Method of evaluating the gate oxide of non-volatile EPROM, EEPROM and flash-EEPROM memories

Abstract: A method employing a test structure (10) identical to the memory array whose gate oxide quality is to be determined, except for the fact that the cells are connected electrically (13-15, 17, 18, 19-21) parallel to one another. The test structure is so stressed electrically as to extract electrons from the floating gate of the defective-gate-oxide cells and so modify the characteristic of the cell while leaving the charge of the non-defective cells unchanged. In this way, only the threshold of the defective cells is altered. A subthreshold voltage is then applied to the test structure, and the drain current through the cells, which is related to the presence of at least one defective cell in the structure, is measured. Measurement and analysis of the current-voltage characteristic provides for determining the number of defective cells. The method is suitable for in-line quality control of the gate oxide of EPROM, EEPROM and flash-EEPROM memories.

Description: The present invention relates to a method of evaluating the gate oxide of non-volatile EPROM, EEPROM and flash-EEPROM memories, according to the preamble of claim 1. The reliability of non-volatile memories of the aforementioned types is known to depend substantially on the quality of the gate oxide, evaluation of which is especially important in the case of flash-EEPROM memories, the cells of which, as shown in the Fig.1 cross section of a flash-EEPROM cell 100, present a thinner gate oxide layer as compared with those of EPROM memories. More specifically, Fig.1 shows a P type substrate 1; N type drain and source regions 2 and 3; a gate oxide layer 4; a floating gate region 5; a dielectric interpoly layer 6; a control gate region 7; and a protective oxide layer 8. The gate oxide layer of flash-EEPROM memories presents a thickness of roughly 110 Å or 11 nm (as compared with roughly 200 Å or 20 nm for EPROM cells) so as to enable erasure of the flash cells by Fowler-Nordheim tunneling current at voltages leaving the cell itself undamaged. Passage of the tunneling current through the gate oxide, which may be defined as the tunnel oxide by virtue of performing the same functions as the tunnel oxide of EEPROM cells, thus enables electron removal from the floating gate. In view of its functions, therefore, the flash cell tunnel oxide must be of guaranteed reliability both in terms of resistance to the electrical stress typical of EPROM memories, and in terms of erasure characteristics. Hence, the importance of an effective, reliable method of evaluating the quality of the tunnel oxide. At present, this is done using tunnel oxide MOS capacitors made specially for the purpose, and wherein the area or perimeter of the capacitors (depending on the side of the cell being considered) is equivalent to that of the arrays used; and the test method itself consists in subjecting the capacitors to voltage or current stress, and evaluating the electric field or total charge at breakdown of the capacitor. At times, the entire current/voltage characteristic obtainable under stress is evaluated. The above diagnosis and evaluation method, however, presents several drawbacks affecting its precision. To begin with, it has not yet been established beyond all doubt whether a correlation exists between the breakdown charge or electric field values and the presence of defective-oxide cells arrays manufactured in the same process. Moreover, not even with the current/voltage characteristics obtained with the above known method is it possible to detect oxide defects when the degree of deficiency of the oxide is such as to affect only some of the cells in the overall array. In fact, the currents responsible for the variation in the threshold of defective-gate-oxide cells, particularly at deficiency levels of a few defects per cm 2 , are rarely measurable, due to the limitation posed by the noise level of the measuring instruments, and the presence at the same time of the gate current of the overall area of the capacitor. This latter factor becomes increasingly more stringent as the deficiency levels for detection get lower, thus requiring the measurement of increasingly higher-area capacitors and so impairing the measuring sensitivity of the method. Even using known redundancy methods, the presence of even a small number of defective cells may result in breakdown of the memory. Consequently, as process efficiency and memory reliability depend on the degree of deficiency involved, it is essential that measuring instruments be made available that are capable of detecting even very low deficiency levels. A method of testing defective cells is disclosed in US-A-5,155,701 wherein the test includes three steps: a stress-applying step, wherein a high positive voltage is applied to the control gates of the cells at high temperature and for some hours; a charge gain test, wherein the cells are tested at high threshold voltage; and a charge loss test, wherein the cells are tested at low threshold voltage. This test method gives results which are dependent from the length of the stress-applying step, the applied voltage and temperature and is not able to detect a wide range of defects. Furthermore it is time consuming, complicated, involves the implementation of complete circuitry and thus cannot be used to rapidly evaluate the effects of variations in the production parameters. US-A-4,841,482 discloses a leakage verification test for flash EEPROMs wherein the voltage applying step is not intended to apply a stress condition, but is the normal voltage applying procedure for erasing EEPROM and flash-EEPROM cells, which is then followed by a measuring step for determining whether the cell is over-erased. A method of the type defined in the preamble of claim 1 is disclosed in US-A-4,963,825 teaching applying an electric field directed from the substrate towards the floating gate of the memory cells to extract electrons from at all cells for erasing all the cells. It is an object of the present invention to provide a method of detecting defective cells due to poor quality gate oxide, and which provides for improved diagnostic performance as compared with known methods and structures currently employed for the purpose. According to the present invention, there is provided a method of evaluating the gate oxide quality of non-volatile EPROM, EEPROM and flash-EEPROM memories, as claimed in Claim 1. In practice, a test structure is proposed which is identical to the memory array whose gate oxide quality is to be determined, which test structure is so stressed electrically as to only extract electrons from the floating gate of defective-gate-oxide cells in the structure. The test structure is then subjected to a subthreshold voltage; the current through the test structure cells, and which is related to the presence of defective cells, is measured; and the current-voltage characteristic is analyzed for determining the number of defective cells. In this way, it is possible to detect the presence of even only one defective cell within the structure. