JP2014022606A - Evaluation method of semiconductor substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating a semiconductor substrate simply by using a charge pumping method.SOLUTION: A gate oxide film 8 is formed on a semiconductor substrate 1, and at least two adjoining dielectric breakdown electrodes 9a, 9b and one gate electrode 4 are formed on the gate oxide film 8. Subsequently, diffusion layers 6a, 6b are formed by diffusing a dopant of a conductivity type opposite from that of the semiconductor substrate 1 to the respective electrodes 9a, 9b and 4. A part of the gate oxide film 8 is subjected to dielectric breakdown by applying an electrical stress between the two adjoining dielectric breakdown electrodes 9a, 9b. While applying a voltage through the gate oxide film 8 subjected to dielectric breakdown, a pulsing voltage is applied to the gate electrode 4, and interface state density is evaluated from a current flowing to the semiconductor substrate 1.

Description

  The present invention relates to a semiconductor substrate evaluation method using a charge pumping method. In particular, the present invention relates to a method for evaluating the interface state density at the silicon / oxide film interface.

  A semiconductor substrate having a high speed and low power consumption is further required as the speed of the system is increased, the integration density is increased, and the development of portable terminals. In addition, the substrate diameter is increasing. In recent years, various elements are formed on such a substrate.

  On the other hand, an evaluation method capable of evaluating the quality of a semiconductor substrate on which such a device is manufactured is also demanded. As an example, there is an interface state density evaluation method using a charge pumping method.

  FIG. 6 is a schematic diagram showing an interface state density measurement system in a conventional MOS (Metal Oxide Semiconductor) transistor. A method for measuring the interface state density using the charge pumping method will be described below. The technique relating to the interface state density measurement method including the measurement system is generally described in G.H. It has been clarified by Groeseneker et al. And Schroder (Non-Patent Documents 1 and 2).

  In such an evaluation method, as shown in FIG. 6, for example, a relatively small constant voltage, for example, about 0 to 2 V is applied to the source electrode 102 and the drain electrode 103 so as to be reverse biased with respect to the silicon substrate 101. A pulse voltage, for example, a frequency: several tens of kHz to several hundreds of kHz, and an amplitude of about 8 V (−4 to +4 V) is applied to the gate electrode 104 using the pulse generator 105. At this time, although related to the principle of the charge pumping method, it is necessary to select a voltage sufficient for the surface to invert or accumulate. By measuring the substrate current under such conditions, the interface state density can be measured.

  This measurement principle will be described in more detail below. FIG. 7 shows a pulse voltage waveform applied to the gate electrode 104, and FIG. 8 shows a semiconductor energy band corresponding to the pulse voltage waveform. In general, it is assumed that interface states are continuously distributed in the band gap, and electrons 111 and holes 112 are captured. When the applied pulse voltage is high (H state in FIG. 7), the semiconductor energy band is in a strong inversion state (FIG. 8A). At this time, the electrons 111 are captured (or already captured) at the interface state.

  Next, when the pulse voltage changes from a high state (H state in FIG. 7) to a low state (L state in FIG. 7), electrons trapped in the interface state slightly below the bottom of the conduction band 113. 111 is emitted to the conduction band 113 (FIG. 8B), and drifts to the source high concentration diffusion layer 106 and the drain high concentration diffusion layer 107 in FIG. 6 together with the conduction band electrons 114 present in the inversion layer. Then, the holes 112 flowing from the silicon substrate 101 to the interface recombine with the electrons 111 remaining at the interface state without being released (FIGS. 8C and 8D). This recombination current becomes a substrate current, so-called charge pumping current, and the charge pumping current Icp is expressed by the following equation (1).

