JP2006013099A - Method for evaluating soi wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an evaluation method that dispenses with a large-scale apparatus and a number of processes, such as a photolithography process, and can quickly, easily, and precisely measure the electrical characteristics of an SOI wafer, and improves the working efficiency of a measuring apparatus for evaluating the SOI wafer efficiently. <P>SOLUTION: In the method for evaluating the SOI wafer using a mercury probe, at least a natural oxide film formed on the surface of the SOI wafer is removed by performing fluoric acid cleaning treatment to the SOI wafer, corona discharge treatment is made to the SOI wafer in which the natural oxide film is removed for placing charges on the surface of the SOI layer in the SOI wafer, and the mercury probe is brought into contact with the SOI wafer that is subjected to corona discharge treatment for evaluating the SOI wafer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an SOI wafer evaluation method for evaluating an SOI layer of an SOI wafer and an interface between the SOI layer and a buried oxide layer using a mercury probe.

  In recent years, SOI wafers having an SOI structure in which an SOI layer (also referred to as a silicon active layer) is formed on an electrically insulating oxide film have been developed to have high speed, low power consumption, high withstand voltage, Due to its excellent environmental properties and the like, it is particularly attracting attention as a high-performance LSI wafer for electronic devices. This is because an SOI wafer has a buried oxide film (hereinafter sometimes referred to as a BOX layer) that is an insulator between the support substrate and the SOI layer, and the electronic device formed in the SOI layer has a withstand voltage. This is because it has a great advantage that it is high and the soft error rate of α rays is low.

  In addition, in a thin film SOI wafer having an SOI layer thickness of 1 μm or less, a MOS (Metal Oxide Semiconductor) type semiconductor device formed on the SOI layer has a source / drain PN junction area when operated in a fully depleted type. Since the capacitance can be reduced, parasitic capacitance can be reduced and device drive speed can be increased. Furthermore, since the capacitance of the BOX layer serving as the insulating layer is in series with the depletion layer capacitance formed immediately below the gate oxide film, the depletion layer capacitance is substantially reduced, and low power consumption can be realized.

  Recently, higher-quality SOI wafers are required for further miniaturization and higher performance of electronic devices. For this reason, the quality of the SOI layer of SOI wafers is actively evaluated. As one method for evaluating the quality of the SOI wafer, a MOS structure is formed on the surface of the SOI layer, and a voltage is applied to the electrode portion to evaluate the quality of the SOI layer.

  However, when a MOS structure is formed on an SOI layer in order to evaluate an SOI wafer, a large-scale apparatus and a large number of processes are required to perform a photolithography process and the like. There were problems such as chipping. Further, this method can evaluate the quality of the surface of the SOI layer, but is incomplete as an evaluation of the interface between the SOI layer and the BOX layer.

  Therefore, an evaluation method has been developed that can more easily evaluate an SOI wafer using a mercury probe without forming a MOS structure on the SOI wafer through a number of conventional processes. As one of them, a Pseudo MOS FET method for evaluating an SOI wafer has been proposed (see, for example, Patent Documents 1 and 2 and Non-Patent Documents 1 and 2). According to this method, the interface state density at the interface between the SOI layer and the BOX layer, the electrical characteristics of the SOI layer, and the like can be measured accurately and simply.

  Here, the Pseudo MOS FET method will be briefly described with reference to the drawings. First, as shown in FIG. 8, a needle probe or a mercury probe is directly contacted as an evaluation electrode on the SOI layer 1 side of the SOI wafer 5 forming a pseudo MOS structure using the BOX layer 2 as a gate oxide film. Is a source electrode 6 and a drain electrode 7. Then, the back surface of the SOI wafer 5, that is, the surface on the support wafer 3 side of the SOI wafer 5 is vacuum-sucked on a stage that is also used as an electrode to form the gate electrode 4, and a voltage is applied between these electrodes. Various electrical characteristics can be obtained. In this case, the gate electrode 4 can be formed, for example, by bringing a needle into contact with the back surface of the SOI wafer 5.

  Further, in the evaluation of the SOI wafer by such a Pseudo MOS FET method, the natural oxide film formed on the surface of the SOI layer can be removed by washing the SOI wafer with an aqueous solution containing hydrofluoric acid before the evaluation. It becomes possible to evaluate the electrical characteristics of the SOI wafer more accurately by eliminating the influence of the natural oxide film.

  Furthermore, in this evaluation method using the Pseudo MOS FET method, if a mercury probe is used as the source electrode and the drain electrode, a probe contact hole generated when the needle probe is brought into contact with the surface of the SOI layer is not formed. In addition, the measurement near the first measurement point can be performed easily and stably.

Then, after forming a Pseudo MOS structure as described above, the relationship between the gate voltage _{V G} and the drain current _{I D} by applying a gate voltage to the positive side while applying a drain voltage, i.e., the _V G -I _D characteristic By measuring, the electron mobility of the SOI layer and the interface state density at the interface between the SOI layer and the BOX layer can be evaluated. On the other hand, by measuring the V _G -I _D characteristic by applying a gate voltage on the negative side while applying a drain voltage, it is possible to evaluate the charge density of the hole mobility and the BOX layer of the SOI layer.

