Selective electrochemical etching

Abstract: A method of selectively etching a body (18) of a semiconductor material, such as single crystalline silicon, having regions (20,22) of n-type and p-type conductivity to remove at least a portion of the n-type region. The body is placed in an etching solution of an etchant (14) having a high pH value and a positive voltage is applied between the body and the etchant. This forms passivating layers on the surfaces of the two regions with the passivating layer on the n-type region being different from that on the p-type region. The voltage is then removed and the body is etched for a period long enough to remove all of the passivating layer from the n-type region and at least a portion of the n-type region, but is not long enough to remove all of the passivating layer from the p-type region. This is allowed by the difference between the passivating layers on the two regions. The steps of forming the passivating layers and etching them is repeated until a desired amount of the n-type region is removed.

Description: The present invention relates to a method of selectively electrochemically etching semiconductor materials of opposite conductivity type and, more particularly, to a method of selectively electrochemically etching n-type conductivity semiconductor material. Heretofore, an anisotropic etching technique has been used to form bodies of silicon using the property that certain crystallographic planes of a semiconductor material will etch differently than other planes. For example, for single crystalline silicon, etching takes place slower along the <111> planes than along the <100> and <110> 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 electrode both immersed in the etchant will also passivate the surface of the body and stop the etching. This phenomena is referred to as "electrochemical etch stop." The bias required to passivate the surface of the silicon body is denoted as the "passivating potential (voltage)." The electrochemical etch stop has been utilized in forming the bodies of silicon by starting with a body having a thin region of n-type conductivity adjacent a region of p-type conductivity with a p-n junction therebetween. 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 positive bias at or more positive than the passivating potential is applied between the starting body and an electrode in the chemical etchant and spaced from the starting body. This positive voltage reverse-biases the p-n junction so that no current flows into the p-type region. The p-type region is anisotropically etched away by the chemical etchant until it is completely removed. When the p-type region is completely removed so as to expose the n-type region, the positive voltage between the n-type region and the electrode causes a passivation layer of silicon oxide 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. An example of a published patent document which discloses this foregoing technique is FR-2,070,873 (Western Electric). A problem with this type of electrochemical etching is that a constant positive bias can cause indiscriminate etch stopping on both p-type and n-type silicon if current flows through the silicon body and into the etchant. Therefore, it is necessary to have a reverse biased junction in order to prevent the flow of current through the p-type region and selectively remove the p-type region while stopping only when the n-type region is reached. However, if there are large reverse leakage currents passing through the junction, the p-type region will be prematurely passivated causing the etching to stop before all of the p-type region is removed. Therefore, it would be desirable to have an electrochemical etching technique which does not rely on the reverse biased junction to control the etching. Also, this technique removes the p-type material leaving the n-type material, so that it cannot be used in cases where a thin layer of p-type material is desired. EP-A-0503693, comprised in the state of the art in accordance with Article 54 (3),(4) EPC, discloses a method for selective etching of p-type regions from a body comprising p-type and n-type regions. A method in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. A method of the present invention includes the step of forming a passivating layer on a surface of a region of each conductivity type in a body of a semiconductor material having regions of opposite conductivity type. The body is then etched in a chemical etchant for a period which removes the passivating layer on the one region and a portion of the one region, and a portion of the passivating layer on the other region. The present invention preferably provides a method of selectively etching a body of a semiconductor material, such as single crystalline silicon, having regions of opposite conductivity type, such as p-type and n-type, by removing some or all of the n-type region so as to leave the p-type region. The method preferably comprises first subjecting the body to a positive voltage bias while immersed in a chemical etchant having a high pH value to form a passivating layer on each portion of the body and then removing the bias to allow etching to take place long enough to remove the passivating layer from the n-type region and a portion of the n-type region while not removing all of the passivating layer from the p-type region. These steps of forming and removing the passivating layers are repeated until a desired amount of the n-type region is removed. The etchant may be KOH, NaOH, LiOH, CsOH, NH 4 OH, ethylenediamine pyrocatechol, hydrazine or the like. An electrode is also immersed in the etchant, and the body and electrode are connected across a voltage source. A positive voltage is applied between the body and the electrode through the etchant for a given period of time. This forms an oxide passivating film on both regions of the body. It has been found that the passivating film formed on the p-type region is different from the passivating film