Icp = q · AG · f · Dit · ΔE (1)
q: Elementary charge AG: Gate area f: Gate applied pulse frequency Dit: Interface state density ΔE: Effectively recombinable energy width (reference numeral 120 in FIG. 8)
Although the interface state density can be quantitatively evaluated from the above equation (1), in practice, it is often obtained using the following equation (2).
Dit = −1 / (q · k · T · AG · f) (dIcp / dln (t _Step )) (2)
k: Boltzmann constant T: Temperature (K)
t _Step = 1 / f

G. Groeseneker et. al. , “A Reliable Approach to Charge-Pumping Measurements in MOS Transistors”, IEEE TRANSACTIONS ON ELECTRON DEVICES, ED-31, 42, JANUARY (1984) D. K. Schroder Semiconductor Material and Device Characterization, New Jersey, 2006. Y. Li and T. P. Ma “A Front-Gate Charge-Pumping Method for Probing Both Interfaces in SOI Devices”, IEEE TRANSACTIONS ON ELECTRON DEVICES, 45, 1329, June (1984).

  As described above, the charge pumping method is a method for obtaining an interface state by measuring a current value generated and flowing at an interface, and the present inventor has conducted intensive research on the charge pumping method. In order to measure the interface state density using this charge pumping method, a MOSFET structure as shown in FIG. 6 is required. Since the MOSFET structure requires the gate electrode and source / drain to be electrically isolated, complicated and time-consuming processes such as the formation of an isolation oxide film and the formation of a metal electrode for contact are required, and the evaluation is completed. It takes time. If it becomes possible to create a structure that can perform charge pumping measurement more easily, the charge pumping method can be used not only for ordinary silicon wafers but also for wafers with an isolation oxide film such as SOI wafers (non-patented). Reference 3), effective in silicon evaluation.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method capable of easily evaluating a semiconductor substrate using a charge pumping method.

  In order to solve the above problems, the present invention is a method for evaluating a semiconductor substrate using a charge pumping method, wherein a gate oxide film is formed on the surface of the semiconductor substrate, and at least adjacent to the gate oxide film. After forming two dielectric breakdown electrodes and one gate electrode, a diffusion layer is formed by diffusing a dopant of a conductivity type opposite to the semiconductor substrate between each of the electrodes, and the two adjacent dielectric breakdowns are formed. An electrical stress is applied between the electrodes for use to break down a part of the gate oxide film, and a voltage in a pulsed state is applied to the gate electrode while applying a voltage through the broken gate oxide film. Provided is a method for evaluating a semiconductor substrate, characterized in that an interface state density is evaluated from a current flowing through the substrate.

  According to the present invention, it is possible to evaluate the interface state density by the charge pumping method by using a simple MOS structure without producing a complicated MOSFET structure, and the number of steps required for the evaluation can be reduced. Time can be shortened. As described above, the semiconductor substrate can be evaluated more easily than the conventional method and with the same high accuracy as the conventional method.

  At this time, it is preferable to form each of the electrodes simultaneously using photolithography.

  With this method, it is possible to perform photolithography only once, without going through the complicated MOSFET process as in the past, when photolithography is performed many times during the evaluation. become. This makes it possible to more easily measure the interface state density by the charge pumping method.

  According to the present invention, it is not necessary to fabricate a complicated MOSFET structure, and it is possible to easily fabricate an evaluation element and measure the interface state density, thereby reducing the process and time required for evaluating a semiconductor substrate. Can do. In addition, the semiconductor substrate can be evaluated with high accuracy equivalent to the conventional method.

It is a cross-sectional schematic explanatory drawing which shows an example of the element for semiconductor substrate evaluation used in the evaluation method of this invention. It is a flowchart which shows an example of the evaluation method of the semiconductor substrate according to this invention. It is a schematic explanatory drawing which shows an example of the manufacturing process of the element for semiconductor substrate evaluation in this invention. It is a graph which shows the relationship between the charge pumping current of an Example and a comparative example, and _tStep . It is a flowchart which shows the evaluation method by the conventional method in a comparative example. It is a cross-sectional schematic explanatory drawing which shows the measurement system of the interface state density in the conventional MOS transistor. It is a figure which shows the pulse voltage waveform applied to a gate electrode in the charge pumping method. It is a figure which shows the energy band of the semiconductor according to a pulse voltage waveform.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. However, the present invention is not limited thereto.