However, when the SOI wafer is evaluated by applying the gate voltage to the negative side as described above, conventionally, the SOI wafer is washed with an aqueous solution containing hydrofluoric acid to remove the natural oxide film on the surface of the SOI layer. since the V _G -I _D characteristic 10-12 hours and then start the measurements, or measurements to be passed more is not stable in, it was not possible to make accurate measurements of V _G -I _D characteristic . Therefore, a very long evaluation time is required to evaluate the SOI wafer, and there is a problem that the efficiency of the SOI wafer evaluation is hindered by reducing the operating rate of the measuring apparatus.

  In this case, since the SOI wafer must be precisely managed so that impurities do not adhere to the wafer after the natural oxide film is removed and until the measurement is completed, there is a problem that the management burden is large. there were.

JP 2001-60676 A JP 2001-267384 A S. Cristoleveanu et al., "A Review of the Pseudo-MOS Transistor in SOI Wafers: Operation, Parameter Extraction, and Applications" IEEE Trans. Electron Dev, 47 1018 (2000). H.J.Hovel, "Si film electrical characterization in SOI substrates by HgFET technique" Solid-State Electronics, 47, 1311 (2003).

  Accordingly, the present invention has been made in view of the above problems, and the object of the present invention is to eliminate the need for a large-scale apparatus such as a photolithography process or the like and a large number of processes, and to shorten the electrical characteristics of the SOI wafer. An object of the present invention is to provide an evaluation method that can be measured easily and accurately in time, and that can efficiently evaluate an SOI wafer by improving the operating rate of the measuring device.

  In order to achieve the above object, according to the present invention, in a method for evaluating an SOI wafer using a mercury probe, at least the SOI wafer is formed on the surface of the SOI wafer by performing a hydrofluoric acid cleaning treatment. After removing the natural oxide film, the SOI wafer from which the natural oxide film has been removed is subjected to a corona discharge treatment so that a charge is placed on the SOI layer surface of the SOI wafer, and then the SOI wafer subjected to the corona discharge treatment. An SOI wafer evaluation method is provided, wherein an SOI wafer is evaluated by bringing a mercury probe into contact therewith (claim 1).

  In this way, after removing the natural oxide film formed on the SOI wafer by the hydrofluoric acid cleaning process, the charge state on the surface of the SOI layer is quickly changed by performing the corona discharge process to place the charges on the surface of the SOI layer. To a steady state. As a result, the time required for measuring the electrical characteristics of the SOI wafer after removing the natural oxide film can be greatly reduced compared to the conventional method, and the evaluation of the SOI wafer can be performed in a very short time. The utilization rate can be improved. In addition, since the present invention can effectively stabilize the charge state on the surface of the SOI layer in a short time before measuring the electrical characteristics of the SOI wafer, the management of the SOI wafer is facilitated. Furthermore, variations in measured values can be reduced, and highly reliable SOI wafers can be stably evaluated.

At this time, it is preferable that the charge placed on the SOI layer surface of the SOI wafer by the corona discharge treatment is a positive charge.
In this way, by making the charge placed on the surface of the SOI layer by corona discharge treatment positive, for example, hydrogen ions such as H ⁺ can be used as a charged gas that exposes the wafer surface. When this hydrogen ion is released from the corona charge electrode during the corona discharge treatment, moisture (H ₂ O) in the air gathers around the hydrogen ion to form (H ₂ O) _n H ^+. It is mounted as a positive charge on the surface of the layer. Since a large amount of moisture combined with hydrogen ions exists in the air, it is not necessary to use a dedicated chamber for performing corona discharge treatment, and charges can be placed on the surface of the SOI layer more easily. .

Furthermore, it is preferable that the amount of electric charge placed on the surface of the SOI layer by the corona discharge treatment is 500 nC / cm ² or more and 50000 nC / cm ² or less (Claim 3).
Thus, by setting the charge amount to be placed on the surface of the SOI layer by the corona discharge treatment to 500 nC / cm ² or more and 50000 nC / cm ² or less, the charge state of the surface of the SOI layer subjected to the hydrofluoric acid cleaning treatment in a short time, It can be stabilized very effectively.

In the present invention, the hole mobility and / or embedding of the SOI layer in the SOI wafer is measured by bringing a mercury probe into contact with the corona discharge treated SOI wafer and measuring the V _{G- ID} characteristics on the hole side. It is preferable to evaluate the charge density of the oxide film.
As described above, in the present invention, the charge state on the surface of the SOI layer can be stabilized in a short time by removing the natural oxide film by the hydrofluoric acid cleaning process and then performing the corona discharge process. By making the mercury probe contact and measuring the V _{G- ID} characteristics on the hole side, the variation in the measured value can be significantly reduced. Therefore, in the past, the measured values were likely to vary, and the hole mobility of the SOI layer and the charge density of the buried oxide film, which required a long time for accurate measurement, were evaluated with high accuracy and high reliability in a short time. can do.

