Method of fabrication of Josephson tunnel junction

© Patent holder (s): Nippon Telegraph and Telephone Corporation of Tokyo, Japan.

© Avg .: Ir. JJH Van kan et al

General Patent Office

Box 645

5600 AP Eindhoven.

© Inventor (s): Osamu Michikami at Naka, Yujiro Katoh, Keiichi Tanabe, Hisataka Takenaka and Shizuka Yoshii alien at Mito, Japan.

© Application No. 8202511.

© Submitted June 22, 1982.

© Claimed priority as of June 22, 1981, July 6, 1981, August 3, 1981, August 3, 1981, August 17, 1981, and February 16, 1982.

© Priority country (s): Japan (JP).

© Number (s) of the priority application (s): Nos. 96129/81, 105345/81, 121577/81, 121578/81, 128439/81 and 24194/82.

© Documented January 17, 1983. © Disclosed April 18, 1994. © Published September 16, 1994.

August 19, 1994.

Method for manufacturing a Josephson tunnel junction

The invention relates to a method of manufacturing a Josephson tunnel junction by patterning a base electrode of a first superconductor metal on a substrate, subjecting a portion of the surface of the base electrode to sputter cleaning in the presence of a plasma forming gas, oxidizing the thus cleaned surface in the presence of a gas mixture of oxygen and noble gas to form an oxide layer on the base electrode portion and forming a counter electrode film of a second superconductor material on the oxide layer.

Such a method is known from IBM Technical Disclosure Bulletin, jrg. 23, No. 8, January 1981, New York (US), RF Broom et al., "Method of producing Nb Josephson tunnel junctions," p. 3886, wherein the first superconductor material is niobium and the second superconductor material is lead.

In the known method, sputter cleaning is physical sputter cleaning in a high-frequency plasma with argon gas.

By using such sputter cleaning agents, in the known method, good transition properties cannot be achieved by damaging the transition surface and the formation of lower oxides serving as the tunnel barrier.

The object of the invention is to propose an improved method which leads to a Josephson tunnel transition with excellent transition characteristics, and for this purpose provides a method of the above-mentioned type, which is characterized in that the surface of the part of the base electrode is subjected to to sputter etching in the presence of one or more hydrocarbon fluorides selected from the group consisting of C in the plasma-forming gas_nF_{2n + 2} (n = 1 - 4), trifluoromethane, trichlorofluoromethane, trifluorobromomethane, dichlorodifluoromethane, trifluoroethane and pentafluoroethane.

The proposed physico-chemical sputter cleaning, or sputter etching, provides a transitional surface of the base electrode of the Josephson tunnel junction which is smooth and free of damage, allowing the Josephson tunnel junction to have excellent transition characteristics.

Further advantages are that the Josephson tunnel transition has low leakage current and excellent thermal cycling properties and can be easily processed or machined to provide an integrated transition structure.

It is to be noted that from EP Patent Application 0.013.130 it is known to perform the oxidation of the portion of the base electrode immediately after its application to the substrate in an HF glow discharge in oxygen at a pressure between 0.13 and 13 Pa. However, the surface of the part in question is not pre-cleaned here.

The invention will now be described in more detail with reference to the drawing, in which drawing: Figures 1A to 1G are explanatory sketches showing the forming steps of a Josephson transition according to the present invention;

Figure 2 is a partial perspective view of the Josephson junction;

Figure 3 is a graph showing V / 1 characteristics of the Josephson junction;

Figure 4 is a sketch showing the relationship between carbon film deposition and sputtering time, and Figure 5 is a graph showing V / 1 characteristics of the Josephson junction.

A method of manufacturing a Josephson tunnel junction of the present invention utilizes a photo-editing technique. The forming steps of the Josephson junction by the photo processing are shown in Figures 1A through 1G.

Figure 1A shows the step of forming a superconductor film serving as a base electrode for the Josephson junction.

A silicon substrate 10 has on its surface an oxidized silicon film formed by a thermal oxidation process. The substrate can be made of sapphire. For example, a superconductor metal film 12 such as Nb is formed by sputtering on the substrate 10, which first superconductor film 12 serves as a base electrode.

Figure 1B shows the patterning step of the superconductor film 12. The base electrode 12 is sequentially subjected to photoresist coating, exposure, development and etching. The etchant used is composed of hydrogen fluoride (HF), nitric acid and lactic acid. Reference numeral 14 indicates the photoresist. The resulting pattern of the base electrode 12 has a width of 20 µm.

Figure 1C shows the pattern of the base electrode 12, from which the photoresist 14 has been removed.

Figure 1D shows the step of forming a raised pattern. A portion of the substrate 10 and a portion of the base electrode 12 are covered with a photoresist figure mold 16. The figure mold 16 may be replaced with another suitable coating or masking.

