulation film  40  are superimposed. Because of that, larger strain appears compared to the case in which the insulation film  40  is not formed. In the large domain in stress/strain, signal processing at actual observation becomes easy as current I changes substantially linearly to stress/strain. The above-mentioned change depends on the structure of the Schottky junction and processing condition. Therefore, the changing quantity of the characteristic is not fixed and shows similar characteristic as piezo resistance depending on a condition. 
     FIGS. 7A to  7 F are sectional views of process steps showing a method of processing said probe-type semiconductor strain sensor of the structure described in FIG. 2A, particularly showing a sectional structure taken by  2 C— 2 C line of FIG.  2 A. 
     First, an N-type semiconductor substrate  31  is etched like the probe shape of FIG. 2, and at all of surface of one surface, resist  81  is applied. Next, a mask is formed removing selectively only a resist of a part equivalent to the N +  contact domain  21  of FIG. 2 by the well-known photo resist technique (FIG. 7A) . Next, N-type impurity (for example, phosphorus) is ion-implanted so as to form an N +  contact domain  21  at a surface of the substrate  31  (FIG. 7B.) 
     Next, SiO 2  film is formed at the surface of substrate for insulation film  30 , and the part equivalent to the N +  contact domain  21  and the part equivalent to the Schottky junction  50  of FIG. 2 are opened so as to expose the N +  contact domain  21  and the Schottky junction domain (FIG. 7C.) 
     Next, the Schottky junction  50  is formed by ohmic contacting an electrode  22  to the N +  contact domain  21  and by Schottky-joining an electrode  32  to the Schottky junction domain (FIG. 7D.) 
     Next, an insulation film  40   a  is formed to always subject the Schottky junction  50  to stress/strain (FIG.  7 E.). Although the insulation film  40  ( 40   a ) is formed in the present invention, the insulation film  40  ( 40   b ) may be formed at the back of the substrate (FIG. 7F.) 
     FIG. 8A is a plane view of a semiconductor probe of a second embodiment according to the present invention. FIG. 8B is a cross sectional view taken by  8 C— 8 C line of FIG.  8 A. The same symbols as said symbols show the same or similar portions. The present embodiment is characterized by that the Schottky junction  50  is actually formed at the whole surface of the arm portion  10   a.    
     That is, in the above-mentioned first embodiment, the area of the Schottky junction is comparatively small as the Schottky junction  50  is formed at only a part of surface of the arm portion  10   a . Because of that, while leakage current is little, it is difficult to obtain high sensitivity. However, the second embodiment is characterized by that high sensitivity is obtained though leakage current increases a little comparing with said first embodiment as the Schottky junction  50  is formed at whole surface of the arm portion  10   a.    
     Next, referring FIGS. 9 to  13 , other embodiments of the present invention will be described. Said insulation film  40  (thin film  8 ) for always subjecting the Schottky junction  50  to stress/strain may be formed on at least one of surface or the back in any embodiment though the film is not shown in each figure. 
     FIG. 9 is a plane view of a third embodiment of the present invention. The same symbols as said symbols show the same or similar portion. The embodiment is characterized by that an electrode  32 , namely a Schottky junction  50 , is generally band-shaped at a center portion of the arm portion  10  so that the Schottky junction is not exposed at an end surface of the arm portion  10   a . Although leakage current generally appears near by the end surface of the PN junction, high sensitivity is obtained while depressing leakage current though the manufacturing process becomes complex a little according to the present invention as the Schottky junction  50  is not exposed at end surface of the probe  10 . 
     FIG. 10 is a plane view of a fourth embodiment, of the present invention. The same symbols as said symbols show the same or similar portion. The present embodiment is done from the point of view that a strain caused by displacement applying to the probe  10  concentrates on a border part of the arm portion  10   a  and the supporting portion  10   b , namely a beam portion of the probe  10 , and that except at these portions, the strain becomes least. 