Furthermore, in the present invention, after the natural oxide film is removed by performing the hydrofluoric acid cleaning treatment, the mercury probe is brought into contact with the SOI wafer from which the natural oxide film has been removed to measure the V _{G- ID} characteristics on the electron side. It is preferable to evaluate the electron mobility of the SOI layer and / or the interface state density between the SOI layer and the buried oxide film, and then perform the corona discharge treatment.
Thus, after removal of the native oxide film by hydrofluoric acid cleaning process, prior to the corona discharge treatment, by measuring the V _G -I _D characteristic of the electron-side contacting the mercury probe in the SOI wafer, an electron transfer It is possible to evaluate the degree and interface state density with high accuracy, and to evaluate the quality of the SOI wafer in more detail.

At this time, after the hydrofluoric acid cleaning treatment, the SOI wafer is subjected to corona discharge treatment before the mercury probe is brought into contact with the SOI wafer to measure the V _{G- ID} characteristics on the electron side. It is preferable to place a negative charge on (Claim 6).
Thus, after the hydrofluoric acid cleaning treatment, by loaded on a negative charge to the corona discharge treatment of the SOI layer surface by performing the SOI wafer, the SOI layer surface before measuring the V _G -I _D characteristic of the electron-side it is possible to stabilize the charge states in a short time, a measurement of V _G -I _D characteristic of the electron-side can be stably performed without causing a variation in measurement value, whereby the electron mobility and the interface The level density can be evaluated with higher accuracy.

  As described above, according to the present invention, when an SOI wafer is evaluated, a charge is placed on the surface of the SOI layer by performing a corona discharge treatment after the hydrofluoric acid cleaning treatment. To a steady state. As a result, the time required to measure the electrical characteristics of the SOI wafer after removing the natural oxide film can be greatly reduced compared to the conventional method, and the evaluation of the SOI wafer can be performed in a very short time. It is also possible to improve the operating rate. Further, in the present invention, since the time from the removal of the natural oxide film to the measurement of the electrical characteristics of the SOI wafer can be greatly shortened, the management of the SOI wafer is facilitated, and the variation in the measured value is further reduced. A highly accurate and highly reliable SOI wafer can be stably evaluated.

Hereinafter, although an embodiment is described about the present invention, the present invention is not limited to these.
Conventionally, when evaluating the hole mobility and BOX layer charge density of an SOI layer in an SOI wafer by the Pseudo MOS FET method, the SOI wafer was washed with an aqueous solution containing hydrofluoric acid to remove the natural oxide film on the surface of the SOI layer. After that, the measured value is not stable unless more than 10 hours elapses. Therefore, there is a problem that a very long time is required for the evaluation of the SOI wafer.

This is for example the case of measuring the V _G -I _D characteristic of the gate voltage, such as evaluation of the hole mobility and BOX layer charge density is applied to the negative side, the positive charge such as H + ions on the SOI layer surface If the surface state is not controlled by adsorbing, the charge on the surface of the SOI layer is not stabilized and accurate measurement cannot be performed. That is, after removing the natural oxide film on the surface of the SOI layer by cleaning with hydrofluoric acid, the electrical state on the surface of the SOI layer is not stable unless 10 hours or more elapses, so that the measurement cannot be performed.

  Therefore, the present inventors, when evaluating an SOI wafer using a mercury probe, evaluate the SOI wafer by performing a process of stabilizing the charge state on the surface of the SOI layer after removing the natural oxide film. We thought that the time could be shortened and repeated diligent experiments and studies. As a result, after cleaning the hydrofluoric acid on the SOI wafer, a corona discharge treatment is performed to place charges on the surface of the SOI layer, and then the SOI wafer is evaluated using a mercury probe, thereby reducing the evaluation of the SOI wafer. The present invention was completed by finding that it can be performed with high accuracy in time.

Hereinafter, an SOI wafer evaluation method of the present invention will be described in detail with reference to the drawings. Here, FIG. 1 is a flowchart showing an example of an SOI wafer evaluation method according to the present invention. The flow chart shown in FIG. 1 shows that after performing hydrofluoric acid cleaning treatment, the V _{G- ID} characteristics on the electron side of the SOI wafer are evaluated to evaluate the electron mobility / interface state density, and then the hole Although the case where the hole mobility / charge density is evaluated by performing the V _{G- ID} characteristics on the side is shown, the present invention is not limited to this at all, and as described in detail below, For the purpose of performing the process from step A to step E only for evaluating the electron mobility / interface state density, or omitting the step C to step E and performing only the evaluation for hole mobility / charge density. It can be changed accordingly.

  First, as shown in FIG. 1, an SOI wafer to be evaluated is prepared (step A). The SOI wafer to be evaluated in the present invention may have any SOI structure in which, for example, a buried oxide film serving as an insulating layer and an SOI layer are formed on a support wafer, and its manufacturing method is particularly limited. It is not something. For example, as an SOI wafer to be prepared, the polished surfaces of two mirror-polished wafers in which a silicon oxide film is formed on at least one silicon wafer surface are bonded to each other, and after heat treatment, one wafer is ground and thinned by polishing. A thing can be used (bonding method). In addition, hydrogen is ion-implanted into one mirror-polished wafer in advance, and another wafer is polished and bonded to the polished surface, followed by heat treatment to peel off one wafer from the hydrogen-ion-implanted layer. Then, after forming the SOI structure, the surface of the thin film that becomes the SOI layer can be polished (hydrogen ion peeling method). Further, a so-called SIMOX (Separated Implanted Oxide) wafer manufactured by performing high-temperature heat treatment after ion implantation of oxygen into one mirror-polished wafer may be used.