Figure 1E shows the step of forming a tunnel barrier film 18 on the base electrode 12. According to an important feature of the present invention, the base electrode 12 is sputtered in the presence of fluorine-containing hydrocarbon to be first cleaned and damage free. Then, the thus cleaned surface of the base electrode 12 is subjected to oxidation to form a good quality oxide film or barrier 18 thereon.

Figure 1F shows the step of forming a second lead superconductor film 20 (Pb) to provide the Josephson tunnel junction, which second superconductor film 20 serves as a top or counter electrode. The counter electrode can be made of the same material as the base electrode 12.

Figure 1G shows the resulting Josephson tunnel junction with the raised photoresist figure mold 16 removed.

Thus, the Josephson tunnel junction thus obtained is composed of the base electrode 12, the counter electrode 20 and the oxide tunnel barrier 18 disposed between the base and counter electrode 12 and 20.

Figure 2 shows a typical example of such Josephson transitions. The insulating oxide barrier 18 has a thickness of 3 to 7 nm. When electric current I passes through the Josephson junction at a temperature below a superconducting junction temperature in the direction indicated by arrows in Figure 2, a voltage V develops between the counter electrode 20 and the base electrode 12. Figure 3 shows a graph illustrating V / 1 characteristics of one example of the Josephson junction. When the base electrode 12 and the counter electrode 20 are connected to a power source, initial current passes through the Josephson junction even at zero voltage. In this region, superconductor electrons are imparted by a tunnel effect and this condition is referred to as a zero voltage state. However, when current above a predetermined value (the maximum Josephson current) flows through the Josephson junction, the Josephson junction transitions to the resistive state. In this region, normally conductive electrons are released by a tunnel effect and this condition is referred to as a limited resistive state. A switching action of the Josephson junction is accomplished by its transition from the zero voltage state to the limited resistive state. When current flows further through the Josephson junction in the limited resistive state, the voltage increases linearly. The resistivity in this limited resistive state is referred to as transition resistance R._NN. When the current in the limited resistive state is reduced, the voltage is held essentially at the Josephson junction focal voltage regardless of the current drop. As the current is further reduced, the voltage decreases linearly. This resistance below the focal voltage is referred to as the sub-gap resistance R._SG.

The Josephson tunnel junction exhibits nonlinear V / l characteristics as described above.

These characteristics largely depend on the quality of the Josephson junction superconductor electrodes and tunnel barrier. The sub-fission voltage reflects the superconductivity of the electrodes at their boundary layers. The superconducting properties of the electrodes can easily be adversely affected by the crystallizability, i.e. the crystal orientation, of the electrodes and the dopants therein. The superconducting current depends to a great extent on the thickness of the tunnel barrier which is extremely thin and is in the order of magnitude of 3 to 7 nm. Electrical characteristics of the Josephson junction are determined by the quality of the tunnel barrier and the uniformity of its thickness.

The Josephson junction with a low sub-gap resistance will develop leakage current. The presence of such a leakage current is due to the degradation of the boundary layers of the Josephson junction and the faulty tunnel barrier. Thus, an ideal Josephson tunnel junction is defined as one containing the two superconductor metal films or electrodes, which has perfect crystallizability, and which has the uniform thickness tunnel barrier sandwiched between the two superconductor films.

Generally, the class or quality of the Josephson junction is represented by the degree of the leakage current. In particular, the quality of the Josephson junction is represented by either the product V_m or l_d and R_SG or the ratio between R._SG and R_NN. The greater Ij.Rsq (V_m) or R_sg/ Rnn becomes, the less the leakage current becomes, thereby improving the Josephson junction.

Niobium, niobium alloys and vanadium compounds can be used as the base electrode.

The niobium alloys are Nb-AI, Nb-Ge, Nb-Sn or Nb-Ga. Niobium compounds are Nb-CN and Nb-N.

Pb-ln and V-Si can also be used as base electrodes. A film of such a material is formed on the substrate by, for example, an electron beam deposition method or a sputtering method to provide the base electrode.

