Anadization System With Remote Voltage Sensing and Active Feedback Control Capabilities

1. Field of the Invention

This invention relates to a system for electrochemical anodization of silicon substrates and, more specifically, to computer controlled anodization of specially prepared porous silicon substrates.

2. Brief Description of the Prior Art

Anodization of bulk silicon generally takes place in order to create silicon having a density which is about half that of the bulk silicon due to the formation of pores in the anodized region. This pore formation provides an increase in surface area of the anodized silicon and permits the anodized region to be oxidized more rapidly than the bulk silicon. Anodization is often used to form isolated islands of silicon within the wafer bulk. An example of the formation of such islands involves starting with N- bulk silicon, forming an N+ layer thereon followed by an N- layer. The isolated island regions are then masked with a layer of silicon nitride followed by a layer of silicon oxide and the unmasked regions are then etched to a level into the bulk to provide a trench exposing the N+ layer. Anodization now forms pores in the N+ layer and subsequent oxidation causes formation of an oxide layer in the region of the former N+ layer as well as along the sidewalls of the trench.

In the prior art of anodization of silicon substrates, it has been necessary to make direct ohmic contact to the substrate being anodized via some metallizing scheme in order to monitor and control the voltage drop thereacross during anodization. Such monitoring is necessary in order to avoid deleterious effects in anodization of the substrate due to changes in the voltage drop across the wafer and, hence, the anodization current density as well as changes due to anodizable area changes. Constant current density is critical to the uniformity of the anodization process and to the subsequent oxidation process since non-uniformities in current density during the anodization process lead to variations in the sizes of the resulting pores in the porous layer, particularly for features 30 microns wide or less. Variable pore size, in turn, leads to oxidation induced stresses which cause unacceptable defect levels in the isolated islands formed. In addition, changes in electrolyte concentration which occur with depletion of the electrolyte can result in electrochemical etching as opposed to anodization. Such prior art related to anodization and methods thereof are set forth in U.S. Pat. No. 4,628,591 and pending application Ser. No. 806,258, filed Dec. 6, 1985 of Zorinsky et al., now abandoned, and Ser. No. 810,001, filed Dec. 17, 1987 of Keen et al., now abandoned, and all assigned to the assignee of the subject application, as well as in the literature referenced in these applications and patent wherein direct contact with the wafer or substrate in some fashion is described. It has been found by the present invention that the problems inherent in systems making direct ohmic contact to the wafer being anodized are lessened by providing a system wherein such direct contact is not required.

Briefly, in accordance with the present invention, the above noted problems of the prior art are minimized and there is provided anodization with substantially uniform porosity throughout the anodized region by a system which precisely controls the electrochemical anodization of specially prepared silicon substrates, wherein regions to be anodized are more highly doped and surround a less doped island therein on which a circuit device is to be fabricated. The invention utilizes remotely placed voltage probes to monitor changes in the potential drop across the wafer as the anodization process proceeds. As the available anodizable area changes, the voltage drop across the wafer and hence the anodization current density is maintained at the desired value by the computer through the use of active feedback provided by these probes. By eliminating the need for voltage control via direct ohmic contact to the wafer, this system makes it possible to design a production process station suitable for use in a front end manufacturing environment. Furthermore, any desired anodization conditions can be programmed into the system using the system software, thereby adding an even greater degree of control over the process. In some cases, it may be necessary to anodize many tens of microns laterally. This system provides the capability to vary the slice potential in a controlled fashion so as to compensate for any electrolyte depletion that occurs. Such depletion adversely impacts the final pore size, causing possible electrochemical etching rather than anodization. Slight modifications to the current density in these situations is highly desirable.

Basically, the anodization system of the present invention includes a closed loop comprising a data aquisition unit, a power supply, an optional current and/or voltage measuring meter between the power supply, data acquisition unit and an anodization tank, and the anodization tank with electrolyte and device to be anodized, such as a semiconductor wafer, in the electrolyte, the device separating the tank into two chambers. A pair of probes is disposed in the electrolyte, one probe on each side of the device, the probes being spaced from the device to measure the voltage drop across the device to provide signals to the data acquisition unit external of the tank indicative thereof. Also disposed within the tank is a pair of electrodes of standard type which is coupled to the data acquisition unit and provides a controlled voltage or current across the tank.

The data acquisition unit provides signals to and receives signals from a computer and controls operation within the loop based upon the signals received from the computer. The computer has a memory and drives a plotter or other display device. The computer is not a part of the loop and is preferably the only element of the system which is software controlled, all other system components being essentially hardward elements. The computer controls system operation via the data acquisition unit.

The power supply provides controlled voltage and/or current across the anodizing tank under control of the data acquisition unit and provides data back to the data acquisition unit to indicate the voltage and/or current being supplied across the tank. An ammeter is optionally located in the path from the power supply to the anodization tank as well as from the power supply to the data acquisition unit to provide an immediate reading of voltage and/or current being supplied to the tank.

The computer is programmed to provide any desired voltage drop across the wafer and/or current therethrough. The anodization characteristics, i.e., the porosity, the rate and the selectivity are determined by the voltage and current applied to the interface at the front of the slice and also to the acid concentration of the electrolyte to a lesser extent.

The characteristics of the anodization process change because of changes in anodizable area and acid concentration deep into the pores being created in the material. It is therefore desirable to change the voltage and/or current as the process proceeds to compensate for these changes.