  Next, the prepared SOI wafer is subjected to a hydrofluoric acid cleaning process using an aqueous solution containing hydrofluoric acid to remove a natural oxide film formed on the SOI layer surface of the SOI wafer (step B in FIG. 1). Normally, a thin silicon oxide film called a natural oxide film is formed on the wafer surface by touching the atmosphere of an SOI wafer. Since this natural oxide film is not uniformly formed on the surface of the SOI wafer, the thickness of the oxide film varies within the wafer surface. Even if an SOI wafer on which such a natural oxide film is formed is evaluated as it is, an accurate evaluation cannot be performed due to variations in the thickness of the natural oxide film and the influence of impurities contained in the natural oxide film. Therefore, first, the prepared SOI wafer is subjected to a hydrofluoric acid cleaning process to remove the natural oxide film on the wafer surface.

  At this time, the hydrofluoric acid concentration in the aqueous solution used for the hydrofluoric acid cleaning treatment is not particularly limited as long as the natural oxide film can be removed. For example, if the hydrofluoric acid concentration is too high, the hydrofluoric acid concentration is interposed between the SOI layer and the supporting wafer. There is a possibility that the BOX layer to be etched will also be etched. Accordingly, the hydrofluoric acid concentration is preferably relatively low. For example, it is preferable to use an aqueous solution having a hydrofluoric acid concentration of 0.5% to 5%, particularly about 1%. Further, the cleaning conditions such as the aqueous solution temperature and the cleaning time when performing the hydrofluoric acid cleaning process only need to be such that the natural oxide film can be removed, and can be changed as needed.

  The SOI wafer subjected to the hydrofluoric acid cleaning treatment with the aqueous solution containing hydrofluoric acid in this way is then rinsed with, for example, pure water and dried. As a drying method, drying may be performed by blowing dry air onto an SOI wafer, or by using a device such as a spin dryer. Alternatively, it may be dried using a chemical solution such as IPA (isopropyl alcohol).

  After the SOI wafer is dried, the SOI wafer is placed on a corona charge device and subjected to a corona discharge treatment, whereby a charge (negative charge) is placed on the surface of the SOI layer of the SOI wafer (step C in FIG. 1). ). FIG. 2 is a schematic configuration diagram illustrating an example of a corona charge device that performs corona discharge treatment on an SOI wafer. The corona charge device 11 shown in FIG. 2 has a stage 12, and this stage 12 is connected to an XY driving motor (not shown) so as to be driven in the XY axis direction. In addition, a charge generation unit 13 made of a metal wire and generating charges is provided above the stage 12. The charge generator 13 is fixed so that the distance between the tip of the charge generator 13 and the SOI wafer W placed on the stage 12 is about 1 to 50 cm, particularly about 20 cm.

When the corona discharge process is performed using such a corona charge device 11, the SOI wafer W is placed on the stage 12 with the SOI layer facing upward, and then the high voltage application power source is applied to the charge generation unit 13. By applying a negative high voltage from (not shown), negative corona ions are poured from the charge generation unit 13 onto the SOI layer surface of the SOI wafer W. At this time, since it is difficult to generate negative corona ions in the air, a closed chamber (not shown) surrounding the corona charge device 11 is set in advance, and the charge is charged after the inside of the chamber is filled with carbon dioxide gas. By applying a negative high voltage to the generator 13, carbon dioxide ions CO ₂ ⁻ can be generated as negative corona ions. Since such negative corona ions are linearly poured from the charge generation unit 13 toward the SOI wafer W, the negative charge is applied to the entire SOI layer of the SOI wafer W by driving the stage 12 in the X or Y direction. (Negative ions) can be placed.

  Thus, by placing negative charges on the SOI layer surface of the SOI wafer, the charge state on the surface of the SOI layer can be quickly stabilized to a steady state. The treatment time of the corona discharge treatment can be appropriately determined according to the amount of charge to be placed on the surface of the SOI layer. For example, the corona discharge treatment is performed for about 1 to 30 minutes, particularly about 10 minutes. Thus, the charge state on the SOI layer surface can be reliably stabilized.

Then, SOI wafer W obtained by loaded on a negative charge on the SOI layer surface as described above, a measurement of V _G -I _D characteristic of the electron-side by using a mercury probe apparatus 21 as shown in FIG. 3 (Step D in FIG. 1). For example, the SOI wafer W is placed so that the SOI layer faces downward, that is, after the surface on the SOI layer side is placed on a stage (not shown) and stored in the apparatus, the opposite side to the surface placed on the stage The surface of the SOI wafer, that is, the surface on the support wafer side of the SOI wafer is adsorbed by the vacuum chuck 22 from above. The vacuum chuck 22 is made of a conductive material such as metal and also serves as a gate electrode.