  FIG. 1 is a schematic cross-sectional explanatory view showing an example of a semiconductor substrate evaluation element used in the evaluation method of the present invention. In this semiconductor substrate evaluation element 10, a gate oxide film 8 is formed on the surface of a semiconductor substrate 1 such as a silicon substrate. On the gate oxide film 8, at least two adjacent dielectric breakdown electrodes 9a and 9b and one gate electrode 4 are formed. Between each of the electrodes, diffusion layers 6a and 6b are formed by diffusing a dopant of a conductivity type opposite to that of the semiconductor substrate 1. Then, an electrical stress is applied between the two adjacent breakdown electrodes, and a part of the gate oxide film 8 is broken down. While applying a voltage through the broken gate oxide film, a pulse voltage is applied to the gate electrode 4 by the pulse generator 5, and the interface state density can be evaluated from the current flowing to the semiconductor substrate.

  Here, the dielectric breakdown electrodes 9a and 9b are electrodes used for applying an electric field for dielectric breakdown of a part of the gate oxide film 8 before the evaluation of the semiconductor substrate 1, and the gate electrode 4 is a semiconductor electrode. It is an electrode used for applying an electric field for evaluation when the substrate 1 is evaluated. The dielectric breakdown electrode 9b is also used for measuring the substrate current. Three or more dielectric breakdown electrodes may be formed as long as they are adjacent to each other, and two or more gate electrodes may be formed. The number of these electrodes can be determined according to the evaluation purpose. Here, a case where there are two dielectric breakdown electrodes and one gate electrode will be described.

  The dielectric breakdown electrodes 9a and 9b and the gate electrode 4 are not particularly limited as long as they are made of a conductive film. However, if they are made of polysilicon, for example, they can be easily processed and become easy to form.

  Next, a method for evaluating a semiconductor substrate in the present invention will be described. FIG. 2 is a process diagram showing an example of a semiconductor substrate evaluation method according to the present invention. Mainly comprising steps (1) to (6), a semiconductor substrate evaluation element 10 as shown in FIG. 1 is manufactured, and the substrate current is measured by a charge pumping method using the element to measure the interface state density. Make an evaluation. FIG. 3 is a diagram for explaining a process for manufacturing the evaluation element.

  First, a semiconductor substrate 1 such as a silicon substrate is prepared as a pre-process, and the semiconductor substrate 1 is oxidized by various methods such as thermal oxidation to form a gate oxide film 8 on the surface of the semiconductor substrate 1 (process of FIG. 2). (1)). The thickness of the gate oxide film 8 is not particularly limited and can be determined each time.

  Next, a plurality of electrodes (two dielectric breakdown electrodes and one gate electrode) are formed on the gate oxide film 8. These electrodes are made of a conductive film. First, a polysilicon film is generally used as the conductive film, and is deposited using, for example, a CVD method (step (2) in FIG. 2). This polysilicon film is generally doped with phosphorus to lower the resistance value. The doping method of phosphorus is not particularly limited, and may be performed by a thermal diffusion method or the like after the conductive film is deposited, or a Doped Poly-Si method in which phosphorus is simultaneously doped when the conductive film is deposited.

  Next, an electrode pattern is formed from the conductive film by photolithography and etching as shown in FIG. 3A (step (3) in FIG. 2). At this time, at least two adjacent dielectric breakdown electrodes 9a and 9b and one gate electrode 4 are formed. Thus, after depositing a conductive film such as polysilicon, the conductive film is partially removed by photolithography and etching, so that the gate oxide film 8 and the respective electrodes 4, 9a, 9b are formed on the surface of the semiconductor substrate 1. A plurality of MOS capacitors having a MOS structure in which are sequentially stacked are manufactured. Thus, the dielectric breakdown electrodes 9a and 9b and the gate electrode 4 can be simultaneously formed by using photolithography. In this way, photolithography can be performed only once without going through a complicated MOSFET process as in the prior art, and interface state measurement by a charge pumping method can be performed more easily.

Thereafter, a diffusion layer 6a, 6b is formed between each of the electrodes by diffusing a dopant having a conductivity type opposite to that of the semiconductor substrate 1 (FIG. 3B, step (4) in FIG. 2). At this time, when the semiconductor substrate 1 is a P-type substrate, a method of depositing phosphorus oxychloride (POCl ₃ ) to diffuse phosphorus is conceivable. When the semiconductor substrate 1 is an N-type substrate, A technique such as boron glass coating diffusion can be considered. What is important here is that the dopant diffused later is diffused through the oxide film in the absence of the MOS capacitor electrode using the MOS capacitor electrode as a mask, resulting in a structure as if a MOSFET was formed. .