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows a cross section of a known flash-EEPROM memory cell; Fig.2 shows a top plan view of the test structure employed according to the present invention; Fig.s 3 to 7 show electric graphs of the characteristic of the Fig.2 structure; Fig.8 shows the stress voltage applied to the Fig.2 structure according to one embodiment of the method according to the present invention; Fig.9 shows the stress voltage according to a further embodiment. Though reference is made in the following description to a flash-EEPROM memory, the present invention may also be applied to EPROM and EEPROM type memories. Fig.2 shows a top plan view of a flash-EEPROM memory test structure 10 identical to a standard memory array and therefore presenting a number of cells 100 as shown in Fig.1. Unlike the standard array, however, and as shown in Fig.2, the drain regions of cells 100 in structure 10 are all mutually connected by metal lines, as are also the source regions and control gate lines. More specifically, Fig.2 shows: part of a wafer 11 of semiconductor material housing field insulating regions 12; metal drain lines 13 connected by a common portion 14 to a single pad 15; metal source line 17 connected to a single pad 18; polysilicon control gate lines 19 connected by a common portion 20 to a single pad 21; drain contacts 23; and source contacts 24. The areas in which field insulating regions 12 are absent and which are not covered by the gate lines constitute the source and drain regions. Structure 10, which is accessible externally by means of pads 15, 18 and 21, is thus equivalent electrically to the parallel connection of all of cells 100. Structure 10 may be formed inside the wafer itself and next to the standard memory arrays to be made operative (one of which is shown schematically by 27 in Fig.2), in which case, structure 10 is fabricated using the same technology and simultaneously with arrays 27, so as to present the same characteristics (particularly as regards gate oxide quality) and give a reliable indication of the quality of the gate oxide of standard array 27. Alternatively, for controlling the quality of the gate oxide, structure 10 may be formed on test wafers, in which case, the fabrication process is shortened by eliminating some of the steps required for forming the circuitry annexed to the standard arrays. For evaluating the quality of the gate oxide, structure 10 is first U.V. erased, so as to render the floating gate of the cells electrically neutral, in which case, the drain current of the structure equals the current of each cell multiplied by the number of cells in the structure. When the structure is stressed electrically in such a manner as to extract electrons from the floating gate of the defective-gate-oxide cells (which thus remains positively charged), and in such a manner as to leave the charge status of the other cells unchanged, the threshold voltage of the defective cells falls, while that of the other (non-defective) cells remains unchanged. Consequently, and particularly if the threshold voltage falls sufficiently (this normally depends on the complexity of structure 10 and is roughly 1 V for one-million cell structures), for gate voltage values below the threshold voltage of the defective cells, the drain or source current of the defective cells is greater than the total drain current of all the other cells. The change in the characteristic of test structure 10 when stressed as described above is shown in Fig.3, wherein curve 30 shows the source current as a function of gate voltage of a U.V. erased test structure 10, measured in the dark and with a zero gate voltage (V G = 0) and a current leakage of less than 1 nA. In general, source leakage, determined solely by the photocurrent generated in the space charge region of the overall source-substrate junction, is highly dependent on light intensity; and drain leakage is also affected by the inverted current of the drain-substrate junction, which depends both on the voltage applied to the drain and, closely, on temperature. On applying a negative voltage to the control gate regions (7 in Fig.1) or a positive voltage to the source regions (3), the electric field in the gate oxide (or tunnel oxide 4) in the floating gate 5-substrate 1 overlap regions (including source and drain diffusions 3 and 2) or respectively in the floating gate 5-source diffusion 3 overlap regions is directed towards floating gate 5. If the voltages applied are low enough to avoid Fowler-Nordheim tunneling through the potential barrier of the gate oxide of non-defective cells, electrons will only be lost from the floating gate of defective cells with an anomalous gate current at low electric fields, to give the structure 10 characteristic indicated by curve 31 in Fig.3, which presents a "tail" 31a due to the presence of defective cells. Fig.4 shows the drain current I d (logarithmic-scale curve 33) and transconductance G (dI/dV G - linear-scale curve 34) of a test structure 10 measured after applying one of the electric stresses described above. More specifically, curve 33 presents a first portion 33a indicating the inverse current of the drain-substrate junction; a second portion 33b indicating the modified portion of the characteristic due to a change in the threshold voltage of a number of cells; and a final portion 33c due to intervention of the overall array. The transconductance curve 34 presents two "humps" 34a and 34b, each due to the drain current produced by a respective defective-oxide cell being turned on at a voltage below the threshold voltage of the other non-defective cells. The transconductance curve thus provides for determining the number of defective cells, and, in particular, for indicating the presence of even only one defective cell. Due to the impossibility, however, of devising an automatic, industrial-scale routine based on measurement and evaluation of the transconductance curve, two alternatives have been devised for evaluating the overall array. According to a first solution, the structure is subjected to stress by a given source or gate