     The present embodiment, as shown in the figure, is characterized by that an electrode  32  (a Schottky junction  50 ) is formed at only said beam portion. According to the present embodiment, high sensitivity is obtained while depressing leakage current as the Schottky junction is not formed at a part not contributing detection of strain. 
     FIG. 11 is a plane view of a fifth embodiment of the present invention. The same symbols as said symbols show the same or similar portion. The present embodiment is characterized by that an electrode  32  (a Schottky junction  50 ) is formed at only the beam portion of the probe  10 , and is generally band-shaped at a center portion of the probe  10  in order to decrease leakage current, as set fourth above for said fourth embodiment. 
     FIGS. 12 and 13 are plane views of a sixth and a seventh embodiment of the present invention. The same symbols as said symbols show the same or similar portion. Each embodiment is characterized by that respective electrode  32  (the Schottky junction  50 ) of said fourth and fifth embodiments is formed at only one of said beam portion. According to the embodiment, leakage current sharply decreases though detecting sensitivity decreases a little. 
     Although it is described that a metal electrode is connected to the N-type substrate  31  to obtain the Schottky junction in each of the above mentioned embodiments, conversely a metal electrode may be contacted to the P-type substrate to obtain the Schottky junction. 
     In the configuration forming the Schottky junction  50  so as not to expose at an end surface of the probe, such as the embodiments described in FIGS. 9,  11 , and  13 , leakage current by spreading of space charge layer  49  increases since reverse bias is applied to the Schottky junction  50  as shown in FIG.  14 . In usual use, leakage current appears at the end portion of the arm portion  10   a . Because of that, a new problem may appears due to a decrease in the measuring sensitivity. Therefore, in each embodiment of the present invention described below, increase of leakage current is depressed. 
     FIG. 15A is a plane view of an eighth embodiment of the present invention. FIGS. 15B and 15C are sectional views taken by  15 B— 15 B line of FIG.  15 A. These figures are particularly shown enlarging near beam portion bending corresponding to displacement of the arm portion  10   a . In the present embodiment, an electrode  32  is generally band-shaped at a center portion of the beam portion of the probe  10  so that the Schottky junction  50  does not expose at both end surfaces of the probe  10  as shown in FIG.  15 A. An N +  contact domain  21   a  is formed at surface of an N-type substrate  31  exposed between an end portion  10   a  and the Schottky junction  50  serving as prevention of enlarging space charge layer and so on. 
     According to the configuration, spread of horizontal direction of the space charge domain  49  or the like appearing at circumference of the Schottky junction is prevented by the N +  contact domain  21   a . Therefore, leakage current is depressed so as to prevent decrease of measuring sensitivity. 
     By forming an N-type semiconductor domain  47  at a main surface of a P-type substrate  48  with an island-shape and by forming the above-mentioned configuration at the N-type semiconductor domain  47 , leakage current is more decreased by separation with a PN junction. 
     FIG. 16A is a plane view of a ninth embodiment of the present invention. FIGS. 16B and 16C are sectional views taken by  16 B— 16 B line of FIG.  16 A. The same symbols as said symbols show the same or similar portion. The present embodiment is characterized by an electrode  32  that has a generally band shape at a center portion of a probe  10  so that the Schottky junction  50  is not exposed at both end surfaces of the probe  10  and an N +  contact domain  21   b  is formed to surround the Schottky junction  50  as shown in FIG.  16 A. 
     In the present embodiment too, spread of horizontal direction of the space charge layer  49  and so on is prevented by the N +  contact domain  21  so as not to reach the end portion of the arm portion  10   a . Therefore, the measuring sensitivity is kept high because an increase of leakage current is depressed. By forming the above-mentioned configuration at an N-type semiconductor domain  47  as shown in FIG. 16C in the present embodiment too, leakage current is more decreased by separation with a PN junction. 