formed on the n-type region. It has not been found whether the difference between the passivating films on the p-type region and n-type region is a matter of film thickness or film composition which has different etching properties. However, it has been found that the passivating films formed on the two regions over the same period of time will be etched away differently in that the passivating film on the n-type region is etched away faster than the film on the p-type region. Using this discovery, the voltage is turned off which causes the passivating films to be etched by the etchant. The etching is carried out for a period of time which removes all of the passivating film from the n-type region and a portion of the n-type region, but does not completely remove all of the passivating film from the p-type region. The positive voltage is then again applied between the semiconductor body and the electrode to reform a passivating film on both the p-type region and the n-type region. The voltage is turned off and the etching step is repeated. The two steps of forming the passivating film on the regions of the semiconductor body and etching completely away the passivating film from the n-type region and a portion of the n-type region are repeated until the n-type region is completely etched away leaving only the p-type region or an n-type region of a desired thickness if required. The present invention will now be described, by way of example, with reference to the following more detailed description, taken with the accompanying drawings, in which:- Figure 1 is a schematic view of an apparatus which can be used to carry out a method of the present invention; Figure 2 is a graph showing the open circuit potential vs. time for semiconductor materials of different conductivity types during the etching process of the present invention; Figure 3 is a graph showing the open circuit recovery time vs. amplitude of applied potential for semiconductor materials of different conductivity types during the process of the present invention; Figure 4 is a graph similar to Figure 3 except that the concentration of the etchant is changed; Figure 5 is a graph similar to Figure 3 except that the temperature of the etchant is changed; and Figure 6 is a schematic view of another apparatus which can be used to carry out a method of the present invention. It should be understood that the drawings are not necessarily to scale. Referring now to Figure 1, there is shown an electrochemical etching apparatus 10 which is useful for etching silicon and which can be used to carry out a 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 body 18 of a semiconductor material, such as silicon, to be etched thereon. The body 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. Wafer support electrode 16 extends across the outer edge of the body 18 so as to make physical and electrical contact to the body 18 while leaving the surfaces of both of the regions 20 and 22 exposed to the liquid chemical etchant 14. A masking layer 26 is coated over the wafer support electrode 16 to protect it from the etching solution. The p-type region 22 is of a thickness equal to the desired thickness of the body to be formed. If only a portion of the n-type region 20 is to be removed, a masking layer may also be coated over the portion of the n-type region 20 which is not to be etched away. In order to further protect wafer support electrode 16 from the etching solution as portions of body 18 are etched away, masking layer 26 can be extended to cover top portions of regions 20 and 22 as is shown by the dashed lines. A counter electrode 28 is within the liquid chemical etchant 14 and is spaced from the body 18. The counter electrode 28 and the wafer support electrode 16 are electrically connected to a potentiostat 30. A reference electrode 32 is within the liquid chemical etchant 14 between the counter electrode 28 and the body 18. The reference electrode 32 is also electrically connected to the potentiostat 30 so as to permit adjustment of the voltage between the wafer support electrode 16 and the counter electrode 28. In using the apparatus 10 to carry out the method of the present invention, the potentiostat 30 is turned on and adjusted to provide a positive voltage greater than zero volts between the (semiconductor) body 18 and the counter electrode 28 for a desired period of time. The voltage is measured between the (semiconductor) body 18 through the wafer support contact 16 and the reference electrode 32 which measurement is used to adjust the potentiostat 30 to achieve a desired voltage between the (semiconductor) body 18 and the counter electrode 28. This forms a passivating film on the surface of both the n-type region 20 and the p-type region 22 with the passivating film on the p-type region 22 being different from that on the n-type region 20. The voltage source is then turned off. This allows the liquid chemical etchant 14 to etch the passivating films. The etching is carried out for a time period which removes all of the passivating film from the n-type region 20 and a portion of the n-type region 20 while not completely removing all of the passivating film from the p-type region 22. These two steps are repeated until either all of the n-type region 20 is removed leaving only the p-type region 22, or a desired amount of the n-type region 20 is removed. Referring to Figure 2, there is shown a graph of open circuit potential (OCP) in volts versus time in seconds for regions of n-type conductivity and p-type conductivity treated in accordance with the method of the present invention. The body 18 was placed in a 20% solution of KOH at 60°C and a potential of 2 volts was applied between the regions 20, 22 and the reference electrode 32 for 1 second. This voltage could be from a few tenths of a volt to many volts. The voltage was turned off and the body was etched while monitoring the open circuit potential between the regions and the electrodes. The open circuit potential reaches a