  Thereafter, an electrical stress is applied between the two adjacent dielectric breakdown electrodes 9a and 9b to cause a dielectric breakdown of a part of the gate oxide film 8 to obtain an electrical contact (step (5) in FIG. 2).

  The dielectric breakdown electrodes 9a and 9b are electrodes disposed on the gate oxide film 8, respectively, but can be broken down by applying a voltage according to the gate oxide film thickness. For example, about 50 V is sufficient for 30 nm. The application of the electrical stress is not particularly limited as long as a part of the gate oxide film can be broken down, and a method of applying a constant voltage or current until a part of the gate oxide film is broken may be used.

  As shown in FIG. 3C, the MOS capacitor electrode (dielectric breakdown electrode 9b) in which the gate oxide film 8 is broken down is connected as if it were a source / drain (that is, the dopant diffusion layer 6b is formed by a conventional method). By connecting the electrode (gate electrode 4) of the MOS capacitor that is not dielectrically broken (as viewed from the source high concentration diffusion layer 106 and the drain high concentration diffusion layer 107) as a gate, a pseudo MOSFET structure can be obtained. This structure enables measurement by the charge pumping method (step (6) in FIG. 2). More specifically, a pulse generator 5 is used for the gate electrode 4 while applying a relatively small constant voltage, for example, about 0 to 2 V, to the electrode that has been destroyed so as to be reverse-biased with respect to the semiconductor substrate 1. By applying a pulse voltage sufficient to invert or accumulate the surface, for example, frequency: several tens of kHz to several hundred kHz, amplitude: about 8 V (−4 to +4 V), and measuring the substrate current, the interface state Measure density.

  According to the present invention, it is possible to evaluate the interface state density by the charge pumping method by using a simple MOS structure without producing a complicated MOSFET structure, and the number of steps required for the evaluation can be reduced. Time can be shortened. As described above, the semiconductor substrate can be evaluated more easily than the conventional method and with the same high accuracy as the conventional method.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, it cannot be overemphasized that this invention is not limited to this.
(Example)
The evaluation method of the present invention as shown in FIG. 2 was performed. As a sample, a P-type silicon wafer having a diameter of 200 mm doped with boron was used. The resistivity is 10 Ω · cm. This wafer was gate-oxidized with a thickness of 25 nm in a dry atmosphere at 900 ° C. to form an oxide film. Poly-Si doped with phosphorus was deposited by CVD. In this case, the thickness of Poly-Si is about 300 nm, and the phosphorus doping amount is 25 ohm / sq. In sheet resistance. I tried to be about. This was subjected to photolithography to form two dielectric breakdown electrodes and one gate electrode, and a MOS capacitor was fabricated on the wafer surface. Poly-Si etching after photolithography was performed in a wet process using hydrofluoric acid. Finally, in order to remove SiO ₂ on the back surface, a resist was applied to the surface, and the back surface treatment was performed by wet etching with dilute HF.
Thereafter, POCl ₃ was deposited on the surface side at 700 ° C. for 30 minutes, and then annealed in a nitrogen atmosphere at 1000 ° C./30 minutes.
Finally, annealing treatment (sintering treatment) was performed at 400 ° C. for 30 minutes in an N ₂ atmosphere to which H ₂ was added in order to stabilize the electrode interface.

The dielectric breakdown of the gate oxide film for the contact may be as long as the gate oxide film can be broken, and a method of applying a constant voltage or current until the dielectric breakdown of the oxide film may be used. This time, we used a method of applying a constant current to breakdown. Adjacent dielectric breakdown electrodes were connected with a probe, and the stress current density was set to J = 0.1 A / cm ² , and electrical stress was applied to perform breakdown. The electrode area this time was 4 mm ² .

Using the evaluation element thus fabricated, the interface state density is measured using an actual charge pumping method. A pulse voltage is applied from the pulse generator 5 to the gate electrode 4 while 2 V is applied to the dielectric breakdown electrode 9 b so as to have a reverse bias. The pulse voltage condition is that the frequency is 100 kHz and the amplitude is 8 V (−4 to +4 V), and the charge pumping current flowing on the substrate side is measured. FIG. 4 shows the relationship between the charge pumping current and t _Step obtained as a result. The interface state density was determined by the above-described formula (2).

