Patent Document

BACKGROUND OF THE INVENTION 
     The present invention generally relates to testing a semiconductor wafer and, more particularly, to measuring a total charge of an insulating layer of the semiconductor wafer using corona charge. 
     The production of insulating layers, particularly, thin oxide layers, is basic to the fabrication of integrated circuit devices on semiconductor wafers. A variety of insulating dielectric layers are used for a wide range of applications. These insulating layers can be used, for example, to separate gate layers from underlying silicon gate regions, as storage capacitors in DRAM circuits, for electrical device isolation and to electrically isolate multilayer metal layers. 
     The devices, however, are very sensitive to induced charges near the silicon surface. In most cases, device performance depends strongly on the concentration of free charges in the silicon. As a result, unwanted variations in device performance can be introduced by charges in the insulating layer and the insulating layer interface. The charges can result, for example, from static charging of the insulating layer surface, poorly forming the insulating layer, excessive ionic contamination within the insulating layer, and metallic contamination within the insulating layer. In addition to degradation of device performance, electrical isolation of individual devices can be impaired by unwanted surface channels due to induced charges. A property of increasing interest, therefore, is total charge Q tot  or sometimes referred to as net charge Q net  of the insulating layer. 
     As illustrated in FIG. 1, there are five principle components of the total charge Q tot  of an oxide layer: surface charge Q s ; mobile charge Q m ; oxide trapped charge Q ot ; fixed charge Q f ; and interface trapped charge Q it . The surface charge Q s  is charge on the top surface of the oxide layer and is frequently static charge or charged contaminants such as metallics. The mobile charge Q m  is ionic contamination in the oxide layer such as potassium, lithium, or sodium trapped near the air/SiO 2  interface or the Si/SiO 2  interface. The oxide trapped charge Q ot  is electrons or holes trapped in the bulk oxide. The fixed charge Q f  is charge at the Si/SiO 2  interface. The interface trapped charge Q it  varies as a function of bias condition. 
     Conventional methods of determining the total charge Q tot  of an oxide layer include capacitance-voltage (CV) surface photovoltage (SPV) with biasing, and SPV analysis. The CV method typically measures each of the individual component charges, except the surface charge Q s  which can be measured by the CV method, with a metal contact formed on the surface of the oxide layer and then obtains the total charge Q tot  by summing up the individual component charges. The SPV with biasing method uses a contacting probe separated from the oxide layer with a Mylar insulator to bias the semiconductor. The total charge Q tot  is determined by measuring the required bias of the probe to force a certain SPV. The SPV analysis method takes SPV measurements and infers the total charge Q tot  via theoretical modeling. 
     While these methods may obtain the total charge Q tot , they each have drawbacks. The CV method requires expensive and time consuming sample preparation. The SPV with biasing method requires a contacting probe which can allow charge transfer from the oxide layer to the probe. The SPV analysis method relies on theoretical modeling and may not be extremely accurate. Additionally, the SPV methods only work over a narrow range of total charge Q tot , when the semiconductor is in depletion. Accordingly, there is a need in the art for an improved method of measuring the total charge of an insulating layer which is contactless, is a direct measurement with no theoretical modeling, is sensitive over a wide range of total charge, and is extremely accurate. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for measuring a total charge of an insulating layer on a substrate which overcomes at least some of the disadvantages of the above-noted related art. According to the present invention, the method includes depositing corona charges on the insulating layer and measuring a surface photovoltage for the insulating layer after depositing each of the corona charges. The method further includes determining a total corona charge required to obtain a surface photovoltage of a predetermined fixed value and using the total corona charge to determine the total charge. 