steady value when all of the passivating film formed on a region is completely etched away. In Figure 2, line 34 is for the n-type region 20 and line 36 is for the p-type region 22. It can be seen from line 34 that the n-type region 20 reached its open circuit potential in about 3 seconds while from line 36 it can be seen that the p-type region 22 did not reach its open circuit potential until about 85 seconds. Thus, it took 3 seconds to remove all of the passivating film from the n-type region 20 while it took 85 seconds to remove all of the passivating film from the p-type region 22. This indicates that for the same period of time for the anodic potential step, the passivating film formed on the n-type region 20 is in some manner different from that formed on the p-type region 22. Thus, if during the method of the present invention, the body 18 is subjected to 2 volts for one second and then the voltage is turned off, the etching step should be for at least 3 seconds but for not longer than 85 seconds. This removes the passivating film completely from the n-type region 20 and a portion of the n-type region, but does not remove all of the passivating film from the p-type region 22. KOH can be used in concentrations of 5% to 95% and within a temperature range of 10°C to 120°C. NaOH, LiOH, CsOH, NH 4 OH, ethylenediamine pyrocatechol and hydrazine at concentration levels of 5% to 95% and with a temperature range of 10°C to 120°C can be substituted for KOH. Referring now to Figure 3, there is shown a graph of open circuit potential (OCP) recovery time in seconds versus amplitude of potential in volts applied between the (semiconductor) body 18 and the electrode. The top line 38 is for the p-type region 22 and the bottom line 40 is for the n-type region 20. The (semiconductor) body 18 was in a 20% solution of KOH at 60°C. It can be seen from this graph that the etching time characteristics of the passivating film formed on the n-type region 20 does not change very much as the voltage is increased up to about 2 volts whereas the passivating film on the p-type region 22 does increase with increasing voltage. Referring now to Figure 4, there is shown a graph similar to that of Figure 3 except that the liquid chemical etchant 14 is a 40% solution of KOH at 60°C. The top line 42 is for the p-type region 22 and the bottom line 44 is for the n-type region 20. As can be seen from this graph, changing the etchant solution does provide a greater change in the etching time characteristics of the passivating film formed on the n-type region 20 with increasing potential and does change the etching time characteristics of the passivating film formed on the p-type region 22 from that obtained in a 20% solution of KOH. Thus, using different solutions of the same liquid chemical etchant changes the etching time characteristics of the passivating films obtained on the regions of the (semiconductor) body 18. It has been found that a 20% solution of KOH is preferred because of the shorter etching time characteristics of the passivating film formed on the n-type region 20. This allows the desired amount of the n-type region to be removed with fewer steps. Referring to Figure 5, there is shown a graph similar to Figure 3 except that the etching solution is at 80°C. Again, the top line 46 is for the p-type region 22 and the bottom line 48 is for the n-type region 20. The method of the present invention can be used to form thin diaphragms of p-type semiconductor material. For this purpose, a thin layer of the p-type semiconductor material is epitaxially deposited on a surface of a substrate of n-type conductivity. The (semiconductor) body so formed is then subjected to the method of the present invention to remove the n-type substrate leaving only the thin layer of p-type semiconductor material. If a support is desired for the diaphragm, only the central portion of the substrate is etched away leaving a thick rim of the n-type substrate around the periphery of the diaphragm. Such semiconductor diaphragms can be used as microsensors in pressure transducers, accelerometers and flow sensors. The method of the present invention can also be used to make wafers for silicon on insulator (SOI) technology. For this purpose, a thin layer of p-type silicon is epitaxially deposited on a substrate of n-type silicon. This is then bonded to a silicon dioxide layer on another silicon substrate with the p-type layer being bonded to the silicon dioxide layer and with a portion of the p-type layer projecting beyond the edge of the silicon oxide so as to be exposed. The device is then subjected to the method of the present invention to remove the n-type substrate from the p-type layer. This leaves the p-type single crystalline silicon layer on a silicon dioxide layer, and hence insulated from the silicon substrate. An integrated circuit or the like can then be formed in the p-type layer. This method of forming an SOI wafer has the advantage that the p-type silicon layer is of good crystalline quality since it was epitaxially grown on a silicon substrate. Thus, there is provided by the present embodiment of the invention a method of selective electrochemical etching to form a thin layer of p-type conductivity semiconductor material, such as single crystalline silicon. The method utilizes the discovery that the passivation oxide layer electrochemically formed on n-type conductivity silicon is different from that formed on p-type conductivity silicon. This difference allows the passivation layer formed on the n-type layer to be etched away more quickly than the passivation layer formed on the p-type layer so that a portion of the n-type layer can be etched away before all of the passivation layer on the p-type layer is removed. By repeating the passivation layer forming step and the etching step, all or any desired amount of the n-type layer can be removed without removing any of the p-type layer. This process does not require the forward biasing of a p-n junction, and, in fact, does not