In the present invention, the ambient gas in which the cleaning of the base electrode transition surface is performed is a fluorine-containing hydrocarbon. Surface cleaning is performed prior to the formation of the tunnel barrier. As the ambient gas of fluorine-containing hydrocarbon or halogenated hydrocarbon, CF₄, C₂F₆, C₃F₈, C₄F₁₀, CHF₃, C₂HF₅, C₂H₃F,

CCI₂F₂, CCI₃F and CBrF₃ are used. Sputter cleaning of the transition surface prior to tunnel barrier formation is performed using ambient gas of fluorine-containing hydrocarbon and / or hydrocarbon containing fluorine, chlorine and bromine. Sputter cleaning is performed using a low voltage discharge. The fluorine-containing hydrocarbon provides F radicals. The F radicals react chemically with niobium or vanadium of the transition surface to give volatile NbF_s and VF₅ to provide. The NbF thus generated_s and VF_S separates from the transition surface so that surface cleaning is effected by this etching operation. This etching by means of the above chemical reaction is much faster compared to sputtering Ar ions. A 10 to 15 nm thick contaminated layer on the transition surface is etched in 30 to 50 minutes. Therefore, damage to the transition surface is limited to a minimum level. In the case of a discharge in fluorine-containing hydrocarbon, carbon is applied to the cleaned transition surface. Since the transition surface is covered with carbon, it is prevented from coming into contact with oxygen at the time of gas exchange in the vacuum chamber. The thickness of the carbon deposition on the transition surface depends on the gas content and the partial pressure, discharge time and discharge voltage of the fluorine-containing hydrocarbon and also depends on its type.

In order to effect the discharge at a low voltage, the required total pressure of the gas is around 1.3 Pa. Generally, when hydrocarbon containing fluorine is used in sputtering, the amount of carbon deposition increases. Therefore, in order to dilute the fluorine-containing hydrocarbon in the ambient gas, a noble gas such as Ar, He, Ne, Kr and Xe and also N₂CO, CO₂ and used methane. When discharge is effected in an atmosphere of a mixture gas of fluorine-containing hydrocarbon diluted by Ar, He, Ne, Kr, Xe, etc., the deposition amount of carbon decreases with an increase in the discharge voltage. The reason is that the deposited carbon is sputtered by ions from the inert gas. In order to obtain uniform quality transition characteristics, it is necessary to control the deposited carbon to a predetermined thickness. When the types of fluorine-containing hydrocarbon and diluent gas, their content and sputtering gas pressure are determined, the carbon deposition amount depends on the discharge voltage and discharge time. Generally, when the discharge voltage is kept at a constant level, the deposition amount of carbon is increased with an extension of the discharge time. However, as the discharge voltage increases, the thickness of the deposited carbon reaches a predetermined level and is maintained at this saturated level. However, it is not desirable to control the amount of carbon deposited by varying the discharge voltage, because the increased discharge voltage causes damage to the transition surface. When the content of fluorine-containing hydrocarbon is reduced by reducing the partial pressure, a long time for cleaning is required because the etching rate of the transition surface is reduced. On the other hand, it is very effective to introduce oxygen into the mixture gas of fluorine-containing hydrocarbon and diluent gas at the time of sputter cleaning.

Oxygen gas reacts with the carbon deposited on the transition surface to produce CO or CO₂ to generate, thereby removing the deposited carbon. When the deposition of carbon and the removal of carbon are in an equilibrium condition, the deposition amount or carbon thickness is kept constant regardless of the discharge time.

For example, when the discharge is performed in a mixture gas of Ar, 7.5% CF₄ and 2% O₂ at a voltage of 200 volts at a pressure of 2.0 Pa, a carbon film of 2 to 4 nm is formed on the transition surface of Nb-AI in one minute and even when the discharge is continued, the thickness of the deposited carbon has not changed substantially. However, the transition surface of the Nb-AI film is etched during the passage of the discharge time. Thus, the addition of oxygen gas serves not only to control the amount of carbon on the transition surface, but also to control the etch rate at a constant level.

In the present invention, the tunnel barrier is formed on the cleaned transition surface of the first superconductor film or base electrode red by oxidation immediately after the above-described cleaning.

When the carbon-coated transition surface is oxidized by oxygen, some of the oxygen reacts with some of the carbon on the transition surface to produce CO or CO₂ to generate, which removes some of the carbon. Another portion of the oxygen diffuses into the carbon on the transition surface to react with the active atoms of Nb or V to form oxides of Nb or V. The oxygen to provide the oxides of Nb or V slowly approaches the superconductor metal of the base electrode to effect the reaction. In the event that the oxide barrier is formed after the transition surface of the superconductor metal is subjected to Ar ion sputter cleaning, oxygen reacts immediately after it is introduced with Nb or V to thereby produce lower oxides, such as NbO, Nb₂O or VO₂ to provide. According to the present invention, the deposited carbon serves to control the oxidation rate of Nb or V so that higher oxides such as Nb₂O₅ and V₂O₅ forming an insulator of stable nature, formed on the transition surface and the low oxides, such as NbO, Nb₂O or VO₂not be formed on the transition surface. Thus, a high quality tunnel barrier can be provided.

The deposited carbon on the transition surface plays an important role in the formation of the stable barrier. The oxidation conditions of the sputter cleaning according to the invention differ significantly from those of the Ar sputter cleaning in that the former uses the deposited carbon.