voltage for fixed intervals of time; and the gate voltages at a predetermined source current (e.g. 300 nA) are measured before and after stress. If the difference in the voltages so measured exceeds a predetermined value, the structure is considered defective. The above solution, however, poses problems as regards minimizing the stress time, and so enabling detection of defective arrays within an acceptable automated time frame, by virtue of the stress time varying widely according to the anomalous gate current, at low electric fields, of the defective cells. A second preferred solution, which provides for overcoming the above problem, consists of a ramped stress routine whereby the stress voltage is applied in steps, and the array characteristic measured after each stress voltage step. The voltage ramp is practically equivalent to an exponential gate current ramp, so that the source current of the defective cells is detected more rapidly as compared with the previous constant-stress solution. The following is a description of the second solution with reference to Fig.s 5 to 8. In the example shown, due to the effect on the drain current of drain-substrate contact leakage, in addition to that caused by light intensity, the characteristic is measured using the source current. To begin with, after U.V. erasing test structure 10, the current leakage of the structure is measured with (Fig.1) V d = 0.05 V, V g = V s = V b = 0 V.; and, if the source leakage exceeds a predetermined threshold (e.g. 100 nÅ), the structure is not tested further. The same also applies to the drain leakage. The characteristic of the structure ("virgin" characteristic) is then measured as shown in Fig.5. More specifically, this phase consists in measuring the VTL and VTH values of the gate voltage at two predetermined source current values I 1 and I 2 (e.g. 200 nA and 1.5 mA), the first affected by the deficiency "tail", and the second representing the intrinsic behavior of the structure, to show the deviation of the characteristic due to the Fowler-Nordheim effect. The structure is then subjected to a first ramped stress phase, e.g. on the gate, wherein the negative gate voltage (V G in Fig.8) is increased gradually from, say, -4 V, and in fixed increments of, say, -0.25 V, each stress voltage being maintained for a predetermined time of, say, 0.1 s. After each application of stress voltage V G , value V 1 of gate voltage V g corresponding to source current value I 1 (Fig.5) is measured and compared with the initial value VTL. If the difference (VTL - V 1 ) is less than a predetermined DV value, e.g. 100 mV (normally depending on the accuracy of the instrument), a higher stress voltage, increased as described above, is applied, and the stress and measuring cycle is repeated. Conversely, if VTL - V 1 ≧ DV, the V G value relative to the stress applied is memorized as VRL, and a second ramped stress phase is performed. As shown in Fig.6, VRL (relative to the (i + 1)-th stress voltage cycle) indicates the stress responsible for a 0.1 V shift in the characteristic ("tail"), in turn due to a shift in the threshold of one or more defective cells. As in the first phase, the second stress phase again consists in applying linearly-increasing voltage levels, and in measuring the characteristic after each application. More specifically, in the second phase, voltage V g corresponding to source current value I 2 is measured, and the resulting value V 2 of gate voltage V g is compared with the initial value VTH. If VTH - V 2 < DV, a higher stress voltage is applied. Conversely, the V G value relative to the stress applied is memorized as VRH. As shown in Fig.7, VRH indicates the stress responsible for the shift in the characteristic due to a variation in the threshold voltage of the overall array; is related to the intrinsic quality of the cell gate oxide; and depends on the fabrication process parameters and technology. At this point, the difference between the resulting VRL and VRH values is calculated, and if this exceeds a predetermined threshold (e.g. 1.5 V), the structure is considered defective. The method may also comprise additional phases for comparing electric parameters and specific characteristic values, and for classifying the structure according to the results. The advantages of the method and structure according to the present invention will be clear from the foregoing description. Firstly, even if the gate current under stress is less than the noise level of the measuring instruments and therefore not measurable directly, the method and structure according to the present invention provide for indirectly measuring the gate current via measurement of the drain or source current of the cell, and so enabling detection of even only one defective cell. Secondly, unlike formerly used capacitors, the structural characteristics of which differ widely from those of the memory arrays, the test structure according to the present invention is physically equivalent to the memory arrays, with which it is formed simultaneously and using the same technology, thus enabling a direct evaluation of the quality of the arrays on the basis of the test structure results. Thirdly, the method described is straightforward and easy to implement, and the test structure easy to produce, with no need for special technology or alterations to existing production processes. To those skilled in the art it will be clear that changes may be made to the method and structure described and illustrated herein without, however, departing from the scope of the present invention. In particular, stress may be applied indifferently to the gate regions of the test structure cells (as in the embodiment described) or to the source regions, with positive voltages in relation to the substrate (as in Fig.9), or to the drain regions. Measurement of the current at subthreshold voltage may involve the drain or source current; while measurement of the characteristic may even involve transconductance as opposed to the drain or source currents. Also, stress may be applied differently from the manner described. Though the above description relates to a flash-EEPROM memory, the method and test structure described may also be employed for evaluating EPROM and EEPROM memories, in which case, the test structure will be physically equivalent to the type of EPROM or EEPROM memory in question, by electrically connecting the control gate, source and drain lines. Finally, the type of stress applied need not necessarily be electrical, e.g. irradiation, providing it results in the removal of electrons from the floating gate of the defective cells.