require a p-n junction at all. Although the method of the present embodiment of the invention has been described with regard to a body having adjacent regions of p-type and n-type conductivity, the two regions can be separated from each other and can be two regions formed along the same surface of a substrate. All that is required is that both regions be simultaneously subjected to the electrochemical passivation layer forming step and to the following etching step. Referring now to Figure 6, there is shown an electrochemical etching apparatus 10a which is useful for etching silicon and which can be used to carry out a method of the present invention. Apparatus 10a is very similar to apparatus 10 of Figure 1 except that a masking layer 26a surrounds three sides of p-type region 22 and portions of n-type region 20 leaving only a portion of region 20 exposed to liquid chemical etchant 14. Apparatus 10a is operated using essentially the same method as apparatus 10 of Figure 1. The result of this operation is that a portion of region 20 which is exposed to etchant is completely etched away so as to expose a portion of region 22 to the etchant 14. This occurs in essentially the same manner as region 20 of apparatus 10 of Figure 1 is etched away. When the p-type region 22 is exposed to the etchant, it forms a silicon dioxide layer which protects region 22 from being etched away in the same manner as occurs with region 22 of apparatus 10 of Figure 1. The result of using the method of the present invention with the apparatus 10a of Figure 6 is a semiconductor body comprised of a p-n junction with a central portion of the n-type region removed so as to provide a thin p-type diaphragm which is useful as a semiconductor sensor. Modifications can be made to the above described arrangement. For example, although KOH has been described as being a preferred etching solution, any of the other etching solutions mentioned above can also be used. Still further, the voltage applied across the body can be varied, and the etching time can be varied as long as it is longer than required to completely remove the passivation layer from the n-type region without completely removing the passivation layer from the p-type region. Still further, other forms of bodies containing p-type and n-type regions may be used as long as both regions can be exposed to the etching solution and have a common voltage placed thereacross. Still further, the method of the present invention can be used to make other devices which require a thin layer of p-type conductivity. Furthermore, it is possible that under certain etchant and bias conditions, the p-type passivating layer etches away faster than the n-type passivating layer. In this case it is possible, using the present invention, to form a thin layer of n-type conductivity.

Claims: A method of selectively etching from a body (18) of a semiconductor material having first and second regions (20,22) of opposite conductivity type at least a portion of the first region (20) of one conductivity type relative to the second region (22) of the other conductivity type, characterised by the steps of: immersing the body (18) in a high-pH chemical etchant (14); applying a positive voltage between each region (20,22) of the body (18) and the chemical etchant (14) of a polarity suitable to form a passivating layer on a surface of each region (20,22) exposed to the chemical etchant (14); and then etching the body (18) in the chemical etchant (14) by removing the voltage from between the body (18) and the chemical etchant (14) for a period of time long enough to etch away all of the passivating layer on the first region (20) of the one conductivity type and a portion of the first region (20) of the one conductivity type, but not long enough to remove all of the passivating layer from the second region (22) of the other conductivity type; wherein the voltage is applied by immersing an electrode (28) in the chemical etchant (14) spaced from the body (18) and connecting the electrode (28) and the body (10) across a voltage source (30) to achieve the desired voltage. A method as claimed in claim 1, in which the voltage is applied to the body (18) with an electrode (16) that contacts both regions (20,22). A method as claimed in claim 1 or claim 2, in which the steps of forming the passivating layer on the regions (20,22) and etching the passivating layer from the first region (20) of the one conductivity type are repeated until a desired amount of the first region (20) of one conductivity type is removed. A method as claimed in claim 3, in which the steps of forming and etching steps are repeated until all of the first region (20) of one conductivity type is removed. A method as claimed in claim 3 or claim 4, in which the one conductivity type is n-type and the other conductivity type is p-type. A method as claimed in claim 5, in which the body (18) is of single crystalline silicon. A method as claimed in claim 6, in which the chemical etchant (14) is selected from the group consisting of KOH, NaOH, LiOH, CsOH, NH 4 OH, ethylenediamine pyrocatechol, and hydrazine. A method as claimed in claim 7, in which the chemical etchant (14) has a concentration of 5% to 95% and a temperature in the range of about 10EC to 120EC. A method as claimed in claim 8, in which the chemical etchant (14) is a 20% solution of KOH at 60EC. A method as claimed in any one of claims 1 to 9, in which the method comprises initially isolating the second region (22) of the other conductivity type from the chemical etchant (14) by a masking layer (26a) which also covers a portion of the first region (20) of the one conductivity type.

Inventors: MCNEIL VINCENT MAURICE, SCHMIDT MARTIN ARNOLD, WANG SU-CHEE SIMON, MCNEIL, VINCENT MAURICE, SCHMIDT, MARTIN ARNOLD, WANG, SU-CHEE SIMON
Applicants: DELPHI TECH INC, MASSACHUSETTS INST TECHNOLOGY, DELPHI TECHNOLOGIES, INC., MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Application year: 1992
Main classification: H01L21
IPC codes: H01L21