     According to one variation of the method according to the present invention, the total corona charge is determined by continuing to deposit the corona charges until the surface photovoltage measured is equal the fixed value. The total corona charge then corresponds to a sum of the corona charges deposited. According to another variation of the method according to the present invention, the total corona charge is determined using a data set of discrete points, preferably by interpolation. The discrete points include the surface photovoltages measured after each of the corona charges and corresponding total corona charges deposited to obtain each of the surface photovoltages. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     These and further features of the present invention will be apparent with reference to the following description and drawings, wherein: 
     FIG. 1 is a diagrammatic view of a semiconductor wafer illustrating principle components of a total charge of an insulating layer; 
     FIG. 2 is a schematic diagram of an apparatus for measuring a total charge of an insulating layer according the present invention; 
     FIG. 3 is an exemplary graph illustrating how the total charge can be determined by incrementally depositing a corona charge until obtaining a surface photovoltage (SPV) equal to a fixed value; and 
     FIG. 4 is an exemplary graph illustrating how the total charge can be determined by interpolating a data set of measured surface photovoltages (SPV) and associated total corona charge densities. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an apparatus  10  for testing a semiconductor wafer  12  according to the present invention. The semiconductor wafer  12  includes a semiconductor substrate  14  and a dielectric or insulating layer  16  disposed on the substrate  14 . The substrate  14  is typically a silicon substrate and the insulating layer  16  is typically an oxide layer. However, it should be understood that the method of the present invention is applicable to a variety of insulating layers grown and/or deposited on substrates of semiconductor materials or metals. An air/dielectric interface  18  is formed at the top surface of the insulating layer  16  and a dielectric/substrate interface  20  is formed between the insulating layer  16  and the substrate  14 . A measurement region  22  of the insulating layer  16  is selected to be tested by the apparatus  10 . 
     The illustrated apparatus includes a wafer chuck  24  for holding the wafer  12  during testing, a contactless calibrated corona discharge source or gun  26  for depositing corona charges, a coulombmeter  28  for measuring deposited corona charges, an SPV device  30  for measuring surface photovoltages, a position actuator  34  for locating various components over the wafer  12 , and a controller  36  for operating the apparatus  10 . The wafer chuck  24  holds the wafer  12  during the measurement process and the wafer  12  is preferably secured to the wafer chuck  24  with a vacuum. 
     The corona gun  26  includes a non-contact corona-charge depositing structure such as one or more needles  38  and an electrode housing  40  which, along with the needles  38 , focuses the corona discharge onto the measurement region  22  of the insulating layer  16 . The needles  38  are preferably disposed a distance above the top surface  18  of the insulating layer  16  to minimize fringing effects and other causes of charge deposition non-uniformity. U.S. Pat. No. 5,498,974, expressly incorporated herein in its entirety by reference, discloses a suitable corona gun for depositing corona charge on an insulating layer and a suitable Kelvin probe for measuring the voltage on the surface of the layer. 
     The needles  38  are connected to a charge biasing means such as a high-voltage power supply  42  via a suitable line. The power supply  42  provides a desired high voltage output (e.g., ±6-12 Kv) to the corona gun  26  to produce positive or negative corona charges depending on the polarity of the supply. The power supply  42  is suitably connected to the controller  36  via an appropriate signal line for feedback control of the power supply  42  during operation of the apparatus  10  as described in more detail hereinafter. 
     The coulombmeter  28  is used to measure the deposited corona charge and preferably includes a first operational amplifier or current-to-voltage converter  44  and a second operational amplifier or charge integrator  46 . The input of current-to-voltage converter  44  is connected via a suitable signal line to the substrate  14  and the wafer chuck  24 . A corona current I c  flows from the corona gun  26  and through the wafer  12  to the current-to-voltage converter  44 . This current I c  is converted by the current-to-voltage converter  44  to a voltage and then integrated by the charge integrator  46  to generate a voltage proportional to the charge Q c  deposited onto the insulating layer  16  by the corona gun  26 . The outputs of the current-to-voltage converter  44  and the charge integrator  46  are each connected to the controller  36  via suitable signal lines to feed the current I c  and the deposited corona charge Q c  information to the controller  36  during operation of the apparatus  10  as described in more detail hereinafter. Note that an electrical contact between the wafer  12  and the chuck  24  because the regulating displacement currents are sufficient to perform the measurement. 
     The SPV device  30  is used to measure surface photovoltages of the insulating layer  16  and preferably includes a very high intensity light source  48  such as, for example, a xenon flash tube. It is noted, however, that other types of SPV devices can be used such as, for example, LED, laser, or AC with lock-in. 