In the formation of the Nb junction, the junction surface, which has been subjected to sputter cleaning in argon atmosphere, is oxidized in the atmosphere of argon gas, which is around 4% oxygen at a pressure of 0.60 Pa at a discharge voltage of 30 to with 40 V, forming a tunnel barrier with a predetermined thickness. On the other hand, according to the present invention, the transition surface that has been subjected to sputter cleaning in an atmosphere of a mixture gas of fluorine-containing hydrocarbon and noble gas is oxidized in an atmosphere of argon gas containing 4 to 20% oxygen at a pressure of 0.67 to 10.6 Pa at a discharge voltage V._CSB from 40 to 150 V, forming the oxide barrier. The oxidation conditions of the present process, namely the oxygen concentration, the gas pressure and the discharge voltage, thus cover broad ranges compared to the oxidation conditions of the conventional process, in which the transition surface is subjected to Ar sputter cleaning. These oxidation conditions of the present invention serve to form the optimal oxide barrier. The reason for this is thought to be that the deposited carbon serves to form stable oxides such as Nb₂O₅ and V₂O₅, on the transition surface during the oxidation as the deposited carbon controls the diffusion of the oxygen.

The method will now be described in detail with regard to specific preferred embodiments thereof.

Example 1

Niobium and aluminum were melted by an arc melting process to produce an Nb-25, 1 atomic percent Al alloy. A 100 millimeter diameter disc was made from the alloy. The Nb-25, 1 atomic percent Al alloy was deposited on an oxidized surface of a silicon substrate, held at 670 ° C by direct current microwave magnetron sputtering in an argon atmosphere at a pressure of 1.3 Pa to form an Nb-AI- film serving as a base electrode. The film was 350 nm thick. The superconducting transition temperature (TJ was measured by an electrical resistivity of the Nb-AI film, using a four-probe method. The Nb-AI film had a T temperature of 16.1 K. Daama, in order to To pattern the Nb-AI film, the film was sequentially subjected to resist coating, exposure, development and etching. A used etchant was composed of a mixture of hydrogen fluoride, nitric acid and lactic acid. Then, an enhancement photoresist figure mold was applied to a portion of the substrate and a portion of the patterned Nb-AI film. The transition region of the Nb-AI film was not covered with the photoresist figure mold. Then the Nb-AI film surface was sputtered in an atmosphere of tetrafluoromethane (CF₄) to clean it and then subjected to plasma oxidation to form an oxide layer or barrier thereon. The sputter cleaning described above was performed at a pressure of 130 to 0.13 Pa at a cathode voltage V._CSB of 150 V for 2 minutes. Sputter cleaning was performed at a cathode voltage V._CSB of 300 V for 2 minutes. Then, the cleaned Nb-AI film was oxidized by plasma oxidation to form the oxide barrier thereon. Plasma oxidation was carried out in an atmosphere of argon and 4 volume percent oxygen at a pressure of 4 Pa, with a cathode voltage V_CSB of 50 V for 8 minutes.

Then, a lead film was applied to the oxide barrier to form a counter electrode, providing a Josephson junction. The Josephson junction thus obtained was composed of the Nb-AI base electrode (width: 2 micrometers, thickness 350 nm, the lead counter electrode, and the Nb-AI oxide barrier sandwiched between the base and counter electrode. Six Josephson5 transitions numbers 12 through 17 in Table 1 were prepared in this manner.

Josephson transition number 11 was prepared according to the procedure described above, except that sputter cleaning was performed in an argon atmosphere.

Josephson transitions numbers 18 and 19 were prepared according to the procedure given above, except that the oxidation of the Nb-AI film to form an oxide barrier thereon was performed by a natural oxidation process. This natural oxidation process was performed in an atmosphere of argon and 20 volume percent O₂ for 10 hours at an atmospheric pressure at room temperature.

Table I shows V / 1 characteristics of the Nb-AI / Pb tunnel junctions numbers 11 to 19 and their manufacturing conditions. They are classified into a tunnel type, a bridge type and a one-particle tunnel type according to their V / l characteristics. The gap voltage and R._Sg /^rnn, representative of the quality of the transition, were obtained by means of the V / l characteristics.

As can be seen from Table I, the Josephson junctions are sputter cleaned in the presence of tetrafluoromethane (CF₄) and subsequent to the plasma oxidation, of the tunnel type. The gap voltage and R._Sg^{/ r}nn depend on the pressure of CF₄. The Josephson transitions with excellent transition properties were obtained using the CF₄pressure from 67 to 6.7 Pa. When the CF₄pressure was below 6.7 Pa, the gas voltage was 3.2 millivolts. This is presumably due to the fact that the pressure is so low that the energy of the sputter particles becomes high enough to cause damage to the transition surface, degrading the superconducting properties. When, on the other hand, the CF₄-pressure exceeds 6.7 Pa, the amount of the deposited carbon is increased excessively. As a result, the etching effect is presumably impaired, so that the contaminated layer of the transition surface cannot be completely removed.