Claims: A method of evaluating the gate oxide of cells of non-volatile EPROM, EEPROM and flash-EEPROM memories, comprising the steps of applying an electric field directed from the substrate (1-3) towards the floating gate (5) of the cells to extract electrons from at least one cell (100) and subsequently measuring the characteristics of said cell, characterized in that said step of applying an electric field comprises applying a such stress as to cause a flow of electrons from the floating gate to the substrate of only defective cells (100). A method as claimed in Claim 1, characterized by the fact that said characteristic measuring step comprises a step of subjecting said test cell (100) to a voltage lower than the standard threshold voltage of a non-defective memory cell. A method as claimed in Claim 2, characterized by the fact that said voltage is a gate voltage. A method as claimed in one of the foregoing Claims, characterized by the fact that said characteristic measuring step comprises a step of measuring the drain current of said test cell (100). A method as claimed in one of Claims from 1 to 3, characterized by the fact that said characteristic measuring step comprises the step of measuring the source current of said test cell (100). A method as claimed in one of Claims from 1 to 3, characterized by the fact that said characteristic measuring step comprises the step of measuring the transconductance of said test cell (100). A method as claimed in one of the foregoing Claims, characterized by the fact that said stress step comprises the step of subjecting said test cell to a stress voltage lower than the tunneling voltage of non-defective standard memory cells. A method as claimed in one of the foregoing Claims, characterized by the fact that said stress step comprises the step of subjecting said test cell to a stress voltage greater than the tunneling voltage of defective-oxide memory cells. A method as claimed in Claim 7, characterized by the fact that said stress voltage application step comprises the step of applying a negative voltage to the gate region with respect to the drain and source regions of said test cell. A method as claimed in claim 7, characterized by the fact that said stress voltage application step comprises the step of applying a positive voltage to the source region with respect to the drain and gate regions of said test cell. A method as claimed in Claim 7, characterized by the fact that said stress voltage application step comprises the step of applying a constant voltage for a given time interval. A method as claimed in Claim 7, characterized by the fact that said stress voltage application step comprises the step of applying a voltage increasing from a minimum to a maximum value. A method as claimed in Claim 12, characterized by the fact that said increasing voltage is increased in steps. A method as claimed in Claim 13, characterized by the fact that it comprises a number of stress and measuring cycles, each cycle including applying said stress voltage and measuring the characteristic of said test cell; the stress voltage in each cycle being increased by a predetermined amount as compared with that of the previous cycle. A method as claimed in one of the foregoing Claims, characterized by the fact that it comprises an initial step of measuring the characteristic of said test cell prior to applying said stress, and comparing said characteristics measured before and after applying said stress. A method as claimed in Claim 15, characterized by the fact that it comprises the step of U.V. erasing said test cell prior to said initial step of measuring the characteristic of said test cell. A method as claimed in any one of the foregoing Claims, characterized by the fact that said test cell forms part of a test structure consisting of an array of cells having drain regions (2) connected electrically (13, 14) to one another and to a single drain pad (15); source regions (3) connected electrically (17) to one another and to a single source pad (18); and control gate regions (7, 19) connected electrically (20) to one another and to a single gate pad (21).

Inventors: CAPPELLETTI PAOLO GIUSEPPE, RAVAZZI LEONARDO, CAPPELLETTI, PAOLO GIUSEPPE, RAVAZZI, LEONARDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: G11C29
IPC codes: G11C29, H01L27, H01L29, H01L21, G11C17