(Comparative example)
A conventional evaluation method as shown in FIG. 5 was performed. As a sample, a boron-doped P-type silicon wafer having a diameter of 200 mm was used. The resistivity is 10 Ω · cm. This wafer was subjected to base oxidation with a thickness of 50 nm in a Pyro atmosphere at 900 ° C., a 140 nm SiN film was formed thereon by CVD, photolithography was performed, and a window was formed in SiN. Thereafter, after oxidation at 300 nm in a Pyro atmosphere at 1050 ° C., SiN was completely removed with phosphoric acid to form an isolation oxide film. After that, gate oxidation of 25 nm was performed in a 900 ° C. dry oxygen atmosphere, and Poly-Si doped with phosphorus by CVD was deposited to obtain an electrode. At this time, the Poly-Si thickness is about 300 nm, and the phosphorus doping amount is 25 ohm / sq. In sheet resistance. I tried to be about. This was subjected to photolithography to produce a MOS capacitor in the wafer surface. Poly-Si etching after photolithography was performed in a wet process using hydrofluoric acid. Thereafter, in order to form a source high concentration diffusion layer and a drain high concentration diffusion layer, phosphorus is ion-implanted around the Poly-Si electrode, and then annealed at 1000 ° C./nitrogen atmosphere for 10 minutes to deposit a CVD oxide film having a thickness of 1 μm. Thus, a separation oxide film between the electrodes was obtained. After performing photolithography for drilling holes for electrode penetration in each part of the source, drain, and gate, AlSi was deposited by sputtering, and finally photolithography was performed again to form electrodes to complete the MOSFET.
Finally, annealing treatment (sintering treatment) was performed at 400 ° C. for 30 minutes in an N ₂ atmosphere to which H ₂ was added in order to stabilize the electrode interface.

In the measurement by the charge pumping method using the evaluation element thus manufactured (FIG. 6), 2 V is applied to the source electrode 102 and the drain electrode 103 so as to be a reverse bias, and a pulse is applied to the gate electrode 104. A pulse voltage was applied from the voltage generator 105. The charge pumping current flowing on the substrate side was measured by setting the pulse voltage condition to frequency: 100 kHz and amplitude: 8 V (−4 to +4 V). The relationship between the charge pumping current and t _Step obtained as a result is shown in FIG. Further, the interface state density was determined by the above formula (2).

  From the results shown in FIG. 4, regarding the charge pumping current, the embodiment in which the present invention is implemented shows almost no difference compared with the comparative example performed by the conventional method. It can be seen that the semiconductor substrate can be evaluated using the charge pumping method with high accuracy according to the present invention because the charge pumping current and the interface state density are very close to each other.

  Further, as shown in FIGS. 2 and 5, it can be seen that the interface state density can be measured by the charge pumping method with a significantly smaller number of man-hours than in the comparative example.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as 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.

DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate, 4 ... Gate electrode, 5 ... Pulse generator, 6a, 6b ... Dopant diffusion layer, 8 ... Gate oxide film, 9a, 9b ... Dielectric breakdown electrode, 10 ... Element for semiconductor substrate evaluation.

Claims (2)

A method for evaluating a semiconductor substrate using a charge pumping method,
Forming a gate oxide film on the surface of the semiconductor substrate;
After forming at least two adjacent dielectric breakdown electrodes and one gate electrode on the gate oxide film,
Between each of the electrodes, a diffusion layer is formed by diffusing a dopant of a conductivity type opposite to the semiconductor substrate,
Applying an electrical stress between the two adjacent breakdown electrodes to break down a portion of the gate oxide film;
A method for evaluating a semiconductor substrate, comprising applying a pulsed voltage to the gate electrode while applying a voltage through the broken gate oxide film, and evaluating an interface state density from a current flowing to the semiconductor substrate. . The method for evaluating a semiconductor substrate according to claim 1, wherein each of the electrodes is simultaneously formed using photolithography.

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