Abstract:
The present invention relates to a method of forming thin bodies of a semiconductor material, such as single crystalline silicon, by selectively etching away a portion of the body until a body of the desired thicknes is obtained. The body includes a p-n junction made up of adjacent regions of p-type and n-type conductivity which are immersed in a chemical etchant with the surface of the p-type region being exposed to the etchant. A time varying voltage waveform having first and second voltage levels is applied through the etchant to the p-n junction. The first voltage level causes a zero forward bias across the p-n junction and the second voltage level causes a reverse bias across the p-n junction. The p-type region is removed by the chemical etchant down to the n-type region. This essentially removes the p-n junction in the area where the p-type region is etched away and resulting current through the n-type region causes a passivating layer to form on the surface of the n-type region which stops further etching.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of selectively electrochemically etching silicon and, more particularly, to a method of electrochemically etching a silicon body which has an automatic stop to control the thickness to which the body is etched. 
     BACKGROUND OF THE INVENTION 
     In the electronic field there are a number of uses for very thin bodies of single crystalline silicon. For example, in the article of J. A. Oakes entitled, &#34;A Pressure Sensor for Automotive Application,&#34; published in the Proceedings of Third International Conference on Automotive Electronics, October, 1981, pages 143-149, there is described a pressure sensor formed of single crystalline silicon and having a thin diaphragm region. In order to form the thin silicon bodies, such as the thin diaphragm region of the pressure sensor of J. A. Oakes, it has been the practice to etch a thicker silicon body down to the desired thickness. 
     Heretofore, an anisotropic etching technique has been used to form the bodies of silicon since such etching technique selectively etches more slowly along certain crystallographic planes, such as the &lt;111&gt; plane, than along other planes, such as the &lt;100&gt; and &lt;110&gt; planes, in etchants with sufficiently high pH values. Typical etchants include KOH, NaOH, LiOH, CsOH, NH 4  OH, ethylenediamine pyrocatechol, and hydrazine. In addition to the crystal orientation selectivity of these etchants, a sufficiently high constant positive bias applied between a silicon body and an electrode both immersed in the etchant results in a current flow which causes the formation of a silicon dioxide layer on the surface of the body. The silicon dioxide layer resists etching and thus effectively prevents etching of the silicon body. This phenomena is referred to as &#34;electrochemical etch stop.&#34; The bias required to passivate the surface of the silicon body is denoted as the &#34;passivating potential (voltage).&#34; 
     The electrochemical etch stop has been utilized in forming the bodies of silicon by starting with a body having a region of n-type conductivity adjacent a region of p-type conductivity which form a p-n junction. The n-type region is formed of the thickness desired for the thin body. The starting body is placed in the chemical etchant with the surface of the p-type region being exposed for etching, and a constant voltage is applied between the n-type conductivity region of the starting body and an electrode in the chemical etchant which is spaced from the starting body. This constant voltage reverse biases the p-n junction so that ideally no current flows into the p-type and n-type regions. The p-type region is anisotropically etched away by the chemical etchant until it is completely removed. After the p-type region is completely removed (etched away), the n-type region is exposed to the etchant. The n-type region now acts as a relatively low resistance resistor with a voltage thereacross which gives rise to a current therethrough. This current flow through the n-type region, which is at a voltage which facilitates the formation of a passivation layer (e.g., silicon dioxide), causes a passivation layer of silicon dioxide to be formed over the surface of the n-type region which stops the chemical etching. This leaves a thin silicon body of the desired thickness. 
     A problem with this type of electrochemical etching is that some reversed biased p-n junctions are much more leaky (i.e., they conduct current therethrough in the reverse direction) than others. This leakage is the result of the reverse resistance of the p-n junction which varies considerably from lot to lot. As a result, the voltage on the surface of the p-type region varies from lot to lot of p-n junctions. With current flow through the p-n junction and a sufficiently positive voltage on the etching surface of the p-type region, a passivation layer can form over the etching p-type region which cuts off further etching and therefore prevents the desired thickness of the semiconductor wafer from being achieved. 