  When the surface of the SOI wafer W on the support wafer side is attracted to the vacuum chuck 22, the stage is moved away from the SOI wafer W. Thereafter, the mercury probe 23 is brought close to the surface of the SOI layer of the SOI wafer W, and only the mercury electrode portion is brought into contact with the SOI layer. At this time, the mercury probe 23 has a structure as shown in FIG. 4, and one of the mercury electrode portions 24 and 25 is used as a source electrode, and the other is used as a drain electrode. In this way, for example, a Pseudo-MOS structure as shown in FIG. 8 can be formed.

A constant drain voltage is applied in a state in which the Pseudo-MOS structure is formed, and in this state, the gate voltage is applied to the positive side to change it, and the change in the drain current is monitored, whereby the gate voltage V _G on the electron side is monitored. a relationship between the drain current _{I D,} i.e., it is possible to measure the _V G -I _D characteristic of the electron side. V _G -I _D characteristic of the measured electron side is displayed as shown in FIG. 6, for example.

Then, by using a formula that has a gradient of C portion and D portion, for example, in Non-Patent Document 1 or 2 in the V _G -I _D characteristic of the measured electron side as shown in FIG. 6, the SOI wafer The electron mobility of the SOI layer and / or the interface state density between the SOI layer and the buried oxide film can be obtained and evaluated (step E in FIG. 1). In this case, in the present invention, before measuring the V _G -I _D characteristic of the electron side of the SOI wafer as described above, the charge state of the SOI layer surface by loaded on a negative charge on the SOI layer surface by corona discharge treatment Therefore, the measurement of the V _{G- ID} characteristics on the electron side can be performed stably without causing variations in measured values, and the evaluation of electron mobility and interface state density can be performed with higher accuracy. Can be done.
The measurement of the V _{G- ID} characteristics on the electron side (step D) and the evaluation of the electron mobility / interface state density (step E) can be performed in about 2 hours in total of the two steps. it can.

Subsequently, after evaluating the electron mobility and interface state density of the SOI wafer as described above, the SOI wafer is subjected to corona discharge treatment to place a positive charge on the surface of the SOI layer (step F in FIG. 1). ). In this way, the corona discharge treatment for placing positive charges on the surface of the SOI layer can be performed using the corona charge device 11 of FIG. 2 described above. For example, an SOI wafer W to be subjected to corona discharge treatment is placed on the stage 12 with the SOI layer facing upward. Thereafter, by applying a positive high voltage to the charge generation unit 13, hydrogen ions (H ⁺ ) are released from the charge generation unit 13, and moisture (H ₂ O) in the air gathers around the hydrogen ions ( The positive corona ions in the state of H ₂ O) _n H ⁺ are poured onto the SOI layer surface of the SOI wafer W, so that positive charges (positive ions) are placed on the SOI layer surface of the SOI wafer W. be able to.

  When a positive charge is placed on the surface of the SOI layer in this way, a large amount of moisture that forms positive corona ions by combining with hydrogen ions exists in the air. For example, the negative charge described above is loaded. There is no need to use a dedicated chamber as in the case of the upper layer, and a positive charge can be easily placed on the surface of the SOI layer. In this way, the corona discharge treatment is performed for about 1 to 30 minutes, particularly about 10 minutes, and positive charges are placed on the SOI layer surface of the SOI wafer, so that the charge state on the SOI layer surface can be changed for a very short time. It can be stabilized to achieve a steady state.

After it loaded on the positive charge on the SOI layer surface as described above, using a mercury probe apparatus 21 shown in FIG. 3 for measuring the V _G -I _D characteristic of the hole side of the SOI wafer (FIG. 1 step G ). For example, after the surface of the SOI wafer W on the support wafer side is adsorbed to the vacuum chuck 22, the mercury probe 23 is brought close to the SOI layer surface of the SOI wafer W, and only the mercury electrode portion is brought into contact with the SOI layer. A Pseudo-MOS structure is formed. Thereafter, a constant drain voltage is applied from the mercury probe 23, and in this state, the gate voltage is applied and changed on the negative side, and the change in the drain current is monitored, for example, on the hole side as shown in FIG. it can be measured V _G -I _D characteristic.

Then, by using the formula shown from the slope of the parts A and B, in Non-Patent Document 1 or 2 in the V _G -I _D characteristic of the measured hole side as shown in FIG. 5, the SOI wafer The hole mobility of the SOI layer and / or the charge density of the BOX layer can be obtained and evaluated (Step H in FIG. 1).

In particular, the present invention, before measuring the V _G -I _D characteristic of the hole side of the SOI wafer as described above, the charge state of the SOI layer surface by loaded on the positive charge on the SOI layer surface by corona discharge treatment since short time to stabilize the can to measure the V _G -I _D characteristic of the hole-side stably without causing variations in measurements, the hole mobility and the BOX layer of the SOI layer The charge density can be evaluated with higher accuracy in a shorter time.
The measurement of V _G -I _D characteristic of the hole-side (Step G) and hole mobility / evaluation of the charge density (step H) is be carried out in approximately 2 hours about the combined two steps it can.