     The position actuator  34  is used to locate the corona gun  26 , and the SPV device  30 , over the measurement region  22  of the water  12 . The position actuator is preferably a high-speed linear translator including a mobile carriage which selectively moves along a track disposed above the wafer chuck  24 . The corona gun  26  and the SPV device  30 , are each suitably spaced apart and attached to the carriage. A control unit is suitably connected to the controller  36  via an appropriate signal line for feed-back control during operation of the apparatus  10  as described in more detail hereinafter. 
     The controller  36  is used to control the operation of the apparatus  10  and preferably includes an input device  62  connected via a suitable line. The controller  36  controls the high-voltage power supply  42 , the SPV device  30 , the Kelvin control  54 , and the position actuator control unit  60  and receives information from the current-to-voltage converter  44  and the current integrator  46 . Based on the method set forth hereinbelow, the controller  36  can provide a measurement of total charge Q tot  of the insulating layer  16 . The controller  36  may be, for example, a dedicated microprocessor-based controller or a general purpose computer. 
     To obtain a total charge Q tot  measurement for an insulating layer  16  of a semiconductor wafer  12  according to a first method of the present invention, the actuator preferably first locates the SPV device  30  over the measuring region  22  of the wafer  12  to obtain an initial SPV measurement V SPV  of the insulating layer  16 . The lamp  48  is flashed and a recording of a peak intensity of the SPV transient is captured by an A/D card of the controller  36 . Because of the high intensity output of the lamp  48 , a measurable SPV can be obtained in both in accumulation and in depletion or inversion. Note that other types of SPV devices such as, for example, LED, laser, or AC lock-in amplifier can be used. 
     The position actuator  34  next locates the corona gun  26  over the measuring region  22  of the wafer  12  to deposit a corona charge Q c  on the measurement region  22  of the insulating layer  16 . The controller  36  provides appropriate control signals for the corona gun  26  to deposit a corona charge Q c . The corona charge Q c  deposited on the insulating layer  16  is measured by the coulombmeter  28  and recorded by the controller  36 . 
     The position actuator then locates the SPV device  30  over the measuring region  22  of the wafer  12  to again measure the SPV V SPV  of the insulating layer  16 . The SPV measurement V SPV  is preferably recorded by the controller  36  and compared to a predetermined target value V SPVtarget  stored in the controller  36 . Preferably, the target value V SPVtarget  is equal to a fixed value (0 volts) which indicates a “flatband condition”. At flatband, no net charge is present on the insulating layer  16  and no space charge imaging is in the silicon substrate  14 . It should be understood that the target value V SPVtarget  can be equal to fixed values other than zero. For example, the target value V SPVtarget  can be equal to a fixed value (typically about ±0.300 V) which indicates a “Midband condition”. At midband, the SPV V SPV  is equal to the fixed value which depends on the doping of the particular substrate  14 . 
     If the SPV measurement V SPV  is not substantially equal to the target value V SPVtarget , the above described steps of depositing the corona charge Q c  and remeasuring the SPV are repeated. If the new SPV measurement V SPV  changes beyond the target value V SPVtarget  from the previous SPV measurement V SPV , the controller  36  provides appropriate control signals for the corona gun  26  to reverse the polarity of the next deposited corona charge Q c . Note that for a target value V SPVtarget  of zero volts, a change in polarity from the previous SPV measurement to new SPV measurement indicates that the polarity of the next deposited corona charge Q c  should be reversed. As required, the controller  36  can adjust the magnitude of the next deposited corona charge Q c  to obtain an SPV measurement V SPV  equal to the target value V SPVtarget . 
     When the SPV measurement V SPV  is substantially equal to the target value V SPVtarget , the controller  36  sums each of the individual corona charge increments Q c  to obtain a total corona charge Q applied@target  applied to the insulating layer  16  to obtain the SPV measurement V SPV  equal to the target value V SPVtarget . The controller  36  then determines the total charge Q tot  of the insulating layer  16  from the total applied corona charge Q applied@target  wherein the total charge Q tot  is the negative of the total applied corona charge Q applied@target , i.e. Q tot =−Q applied@target . 