TABLE

<td rowspan="2"></td><td rowspan="2"> 11</td><td rowspan="2"> 12</td><td rowspan="2"> 13</td><td colspan="3">Experiment number</td><td rowspan="2"> 17</td><td rowspan="2"> 18</td><td rowspan="2"> 19</td>

<td> 14</td><td> 15</td><td> 16</td>

<td>Sputification</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Argon: 1.3 Pa V_CBS</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>(V)</td><td> 300</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td> 300</td>

<td>Time (min)</td><td> 10</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td> 5</td>

<td>Tetrafluoromethane</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Pressure (Pa)</td><td></td><td> 133</td><td> 67</td><td> 13,3</td><td> 1,3</td><td> 0,7</td><td> 0,13</td><td> 13,3</td><td> 13,3</td>

<td>V_CBS (V)</td><td></td><td> 150</td><td> 150</td><td> 150</td><td> 150</td><td> 150</td><td> 150</td><td> 150</td><td> 150</td>

<td>Time (min)</td><td></td><td> 2</td><td> 2</td><td> 2</td><td> 2</td><td> 2</td><td> 2</td><td> 2</td><td> 2</td>

<td>oxidation:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Plasma oxidation</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Ar-4 vol. % O₂</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>4.0 Pa</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>V_CBS (V)</td><td> 50</td><td> 50</td><td> 50</td><td> 50</td><td> 50</td><td> 50</td><td> 50</td><td></td><td></td>

<td>Time (min)</td><td> 8</td><td> 8</td><td> 8</td><td> 8</td><td> 8</td><td> 8</td><td> 8</td><td></td><td></td>

<td>Oxidation room temperature</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>perature</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Time (h)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td> 10</td><td> 10</td>

<td>Ar-20 vol. % O₂</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>one atmosphere</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Electric</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>characteristics</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>of Nb-AI / Pb-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>tunnel crossing</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>V / L characteristics</td><td colspan="3">Bridge type Bridge type Tunnel</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Bridge-</td><td>Tunnel-</td><td>Tunnel-</td>

<td></td><td></td><td>or</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td></td><td></td><td>tunnel-</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td></td><td></td><td>type</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

TABLE I (continued)

<td rowspan="2"></td><td rowspan="2"> 11</td><td rowspan="2"> 12</td><td rowspan="2"> 13</td><td colspan="2">Experiment number</td><td rowspan="2"> 17</td><td rowspan="2"> 18</td><td rowspan="2"> 19</td>

<td> 14 15</td><td> 16</td>

<td>Crush tension</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>(mV)</td><td> -</td><td> 3</td><td> 3,5</td><td> 4,1 3,8</td><td> 3,2</td><td> -</td><td> 4,0</td><td> 4,0</td>

<td>Rsg / Rnn</td><td> —</td><td> —</td><td> 4</td><td> 8 10</td><td> 7</td><td></td><td> 8,5</td><td> 7,5</td>

Example 2

Nb-AI / Pb junctions were prepared in the same manner as described in Example 1 except that sputter cleaning halogenated hydrocarbons such as hexafluoroethane C₂F₆, octafluoropropane C₃F₈decaf luorbutane C₄F₁₀trifluoromethane CHF₃pentafluoroethane C₂HF₅, trifluoropropane C₂H₃F₃trichloroforomethane CCI₃F, CF₄ - 50 full. % CCI₃F, C₃F₈ - 80 full. % CBrF and C₂F₆ - 20 vol. % CF₄were used. Sputter cleaning was performed at a pressure of 2.7 Pa at a cathode voltage V._GSBof 150 V for 5 minutes. Then, the cleaned NB-AI film was oxidized by plasma oxidation to form an oxide barrier thereon. Plasma oxidation was performed in an atmosphere of argon and 4 volume percent oxygen at a pressure of 4.0 Pa at a cathode voltage V_CSB of 150 V for 5 minutes. A lead film was applied to the oxide barrier to form a counter electrode.

Table 2-1 and Table 2-2 show V / L characteristics of tunnel crossings numbers 21 to 30 and the type of halogenated hydrocarbons used in sputter cleaning. The pressure of the halogenated hydrocarbons was 2.7 Pa, as can be seen from the table. 2-1 and Table 2-2. The gap voltage and R._Sq / Rnn were obtained by the V / l characteristics and the Josephson junctions numbers 21 to 30 were tunnel type.