Here, FIG. 7 shows the result of investigation by the inventors of the present invention on the relationship between the amount of charge to be placed on the surface of the SOI layer by corona discharge treatment and the drain current measured by forming the Pseudo-MOS structure. Figure 7 performs the measurement of the _V G -I _D characteristic by changing the amount of charge to form a Pseudo-MOS structure in a variety of SOI wafer was subjected to corona discharge _treatment, was measured at V G = -10 V drain It is the graph which plotted the value of electric current _ID .

As shown in FIG. 7, it can be seen that the drain current is very stable when the amount of charge to be placed on the surface of the SOI layer by the corona discharge treatment is 500 nC / cm ² or more. Therefore, it is preferable that the amount of charge to be placed on the surface of the SOI layer by the corona discharge treatment is 500 nC / cm ² or more, whereby the charge state on the surface of the SOI layer can be stabilized very effectively in a short time. . On the other hand, even if the amount of charge placed on the surface of the SOI layer is excessively large, it can be assumed that the drain current is stable at a constant value and it is difficult to obtain further effects. It is conceivable that the load on the corona charging device to be used increases and the burden on the cost also increases. Therefore, it is preferable that the amount of electric charge placed on the surface of the SOI layer by the corona discharge treatment is 50000 nC / cm ² or less, particularly 3000 nC / cm ² or less.

As described above, according to the method for evaluating an SOI wafer of the present invention, hydrofluoric acid cleaning treatment is performed on the SOI wafer to remove the natural oxide film on the SOI wafer surface, and then corona discharge treatment is performed to charge the SOI layer surface. , The charge state on the surface of the SOI layer can be quickly stabilized to a steady state, and thereafter, a mercury probe is brought into contact with the SOI wafer having a stable charge state on the surface of the SOI layer. _{The G- ID} characteristics can be measured, and the electrical characteristics of the SOI wafer such as electron mobility, interface state density, hole mobility, and BOX layer charge density can be evaluated. Thereby, without requiring a large-scale equipment and a large number of steps such as a photolithography process, to be able to very easily carry out the evaluation of an SOI wafer by using a mercury probe, also, V _G -I _D characteristic Since the charge state on the surface of the SOI layer is very stable when measuring the measurement, variations in the measured value can be reduced, and highly reliable SOI wafers can be evaluated with high accuracy and stability.

In particular, when evaluating hole mobility and BOX layer charge density of the SOI wafer before the conventional for measuring the _V G -I _D characteristic due to the mercury probe, charge of the SOI layer surface to be placed over 10-12 hours While the state could not make an accurate measurement of the V _G -I _D characteristic for not stable, according to the present invention, to stabilize the charge states in a short as 10 minutes time by performing a corona discharge treatment be able to. Therefore, the present invention can significantly reduce the evaluation time of SOI wafers compared to the prior art, especially when evaluating hole mobility and BOX layer charge density. The evaluation of the SOI wafer can be performed very efficiently, and the management of the SOI wafer when performing the evaluation is much easier than before.

Note that the SOI wafer evaluation method of the present invention is not limited to the process shown in the flow chart of FIG. 1, for example, and the process can be changed or deleted as needed.
For example, if it is necessary to evaluate the electron mobility, interface state density, hole mobility, and BOX layer charge density in an SOI wafer at a lower cost, the negative charge shown in step C of FIG. by omitting the corona discharge treatment, it may be measured V _G -I _D characteristic of hydrofluoric acid cleaning process immediately after the electronic side. In order to perform this negative charge corona discharge treatment, a closed chamber for filling carbon dioxide gas (for example, CO ₂ ⁻ ) is required around the corona charge device as described above. The device manufacturing cost can be greatly reduced. In addition, since the V _{G- ID} characteristics on the electron side are relatively stable if measured immediately after removal of the natural oxide film, the electron mobility and / or interface state density can be measured correctly. Is possible. Furthermore, in some cases, in the corona discharge treatment in step C of FIG. 1, a positive charge, not a negative charge, can be placed on the surface of the SOI layer.

Furthermore, for example, when only evaluating the electron mobility and / or interface state density of an SOI wafer, the steps A to E in FIG. 1 may be performed in order, while the hole mobility of the SOI wafer is performed. If only the charge density of the BOX layer is to be evaluated, Steps F to H may be performed without performing Steps C to E after performing Steps A and B. In this case, V _G -I _D characteristic of the hole side as described above, since the measured value and measured immediately after removal of the native oxide film takes a long time to stabilize, be corona discharge treatment step F is necessary.

  In addition, if necessary, two SOI wafers manufactured under the same conditions in Step A are prepared, and one of the SOI wafers is sequentially processed from Steps B to E to evaluate electron mobility and / or interface state density. Further, the hole mobility and / or the charge density of the BOX layer may be evaluated by sequentially performing the steps B and F to H on the other SOI wafer. Thus, by preparing two SOI wafers and evaluating each separately, the gate voltage sweeps only on the positive side or the negative side, so that the SOI wafer can be evaluated in a shorter time. In addition, the stress applied to the SOI wafer can be reduced.