     It is desirable to have an electrochemical etching technique for use with p-n junctions which is relatively insensitive to variations in reverse bias leakage through the p-n junctions. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method of selectively electrochemically etching a body of semiconductor material in which a time varying voltage having first and second voltage levels is applied across the body while it is subjected to the chemical etch. This selectively removes a portion of the body of p-type conductivity type while stopping when a region of n-type conductivity type is reached. 
     Viewed from one aspect, the present invention is directed to a method of removing portions of a p-type conductivity semiconductor layer of a p-n junction comprising an n-type semiconductor layer with a p-type layer adjacent thereto. The method comprises the steps of placing the p-n junction in a chemical etchant, and applying a time varying voltage waveform across the p-n junction which varies in voltage level from a first level that provides an essentially zero forward bias across the p-n junction to a second level which provides a reverse bias voltage across the p-n junction so as to etch away a portion of the p-type layer. 
     Viewed from another aspect, the present invention is directed to a method of removing portions of a p-type conductivity semiconductor layer of a p-n junction comprising an n-type semiconductor layer with a p-type layer adjacent thereto. The method comprises the steps of placing the p-n junction in a chemical etchant, and applying a time varying voltage waveform across the p-n junction which varies in voltage level from a first level which is less than a potential required to passivate both the n-type and p-type layer to a second level which is greater than a potential required to passivate the n-type layer. 
     The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an apparatus which can be used to carry out the method of the present invention. 
     FIG. 2 shows a voltage waveform which is applied to the apparatus of FIG. 1 in accordance with the method of the present invention. 
     FIG. 3 is an electrical equivalent of the apparatus of FIG. 1 when the method of the present invention is utilized. 
     FIG. 4 is a schematic view similar to FIG. 1 showing the semiconductor body after it has been etched. 
     It should be understood that the drawings are not necessarily drawn to scale. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown an electrochemical etching apparatus 10 which is useful for etching silicon and which can be used to carry out the method of the present invention. Apparatus 10 comprises a container 12 which contains a liquid chemical etchant 14. A wafer support electrode 16 is in the container 12 and has a wafer 18 of a semiconductor material, such as silicon, to be etched thereon. The wafer 18 has thereacross a region 20 of n-type conductivity and an adjacent region 22 of p-type conductivity which form a p-n junction 24. Electrode 16 makes physical and electrical contact to region 20 but is isolated from the etchant 14 by a masking layer 25. The n-type region 20 is of a thickness equal to desired thickness of the body to be formed. The wafer 18 is mounted on the electrode 16 with the n-type region 20 seated against the electrode 16 and the p-type region 22 being exposed along a surface 22a thereof to the chemical etchant 14 If only a portion of the p-type region 22 is to be removed, the masking layer 25 is coated over the periphery of the wafer 18 except for a portion of the surface 22a of the p-type region 22. 
     A counter electrode 26 is within the chemical etchant 14 and is spaced from the wafer 18. The counter electrode 26 and the support electrode 16 are electrically connected to a potentiostat 28. A reference electrode 30 is within the chemical etchant 14 between the counter electrode 26 and the wafer 18. The reference electrode 30 is also electrically connected to the potentiostat 28 so as to permit adjustment of the voltage between the support electrode 16 and the electrode 26. The voltage between electrodes 16 and 26 is adjusted to reach a desired voltage between electrodes 16 and 30. 
     In a prior art operation of apparatus 10, a fixed bias (voltage) is applied between electrodes 16 and 26 so as to reverse bias p-n junction 24. Ideally, there is little current flow through the p-n junction 24 and thus surface 22a does not have a passivation layer formed thereon. Under these conditions, the surface portion 22a of the p-type layer 22 etches away until a surface of the n-type region 20 is exposed to the chemical etchant 14. As soon as a surface of n-type region 20 becomes exposed to the chemical etchant 14, the p-n junction 24 is eliminated and current begins to flow from electrode 16, through the n-type region 20, and into the chemical etchant 14. If there is current flow through the n-type region 20 into the chemical etchant 14 and the voltage of exposed surface of region 20 is greater than the passivation potential of n-type silicon, then a protective silicon dioxide layer forms on the exposed surface of region 20. This silicon dioxide layer prevents further etching of region 20 since it is resistant to etching. One serious problem with this prior art method of operation of the apparatus 10 of FIG. 1 is the existence of reverse bias leakage through the p-n junction. This reverse bias leakage is difficult to predict and is indicative of the reverse bias resistance of the p-n junction being lower in some cases than is desired. As a result of the decreased reverse bias resistance, the voltage drop across the p-n junction decreases and thereby results in the voltage at the surface 22a becoming more positive than the passivation potential of p-type silicon. This results in a passivating silicon dioxide layer being formed on surface 22a which prevents further etching and thus does not allow the desired end result which is the complete removal of a portion of region 22 so as to expose a surface of region 20. 