The system has a real time feedback loop wherein, as the area of the wafer available for anodization changes, the current density can be controlled by controlling the voltage through the feedback loop. The anodization process is self-limiting. Uniform porosity is important because, if there is a porosity gradient within the porous layer, a great deal of stress is induced after oxidation of the material. It is this stress which provides the defects in the isolated material which is fatal to the production of bipolar devices and detrimental to the production of MOS devices.

In operation, a wafer which has been processed to include a trench with an N+ layer therein exposed at a surface of the wafer and which is to be anodized is placed in an anodizing tank filled with appropriate electrolyte, preferably hydrofluoric acid (HF) electrolyte in the range of 10 to 40% and preferably 20%. Other electrolytes which can be used are combinations of hydrofluoric acid and materials which will better wet the surface and reduce the surface tension so that bubble formation is not as much of a problem, the wafer performing an electrical and physical separation between the two half cells of the tank to form two chambers. A predetermined voltage is initially provided across the tank electrodes, this voltage being determined by the voltage programmed across the wafer at that time by the computer, via the data acquisition unit. The electrolyte causes pores to form at the surface of the N+ layer and gradually work their way deeper into the silicon layer and forms new pores. This process continues for the duration of the anodizing period. The silicon removed from the pore region goes into solution in the electrolyte. As the pores form, the voltage across the probes changes due to the changes in anodizable area. As the anodization proceeds, the acid deep in the pores may become depleted, thereby changing the effective electrolyte concentration. This, in turn, may cause a change in porosity. It is therefore necessary to modulate the voltage across the wafer to compensate for these changes in order to insure the formation of uniformly porous material under all device islands. It is also desirable to lower the voltage substantially near the end of the anodization cycle to provide for minimum surface continuity at the interface with the anodized region.

A data base of porosity as a function of acid concentration and current density, the lateral anodization rate at each current density and each acid concentration is used to determine the appropriate program parameters for each device type. The changes in voltage programmed across the anodizing tank are based upon the data base and the desired rates originally programmed into the system. The data base is empirical in nature.

FIG. 1 is a block diagram of the system configuration of an anodization system in accordance with the present invention; and

FIG. 2 is an enlarged detailed schematic diagram of of the anodization tank of FIG. 1.

In Appendix B, the figures are as follows:

drawing A shows the wiring and external connections of the ammeter 5 and power supply 3;

drawing B shows the external connections of the data acquisition unit 1; and

drawings C and D show the wiring of the plugs of drawing B.

Referring first to FIG. 1, there is shown a block diagram of the anodization system in accordance with the present invention. The system comprises a closed loop control including a data acquisition system 1, which is preferably a Hewlett-Packard Model HP 3497A and which is coupled to a power supply 3 via a two way lead 13 as well as a lead 15 coupled back to the acquisition unit 1. The power supply is also coupled to an optional ammeter 5 via conductor 17, the ammeter being coupled to the data acquisition unit 1 via conductor 19 and providing the power to the positive platinum electrode 23 in the anodizing tank 7 via the conductor 21. The negative electrode 25 in the anodizing tank 7 is coupled to the data acquisition unit 1 via reference voltage lead 27 and lead 29 whereas the positive electrode 23 is coupled to the data acquisition unit 1 via the lead 31. Probes 33 an 35 are disposed in opposite portions of the tank 7 and are connected to the data acquisition unit 1 via leads 37 and 39. A semiconductor wafer 41 is positioned in the tank 7 and separates the electrolyte in the tank into two separate chambers as will be explained in more detail hereinbelow. Probes 33 and 35 are on opposite sides of the wafer 41 and measure the voltage across the opposite sides of wafer 41. That voltage measurement provides data that is used to control the anodization process.

The data acquisition unit is driven by a computer 43, preferably a Hewlett Packard Model MP 98165 which operates in accordance with the program set forth in Appendix A herein which is preferably stored in the disk drive 9, preferably a Hewlett Packard Model HP 9121D, which sends data to and receives data from the data acquisition unit 1 along the line 47. Also shown in FIG. 1 is a display device in the form of a plotter 11 coupled to the computer 45 via the conductor 49.

Referring now to FIG. 2, there is shown an enlarged and more detailed cross section of the anodization tank 7 of FIG. 1. The tank includes center partitions 53 and 55 and retaining rings 54 and 56, the partitions and retaining rings each having retaining means including O-rings 57 for sealingly securing the wafer 41 thereto to isolate electrolyte in tank chamber 59 from electrolyte in tank chamber 61. A reference electrode 63 having a voltage V1 is positioned in the electrolyte in chamber 59 and a reference electrode 65 having a voltage V4 is positioned in the electrolyte in chamber 61. Electrolyte in chamber 59 is recirculated into and out from that chamber by a first pump (not shown) via inlet 67 and outlet 69 whereas electrolyte in chamber 61 is recirculated into and out from that chamber by a second pump (not shown) via inlet 71 and outlet 73.

In operation, the tank 7 is filled with electrolyte to a level above the top of the wafer 41 and the wafer is secured by retaining ring 55 and center partition 53 in the tank to form the separate and isolated chambers 59 and 61. The computer 45 is then controlled externally to provide the required data as set forth in the program in Appendix A to provide the required current and voltage in the tank 7 via the electrodes 23 and 25 until complete anodization takes place. Anodization then takes place either by timing the process to completion using the rate thereof at a given current density along with the width of the largect feature requiring isolation or, by using a minimum current value (i.e., when all features have anodized, only regions in the field continue to anodize, resulting in constant (small) current. At the completion of anodization, the system will turn off and the wafer 41 is removed from the tank 7. The procedure is then repeated for the next wafer 41.

Though the invention has been described with respect to a specific preferred embodiment thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications. ##SPC1##