     FIG. 17A is a plane view of a ninth embodiment of the present invention. FIGS. 17B and 17C are sectional views taken by  17 B— 17 B line of FIG.  17 A. The same symbols as said symbols show the same or similar portion. The present embodiment is characterized by that an electrode  32  is formed at center portion of a probe  10  so that the Schottky junction  50  is not exposed at both end surfaces of the probe  10  and an N +  contact domain  21   c  is formed to surround the Schottky junction  50  from three directions. 
     In the present embodiment too, leakage current appearing at a circumference of the Schottky junction  50  is depressed by the N +  contact domain  21   c  and the leakage current does not reach the end portion of the probe  10 . Therefore, a measuring sensitivity is kept high because an increase of leakage current is depressed. By forming the above-mentioned configuration at an N-type semiconductor domain  47  as shown in FIG. 17C in the present embodiment too, leakage current is more decreased by separation with a PN junction. 
     FIGS. 18A and 18B show an example in which the sensor of the ninth embodiment of the present invention (FIG. 16) is actually formed on a rectangular arm portion. FIG. 18A is a plane view of the embodiment. FIG. 18B is a sectional view taken by  18 B— 18 B line of FIG.  18 A. The same symbols as said symbols show the same or similar portion. In the eighth to tenth embodiments, a width of the sensor becomes wide as a Schottky junction  50  is formed at a center portion of the sensor portion, and at a circumference portion, an N +  contact domain  21  is formed. Therefore, in order to make a width of the arm portion  10   a  small, a rectangular arm is easier than a U-shape arm as a design for the arm portion. 
     Although it is described that a thin film is used for the insulation film  40  in each embodiment, the present invention is not so limited. The thin film for subjecting the Schottky junction to stress/strain may be a conductive film or a semiconductive film so long as the insulation film is previously formed at a surface of substrate without relation to stress/strain and the thin film is formed at the insulation film. 
     FIG. 19 is a block diagram showing a configuration of the scanning probe microscope using the present invention. A sample  52  is set on a three dimensions sample stage  55 , and an exploring needle  31   c  of the probe  10  configured like the above-mentioned is arranged facing the stage. A diode characteristic of a Schottky junction formed at the probe  10  is detected at a measuring portion  71  and inputted to a non-inversion input terminal (+) of a differential amplifier  75  as a bending signal S 1 . 
     A reference value relative to bending of the probe  10  is inputted to an inversion input terminal (−) of the differential amplifier  75  so that output of the differential amplifier  75  becomes zero when bending is zero for example. An error signal S 2  outputted from the differential amplifier is inputted to a control portion  76 . The control portion  76  controls an actuator driving amplifier  70  so that the error signal S 2  approaches zero. Output signal of the control portion  76  is supplied to a CRT as a luminance signal. A scanning signal generating portion  78  supplies a differential signal for slightly moving the sample  52  toward XYZ directions to the actuator driving amplifier  70 , and a raster scanning signal is supplied to the CRT. 
     As above-mentioned, the present invention has the following advantages: 
     (1) As a Schottky junction in which an electric characteristic sensitively changes in response to strain is formed at a probe so as to detect bending of the probe as a change in the electric characteristic of Schottky junction, not only sensitivity to bending of the probe increases, but also a configuration of a detecting circuit connected to the next stage. 
     (2) As a high concentration contact domain is formed between a Schottky junction and end portion of a probe so that space charge layer and so on appearing at the Schottky junction does not reach the end portion of the probe, measuring sensitivity can be kept high and increase of leakage current is suppressed. 
     (3) By forming a thin film at main surface of the probe so as to subject to Schottky junction domain to stress/strain when the probe bends always have stress/strain, sensitivity to deflection of the probe can be improved. 
     (4) By using the semiconductor strain sensor of the present invention as a probe of a scanning probe microscope, a surface shape of the sample detected as a strain value of the probe is regarded as a change of an electric characteristic of the Schottky junction. Therefore, the surface shape of the sample can be observed with high sensitivity.