     The method of the present invention comprises placing in a liquid chemical etchant a body of a semiconductor material, such as single crystalline silicon, having adjacent regions of opposite conductivity type, i.e., n-type conductivity and p-type conductivity, forming a p-n junction therebetween. The body is positioned with the p-type region exposed to the chemical etchant. A time varying waveform (bias) is applied between the body and an electrode which is in the chemical etchant and is spaced from the body. The waveform is varied between two voltage levels, such as, for example, -1.6 volts and 0 volts using a Ag/AgCl electrode 30. The voltage can be scanned between the two values at any rate at which the p-type region etches and the n-type region does not etch. This rate can vary from 0.1 mV/sec. to 100 V/sec. At different scan rates, the selectivity of etching between the p-type region and the n-type region may vary. The voltage may be scanned along different waveforms, such as triangular, sinusoidal or pulsed. The particular etchant, its concentration and temperature may vary, as long as the etchant effectively etches the semiconductor material. For silicon etchants such as KOH, NaOH, LiOH, CsOH, NH 4  OH, ethylenediamine pyrocatechol and hydrazine at concentrations of 5 to 95% and at a temperature of between 10° C. and 120° C. may be used satisfactorily. 
     In the operation of the etching apparatus 10 of FIG. 1 in accordance with the method of the present invention, the exposed area of the p-type region 22 of the wafer 18 is etched by the chemical etchant 14. FIG. 2 graphically shows a voltage versus time illustrative waveform which is applied to the electrode 16 by the potentiostat 28 referenced to the electrode 26. The voltage levels of the waveform are -0.5 volts and -1.6 volts referenced to the electrode 30 when same is a Ag/AgCl electrode. The -1.6 volt level results in the p-n junction 24 having a zero forward bias thereacross and there being essentially no current flow therethrough. The -0.5 volt level results in the p-n junction 24 being reverse biased by about 0.9 volts. As such, p-n junction 24 can have leakage current flow therethrough from region 20 to region 22. The voltage levels of the waveform applied to electrode 16 can be -1.6 volts and 0 volts. 
     Referring now to FIG. 3, there is shown an approximate equivalent electrical circuit of the apparatus 10 of FIG. 1 when it has the voltage waveform of FIG. 2 applied to electrode 16 and 0 volts applied to electrode 26. The equivalent circuit comprises the reverse bias resistance Rrb of the p-n junction 24 in series with the parallel combination of the contact resistance Rc of surface 22a with the etchant 14 and the capacitance Ce-p between the surface 22a of p-type layer 22 and the etchant 14 which are in series with the resistance Re of the etchant 14. The arrow through Rrb indicates that the resistance through a reversed biased p-n junction varies considerably from lot to lot of p-n junctions. It is to be noted that there is a capacitance of the reverse biased p-n junction in parallel with Rrb which is not shown since same is typically of negligible value in this application. 
     In the prior art method discussed hereinabove, Ce-p plays essentially no role in the operation since a fixed DC voltage is applied between electrodes 16 and 26. Typically, Rrb and Rc are of relatively high value resistances and Re is a relatively low value resistance. Accordingly, variations in Rrb can easily cause an undesirably large change in the voltage of surface 22a when current flows through the p-n junction 24 in the reverse direction. This current flow, when coupled with region 20 being at a voltage which is greater than the passivation potential of p-type silicon, can result in the forming of a passivating layer on p-type layer 22 which then prevents further etching therethrough. 