TABLE 2-1

<td rowspan="2"></td><td rowspan="2"> 21</td><td rowspan="2"> 22</td><td colspan="2">Experiment number</td><td rowspan="2"> 25</td>

<td> 23</td><td> 24</td>

<td>Sputter cleaning gas Fluorocarbon at a pressure of 2.7 Pa An electrical characteristic of the Nb-AI / Pbtunnel junction</td><td>C₂F₆</td><td> 8</td><td>C₄F io</td><td>CHF₃</td><td>C, HF_S</td>

<td>Slit voltage (mV) Transition quality</td><td> 4,1</td><td> 4,0</td><td> 3,9</td><td> 4,0</td><td> 4,1</td>

<td>Rsg / Rnn</td><td> 10,7</td><td> 8,8</td><td> 8,2</td><td> 7,8</td><td> 9,3</td>

TABLE 2-2

Experiment number

<td></td><td> 26</td><td> 27</td><td> 28</td><td> 29</td><td> 30</td>

<td>Sputter cleaning gas Fluorocarbon at a pressure of 2.7 Pa</td><td>C₂H₃F₃</td><td>CC €₃F</td><td>cf₄⁺50 full. % CC €₃F</td><td>c₃f₈* 80% vol. % CBrF₃</td><td>c₂f₆⁺20% vol. % cf₄</td>

An electrical characteristic of the Nb-AI / Pbtunnel junction

TABLE 2-2 (continued)

<td rowspan="2"></td><td rowspan="2"> 26</td><td rowspan="2"> 27</td><td colspan="2">Experiment number</td><td rowspan="2"> 30</td>

<td> 28</td><td> 29</td>

<td>Slit voltage (mV)</td><td> 3,9</td><td> 3,8</td><td> 3, 9</td><td> 3,8</td><td> 4,0</td>

<td>Transitional quality</td><td></td><td></td><td></td><td></td><td></td>

<td>Bsg / Rnn</td><td> 6,7</td><td> 8,4</td><td> 9,1</td><td> 8,7</td><td> 9,0</td>

Example 3

Nb-AI / Pb junctions were prepared in the same manner as described in Example 1, except that in sputter cleaning instead of CF₄ a mixture gas of CF₄ and a diluent gas was used. The diluent gas used therein was helium, neon, argon and carbon monoxide. Sputter cleaning was performed at a pressure of 0.02 Torr at a cathode voltage V._CSB of 200 V for 10 minutes.

Table 3-1 shows the content of CF₄ in helium and V / l characteristics of Nb-AI / Pb tunnel junctions from numbers 31 to 36. Table 3-2 shows the content of CF₂ in neon and V / l characteristics of Nb-AI / Pb tunnel crossings from numbers 37 to 42.

Table 3-3 shows the content of CF₄ in argon and V / l characteristics of Nb-AI / Pb tunnel crossings from numbers 43 to 48. Table 3-4 shows the content of CF₄ in carbon monoxide and V / l characteristics of Nb-AI / Pb tunnel junctions from numbers 49 to 54. The cleaned film was oxidized by plasma oxidation to form an oxide barrier thereon. Plasma oxidation was performed in an atmosphere of argon and 4 vol. % oxygen at a pressure of 2.7 Pa. A lead film was then applied to the oxide barrier to provide a Josephson junction.

As can be seen from Tables 3-1 through 3-4, the Josephson transitions are subject to sputter cleanings in the presence of a mixture gas of 5 to 40 volume percent CF₄ and a diluent gas, such as helium, neon, argon, carbon monoxide and then plasma oxidation, of the tunnel type.

As seen from the data in Tables 3-1 through 3-4, Josephson transitions with excellent transition properties were obtained using the mixture gas with the CF₄content from 5 to 40% by volume. As can be seen from Tables 3-1 through 3-4, R._SB/ R_NN of the transitions above 8.5. Josephson tunnel type transitions. When the CF₄content below 5 vol. %, the transitions did not have good transition properties. The reason for this is that the cleaning of the transition surface is not complete. When, on the other hand, the CF₄-content 40 vol. %, the amount of the deposited carbon is excessively increased, so that good transition properties are not obtained. As a result, the etching defect is presumably adversely affected, so that the contaminated layer on the transition surface cannot be completely removed.

Nb-AI / Pb transitions were prepared in the same manner as described in this example, except that xenon, krypton, methane and carbon dioxide were used as the diluent gas in the mixture gas and that niobium nitride (Nb-N) or an Nb-Sn alloy was used as the base electrode.

The transitions were sputter cleaned in the presence of the mixture gas with the CF.₄-content from 5 to 40 vol. % and then to the plasma oxidation. The value of R._Sg / Rnn, obtained by the V / l characteristics, was above 8.