  Furthermore, in the present invention, the SOI wafer may be evaluated by replacing the processes C to E and the processes F to H in FIG. That is, after preparing an SOI wafer and performing a hydrofluoric acid cleaning treatment (steps A and B), the steps F to H are performed first to evaluate the hole mobility / BOX layer charge density, and then the step C. -E can be performed to evaluate the electron mobility / interface state density. In this case, for example, after evaluating the hole mobility / BOX layer charge density in Step H, the SOI wafer surface is cleaned to remove positive charges on the SOI layer, and Steps C to E are performed. It is preferable. In this way, after performing Steps F to H and before performing Steps C to E, by removing the positive charge on the surface of the SOI layer, the electron mobility and interface state density can be increased with higher accuracy. It can be evaluated with high reliability. When the SOI wafer is evaluated by exchanging the steps C to E and the steps F to H in FIG. 1, for example, the negative charge corona discharge process in the step C is omitted in order to reduce the cost. can do.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.
(Example)
By using a silicon wafer having a p-type conductivity, a diameter of 200 mm, and a crystal orientation <100> as a bond wafer for forming a support wafer and an SOI layer, an SOI wafer is manufactured by a hydrogen ion delamination method. An SOI wafer was prepared (step A). Boron was used as a dopant for making the conductivity type of the wafer p-type. Moreover, the thicknesses of the SOI layer and the BOX layer of the manufactured SOI wafer were about 100 nm and 145 nm, respectively.
The prepared SOI wafer was subjected to a hydrofluoric acid cleaning treatment for 1 minute using an aqueous solution containing 1% by weight of hydrofluoric acid (step B). Then, after rinsing with pure water, moisture was removed by drying air on the SOI wafer to dry it.

After drying, the corona discharge treatment (step C) for placing a negative charge on the surface of the SOI layer is not performed, and the SOI wafer is immediately set in a mercury probe device (CVmap 92 manufactured by Four DIMENSIONS) and V _G -I on the electronic side. _D characteristics were measured (step D). The measurement results are shown in FIG. At this time, the measurement of V _G -I _D characteristic of the electron-side can be performed by monitoring the drain current causes the gate voltage is changed while applying a constant drain voltage from the drain electrode. Then, the measurement result of the V _G -I _D characteristic of the electron side shown in FIG. 9, by using the formula shown in Non-Patent Documents 1 and 2, the electron mobility and / or SOI layer of the SOI layer And the interface state density of the buried oxide film can be obtained and evaluated (step E). At this time, from the start of measurement of the V _{G- ID} characteristics on the electron side to the determination of electron mobility and interface state density, it could be performed in about 2 hours.

Subsequently, using KG101 manufactured by SEMILAB as a corona charge device, the SOI wafer was subjected to a corona discharge treatment for 10 minutes, and a positive charge was placed on the SOI layer surface of the SOI wafer with a charge amount of 3000 nC / cm ² ( Step F). Immediately thereafter, the SOI wafer was set in a mercury probe apparatus (CVmap 92 manufactured by Four DIMENSIONS), and the V _{G- ID} characteristics on the hole side were measured (step G). The measurement results are shown in FIG. Then, the measurement result of the V _G -I _D characteristic of the positive hole side shown in FIG. 10, by using the formulas disclosed in Non-Patent Documents 1 and 2, the hole mobility and / or BOX of the SOI layer The charge density of the layer can be determined and evaluated (Step H). At this time, from the start of the measurement of the V _G -I _D characteristic of the hole side to determine the charge density of the hole mobility and the BOX layer of the SOI layer, it could be done in about 2 hours.
Therefore, in the evaluation of the SOI wafer performed in this example, the electron mobility, the interface state density, the hole mobility, and the BOX layer are obtained in a short time of about 4 hours after the hydrofluoric acid cleaning treatment is performed on the SOI wafer. The charge density could be evaluated.

(Comparative Example 1)
As Comparative Example 1, an SOI wafer manufactured under the same conditions as in the example was prepared, and this SOI wafer was subjected to a hydrofluoric acid cleaning treatment for 1 minute with an aqueous solution containing 1% by weight of hydrofluoric acid. After rinsing, moisture was removed by drying air on the SOI wafer to dry it.

After drying the SOI wafer, loading of negative charges on the SOI layer by corona discharge treatment, measurement of V _{G- ID} characteristics on the electron side, and evaluation of electron mobility / interface state density were not performed. The SOI wafer was immediately set on a mercury probe device without carrying a positive charge on the SOI layer by corona discharge treatment, and the V _{G- ID} characteristics on the hole side were measured. The measurement results are shown in FIG. The measurement results shown in FIG. 11, when compared with V _G -I _D characteristic of the hole side of Figure 10 measured in Example, that the I _D becomes generally lower for V _G Recognize. This is considered to be caused because of the measurement of the V _G -I _D characteristic of the hole-side charge state of the surface of the SOI layer of the SOI wafer without stability.

(Comparative Example 2)
As Comparative Example 2, an SOI wafer manufactured under the same conditions as in the examples was prepared, and this SOI wafer was subjected to a hydrofluoric acid cleaning treatment for 1 minute with an aqueous solution containing 1% by weight of hydrofluoric acid. After rinsing, moisture was removed by drying air on the SOI wafer to dry it.