     FIG. 3 illustrates an example of this first method wherein the target value V SPVtarget  is zero volts, or flatband condition. A first corona charge Q c  of −0.20E −07  C/cm 2  is applied on the insulating layer and an SPV measurement V SPV  of about 0.090 volts is obtained. A second corona charge Q c  of −0.20E −07  C/cm 2  is then applied on the insulating layer  16  such that the total corona charge Q applied  is −0.40E −07  C/cm 2 . The second SPV measurement V SPV  is about 0.100 volts. A third corona charge Q c  of +0.40E −07  C/cm 2  is applied on the insulating layer  16  such that the total corona charge Q applied  is 0.00E −07  C/cm 2 . The third SPV measurement V SPV  is about 0.060 volts. Note that the polarity of the third deposited corona charge Q c  was changed, because the SPV measurements V SPV  were going away from the target value (zero) and the magnitude of the third deposited corona charge Q c  was changed, specifically increased or doubled, to avoid duplicating the first measurement. A fourth corona charge Q c  of +0.20E −07  C/cm 2  is applied on the insulating layer  16  such that the total corona charge Q applied  is +0.20E −07  C/cm 2 . The fourth SPV measurement V SPV  is about −0.100 volts. A fifth corona charge Q c  of −0.10E −07  C/cm 2  is applied on the insulating layer  16  such that the total corona charge Q applied  is +0.10E −07  C/cm 2 . The fifth SPV measurement V SPV  is about 0.000 volts and substantially equal to the target value V SPVtarget . Note that the polarity of the fifth deposited corona charge Q c  was changed because the fourth SPV measurement V SPV  went past the target value (zero) V SPVtarget  and the magnitude of the fifth deposited corona charge Q c  was changed, specifically reduced by half, to avoid duplicating the third measurement. Therefore, the total applied corona charge Q applied@target  to obtain the target value V SPVtarget  is +0.10E −07  C/cm 2 . The controller  36  then determines the total charge Q tot  of the insulating layer is +0.10E −07  C/cm 2 . 
     In a second method of measuring the total charge Q tot  of the insulating layer  16  according to the present invention, the position actuator  34  alternately locates the corona gun  26  and the SPV device  30  over the measuring region  22  of the wafer  12  to deposit increments of corona charge Q c  on the insulating layer  16  and to obtain SPV measurements V SPV  of the insulating layer  16 . The controller  36  records each SPV measurement V SPV  and determines and records the total corona charge Q applied  applied to the insulating layer  16  to obtain that SPV measurement V SPV . Therefore, a data set is obtained containing the plurality of SPV measurements V SPV  along with the corresponding total applied corona charges Q applied . The controller  36  then determines the total applied corona charge Q applied@target  required for the SPV measurement V SPV  to be substantially equal to the target value V SPVtarget  from the data set. The value Q applied@target  is preferably interpolated from the data set of discrete points. The controller  36  then determines the total charge Q tot  of the insulating layer  16  from the total applied corona charge Q appliedatarget  wherein the total charge Q tot  is again the negative of the total applied corona charge Q applied@target , i.e. Q tot =−Q applied@target . FIG. 4 illustrates an example of this second method wherein the target value V SPVtarget  is zero volts, or flatband condition. A data set is obtained by incrementally depositing a plurality of corona charges Q c  on the insulating layer and obtaining a SPV measurement V SPV  for each incremental deposition. The illustrated data set contains 19 discrete points containing the SPV measurements V SPV  and the corresponding total applied corona charges Q applied . The controller  36  interpolates the discrete points to determine that the total applied corona charge Q applied@target  at the target value V SPVtarget  is about +0.10E −07  C/cm 2 . The controller  36  then determines the total charge Q tot  of the insulating layer is +0.10E −07  C/cm 2 . 
     When the target value V SPVtarget  is zero volts, each of the SPV measurements V SPV  are preferably corrected with a small Dember Voltage correction in either of the methods. The Dember Voltage correction is a small “second order” correction which can be applied via well known equations. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Technology Category: g