Nb-AI / Pb transitions were prepared in the same manner as described in this example, except that instead of CF₄ trifluoromethane (CHF₃) or hexafluoroethane (C₂F₆) were used in sputter cleaning.

The Nb-AI film surface was sputtered in an atmosphere of a CHF mixture gas₃ or C₂F₆ and diluent gas, such as helium, neon, argon, carbon monoxide, carbon dioxide and methane, to clean it and was then subjected to plasma oxidation to form an oxide barrier thereon. A lead film was then applied to the oxide barrier to provide a Josephson junction. The Josephson transitions thus obtained exhibited transition characteristics similar to those of the transitions subjected to sputter cleaning in the presence of CF₄ and the diluent gas.

TABLE 3-1

Experiment number

<td></td><td> 31</td><td> 32</td><td> 33</td><td> 34</td><td> 35</td><td> 36</td>

<td>CF₄content in helium (vol.%)</td><td> 2</td><td> 5</td><td> 15</td><td> 40</td><td> 60</td><td> 100</td>

<td>V / L characteristics</td><td>a-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel type</td>

<td></td><td>particle tunnel- type</td><td>type</td><td>type</td><td>type</td><td>type</td><td></td>

<td>Slit voltage (mV)</td><td> 3,2</td><td> 3,8</td><td> 3,9</td><td> 3,8</td><td> 3,7</td><td> 3,6</td>

<td>Rsg / Rnn</td><td> 3</td><td> 8,5</td><td> 11</td><td> 9</td><td> 8</td><td> 8</td>

TABLE 3-2

Experiment number

<td></td><td> 37</td><td> 38</td><td> 39</td><td> 40</td><td> 41</td><td> 42</td>

<td>CF₄content in neon (vol.%)</td><td> 2</td><td> 5</td><td> 10</td><td> 20</td><td> 40</td><td> 70</td>

<td>V / L characteristics</td><td>a-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td>

<td></td><td>particle</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>Slit voltage (mV)</td><td>tunnel- type 2.9</td><td> 3,7</td><td> 3,8</td><td> 3,9</td><td> 3,8</td><td> 3,6</td>

<td>Rsg / Rnn</td><td> 4</td><td> 9</td><td> 12</td><td> 10</td><td> 9</td><td> 8</td>

TABLE 3-3

Experiment number

<td></td><td> 43</td><td> 44</td><td> 45</td><td> 46</td><td> 47</td><td> 48</td>

<td>CF₄content in argon (vol.%)</td><td> 2</td><td> 5</td><td> 10</td><td> 20</td><td> 40</td><td> 70</td>

<td>V / L characteristics</td><td>a-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td>

<td></td><td>particle</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>Slit voltage (mV)</td><td>tunnel- type 2.9</td><td> 3,7</td><td> 3,9</td><td> 3,9</td><td> 3,9</td><td> 3,9</td>

<td>Rsq / Rnn</td><td> 4</td><td> 9,5</td><td> 13</td><td> 11</td><td> 9</td><td> 9</td>

TABLE 3-4

<td rowspan="2"></td><td rowspan="2"> 49</td><td colspan="5">Experiment number</td>

<td> 50</td><td> 51</td><td> 52</td><td> 53</td><td> 54</td>

<td>CF₄content in carbon monoxide (vol.%)</td><td> 2</td><td> 5</td><td> 10</td><td> 20</td><td> 40</td><td> 70</td>

<td>V / L characteristics</td><td>a-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td><td>tunnel-</td>

<td></td><td>particle</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>Slit voltage (mV)</td><td>tunnel- type 3.1</td><td> 3,7</td><td> 3,9</td><td> 3,9</td><td> 3,7</td><td> 3,6</td>

<td>Rsg / Rnn</td><td> 3,8</td><td> 8,5</td><td> 11,5</td><td> 9,7</td><td> 9</td><td> 8</td>

Example 4

A 100 millimeter diameter disc was made of Nb with a purity of 99.99%.

Nb was applied to an oxidized surface of a silicon substrate by direct current microwave magnetron sputtering in an argon atmosphere to form an Nb film serving as a base electrode. Nb / Pb junctions were prepared in the same manner as described in Example 1 except that sputtering was performed in an atmosphere of argon, CF₄ and oxygen.

The Nb film was sputtered in an atmosphere of a mixture gas of argon-7.5 volume percent CF₄ or a mixture gas of argon-7.5 volume percent CF₄ - 4 volume percent oxygen for cleaning its surface. The sputter cleaning described above was performed at a pressure of 4.0 Pa at a cathode voltage V._CSB of 150 V at different times for the transitions.