Next, after exposing the SOI wafer during 12 hours air, negative charge loaded on the measurement of V _G -I _D characteristic of the electron-side, the electron mobility / interface state density to SOI layer by corona discharge treatment In addition, without setting the positive charge on the SOI layer by corona discharge treatment, it was set in a mercury probe device and the V _{G- ID} characteristics on the hole side were measured. The measurement results are shown in FIG. Then, the hole mobility of the SOI layer and the charge density of the BOX layer were obtained from the measurement results of FIG. 12 and evaluated. At this time, from the start of the measurement of the V _G -I _D characteristic of the hole side to determine the charge density of the hole mobility and the BOX layer of the SOI layer, it could be performed in about 1 hour.

The measurement results shown in FIG. 12, it was found that V _G -I _D characteristic of the positive hole side measured in Example (FIG. 10) is substantially the same. However, in Comparative Example 2, the evaluation of the electron mobility and the interface state density is not performed, but after the hydrofluoric acid cleaning treatment is performed on the SOI wafer, the hole mobility and the charge density are obtained. It took 13 hours or more and took about three times as long as the above example.

From the above results, since the charge state on the surface of the SOI layer can be stabilized in a short time by performing corona discharge treatment when evaluating the SOI wafer, the time required for the evaluation of the SOI wafer, particularly hydrofluoric acid cleaning it was possible to confirm that reliably shorten the time to measure the V _G -I _D characteristic of the positive hole side after performing a process. In addition, there are few disturbance factors such as wafer contamination as in the case where the wafer is exposed for 12 hours or more, and the reliability of the data is high.

  The present invention is not limited to the above embodiment. The above embodiment is merely an example, and the present invention has the same configuration as that of the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

It is a flowchart which shows an example of the evaluation method of the SOI wafer of this invention. It is the structure schematic which shows an example of the corona charge apparatus which performs a corona discharge process. 1 is a schematic configuration diagram of a mercury probe apparatus. It is an electrode top view of the mercury electrode of a mercury probe apparatus. An example of a V _G -I _D characteristic of the hole side which is measured in the evaluation method of an SOI wafer of the present invention is a graph showing. An example of a V _G -I _D characteristic of the electron-side measured in the evaluation method of an SOI wafer of the present invention is a graph showing. It is a graph which shows the relationship between the electric charge amount mounted on the SOI layer surface by corona discharge treatment, and the drain current measured by forming a Pseudo-MOS structure. It is a schematic diagram which represents typically the Pseudo MOS FET method. Is a graph showing the V _G -I _D characteristic of the electron-side measured in Example. Is a graph showing the V _G -I _D characteristic of the positive hole side measured in Example. 6 is a graph showing V _{G- ID} characteristics on the hole side measured in Comparative Example 1. FIG. 6 is a graph showing V _{G- ID} characteristics on the hole side measured in Comparative Example 2. FIG.

Explanation of symbols

1 ... SOI layer, 2 ... BOX layer (buried oxide film),
3 ... support wafer, 4 ... gate electrode, 5 ... SOI wafer,
6 ... Source electrode, 7 ... Drain electrode,
11 ... Corona charge device, 12 ... Stage, 13 ... Charge generator,
21 ... Mercury probe device, 22 ... Vacuum chuck,
23 ... Mercury probe, 24, 25 ... Mercury electrode,
W ... SOI wafer.

Claims (6)

  In a method of evaluating an SOI wafer using a mercury probe, at least the SOI wafer is subjected to a hydrofluoric acid cleaning treatment to remove a natural oxide film formed on the surface of the SOI wafer, and then the natural oxide film The SOI wafer from which the wafer has been removed is subjected to a corona discharge treatment so that charges are placed on the surface of the SOI layer of the SOI wafer, and then the mercury wafer is brought into contact with the SOI wafer subjected to the corona discharge treatment to evaluate the SOI wafer. SOI wafer evaluation method characterized by the above.   2. The method for evaluating an SOI wafer according to claim 1, wherein a charge placed on the surface of the SOI layer of the SOI wafer by the corona discharge treatment is a positive charge. 3. The method for evaluating an SOI wafer according to claim 1, wherein an amount of electric charge placed on the surface of the SOI layer by the corona discharge treatment is set to 500 nC / cm ² or more and 50000 nC / cm ² or less. By contacting a mercury probe with the corona discharge treated SOI wafer and measuring the V _{G- ID} characteristics on the hole side, the hole mobility of the SOI layer and / or the charge density of the buried oxide film in the SOI wafer is measured. The method for evaluating an SOI wafer according to claim 1, wherein: After removing the natural oxide film by performing the hydrofluoric acid cleaning process, the mercury probe is brought into contact with the SOI wafer from which the natural oxide film has been removed, and the V _{G- ID} characteristics on the electron side are measured to thereby determine the electrons in the SOI layer. 5. The SOI wafer according to claim 1, wherein mobility and / or interface state density between the SOI layer and the buried oxide film is evaluated, and then the corona discharge treatment is performed. Evaluation method. After the hydrofluoric acid cleaning process, before the mercury probe is brought into contact with the SOI wafer and the V _{G- ID} characteristics on the electron side are measured, the SOI wafer is subjected to a corona discharge process to cause negative charges on the surface of the SOI layer. 6. The method for evaluating an SOI wafer according to claim 5, wherein:

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