     The impedance of Ce-p is a function of the type, temperature and concentration of the etchant used and is proportional to the inverse of the frequency of the waveform applied to electrodes 16 and 26. Accordingly, proper selection of the etchant 14 and the frequency of the applied waveform controls the impedance of Ce-p. Since Ce-p is in parallel with Rc, the impedance of the parallel combination is controlled by the element with the lower impedance. Thus, if the impedance of Ce-p is low, then the parallel combination of Ce-p and Rc is even lower. The low impedance of this parallel combination is in series with the relatively low impedance of Re and thus the equivalent impedance is relatively low compared to the impedance of Rrb. Accordingly, the voltage of surface 22a closely follows that of electrode 26 even with variations in the value of Rrb. It is thus easier to maintain the voltage of surface 22a at a level more negative than the passivation potential of p-type silicon using the method of the present invention as compared to the prior art method and thus to increase yield and thereof lower the cost of manufacture. 
     Applicants&#39; recognition of Ce-p, and the use of an AC biasing waveform so as to utilize Ce-p, allows etching completely through layer 22 before a passivating layer is formed and thus increases yield and reduces cost of manufacture. 
     Referring now to FIG. 4, there is shown the apparatus 10 after the p-type region 22 has been completely etched through using the method of the present invention. When the voltage across electrodes 16 and 26 is greater than the passivation potential of n-type silicon, a current flows through the n-type region 20 and these conditions result in the formation of a passivating layer (e.g., silicon dioxide) 32 on the surface of the n-type region 20. The voltage on electrodes 16 and 26 now result in a current through the n-type region 20 which forms a passivating layer (e.g., silicon dioxide) 32 of silicon dioxide on the surface of the n-type region 20. This stops the etching to leave a portion of the wafer 18 of the desired thickness. The frequency of the voltage waveform of FIG. 2 has to be great enough so that there is insufficient time at or near the lower voltage level for the passivating layer 32 to completely etch away and expose the silicon therebelow to the etchant. A wafer 18 with a central portion of a p-type layer 22 removed therefrom can serve as a basic portion of a pressure sensor with the exposed portion of the n-type region acting as a diaphragm. 
     We placed a wafer of single crystalline silicon of p-type conductivity on a support electrode 16 in a chemical etchant 14 of a 50% aqueous solution of CsOH at a temperature of 60° C. A platinum counter electrode 26 and a Ag/AgCl reference electrode 30 were immersed in the chemical etchant. A bias voltage was applied between the support electrode and the counter electrode and, using the potentiostat 28, the voltage was varied to provide a scanning along a triangular waveform of between -1.6 volts and -0.05 volts as measured between the support electrode and the reference electrode. The potentiostat 28 used is an EG&amp;G model number 273. The scan rate was 20 mV/sec. and was controlled by the potentiostat. Under continuous scanning, the p-type wafer did not passivate in the 50% solution of CsOH and etched at a rate of 11 micrometers per hour. This etch rate is identical to that measured in 50% CsOH at 60° C. without an applied voltage. However, an n-type wafer treated under the same conditions was passivated under continuous scanning and its etch rate was negligible. Therefore, when a substrate having contiguous p-type and n-type regions is subjected to the chemical etch with the continuous scanning of the voltage, the p-type layer should be completely removed and the etching should stop at the n-type region when the passivating layer is formed. 
     The structure of the partially etched away silicon wafer 18 shown in FIG. 4 is obtainable using an other method invented by the same inventors which is disclosed and claimed in a co-pending U.S. patent application Ser. No. 07/670,119, now abandoned, entitled, &#34;Selective Electrochemical Etching,&#34; and filed concurrently with the present application and having a common assignee. 
     It is to be appreciated and understood that the specific embodiments of the invention are merely illustrative of the general principles of the invention. Various modifications may be made consistent with the principles set forth. For example, instead of a body of silicon, the semiconductor body may be any other semiconductor material which can be chemically etched and which will selectively form a passivating layer when a current is passed therethrough.