Then, the cleaned Nb film surface was subjected to plasma oxidation to form an oxide barrier thereon. The plasma oxidation was carried out in an atmosphere of argon and 5 vol. For five minutes. percent oxygen at a pressure of 4.0 Pa at a cathode voltage V._CSB of 50 V. Then, a lead film was applied to the oxide barrier to provide the Josephson junction.

Table 4 shows V / 1 characteristics of the Nb / Pb tunnel junctions numbers 55 to 60 and their manufacturing conditions. V / L characteristics of the Nb / Pb tunnel crossings were represented by both the product of transition surface A and R_NN as by V_m (Ij-R_Sg) · Each transition has a slit voltage of 2.7 millivolts. Figure 4 shows a relationship between the sputtering time and the thickness of the deposited carbon film, measured by an elliptometric method. In Figure 4, a lower or saturation curve represents a mixture gas of argon-7.5 volume percent CF₄- 4 volume percent oxygen and a top curve represents a mixture gas of argon - 7.5 volume percent CF₄. When sputter cleaning was performed under an atmosphere of argon and 7.5 volume percent CF₄ to clean the Nb film surface, the amount of the deposited carbon on the Nb film was increased as the sputtering time increased. On the other hand, when sputter cleaning was performed under an atmosphere of argon, 7.5 volume percent CF₄ and 40 volume percent oxygen for cleaning the Nb film surface, the amount of the deposited carbon approached the saturation curve. In this saturation condition, the thickness of the deposited carbon film was around 2 nm.

As can be seen from Table 4, sputter cleaning in the presence of argon mixture gas exhibits -7.5 vol. % CF₄-4 vol. % oxygen for 15 to 25 minutes, the Josephson transitions subject substantially similar transition characteristics. This is presumably so due to the fact that the amount of the deposited carbon is in the saturation state. The addition of oxygen serves to control the resistivity of the transition, because a constant amount of the carbon can be deposited by the addition of an optimal amount of oxygen in C₂F₆ or C₃F₈regardless of the sputtering time.

Figure 5 shows V / l characteristics of Nb / Pb tunnel junction number 58, which shows no knee in the V / l curve and has less leakage current, which is advantageous.

Also, Nb / Pb tunnel junctions were prepared in the same manner as described above in this example, except that the oxygen content was varied. When the oxygen content was below 14 volume percent, the value of the superconducting current Ij was not zero. When the oxygen content exceeded 35 volume percent, the Josephson junctions subjected to sputter cleaning and then plasma oxidation showed one-particle tunnel type. The reason for this is that the oxidation of the Nb film surface proceeds excessively due to excessive oxygen content. Nb / Pb tunnel crossings, subject to sputter cleaning in the presence of argon-CF mixture gas₄-5 to 35 full. % oxygen and then not subject to plasma oxidation have an oxidized barrier with a thickness of 2 to 3 nm.

TABLE 4

Experiment number

56 57 58 59 60

Sputter cleaning gas: Argon + 7.5 vol. % CF₄then Argon + 7.5 vol. % CF₄ + 4 vol. % subject to plasma oxygen, without plasma oxidation oxidation

Sputtering times (min) 15 20 25 15 20 25

TABLE 4 (continued)

<td rowspan="2"></td><td rowspan="2"> 55</td><td rowspan="2"> 56</td><td colspan="3">Experiment number</td><td rowspan="2"> 60</td>

<td> 57</td><td> 58</td><td> 59</td>

<td>V / L characteristics</td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>AR_nn (Q.prn²)</td><td>7x10⁴</td><td>2x10⁴</td><td>6x10³</td><td>4.8x10⁵</td><td>4.8x10⁵</td><td>4.8x10^s</td>

<td>V_m (mV)</td><td> 26</td><td> 24</td><td> 22</td><td> 29</td><td> 29</td><td> 29</td>

Example 5

Nb / Pb tunnel junctions were prepared in the same manner as described in Example 1, except niobium of 99.99% purity was used as a base electrode. The Nb film surface was sputtered for 20 minutes in an atmosphere of argon and 10 vol. % CF₄ at a pressure of 4.0 Pa at a cathode voltage V_CSB from 20 to 400 V to clean it, and then the cleaned Nb film was subjected to plasma oxidation to form an oxide barrier thereon. Plasma oxidation was performed for 5 minutes at a cathode voltage V._CSB of 50 V in an atmosphere of a mixture gas of argon and 5 vol. % oxygen at a pressure of 4.0 Pa. Then, a lead film was applied to the oxide barrier to provide a counter electrode.

Table 5 shows V / l characteristics of the Nb / Pb tunnel junctions numbers 51 to 67 and cathode voltage V_CSB of sputter cleaning. The Nb / Pb-Josephson junctions are of the tunnel type and have a focal voltage of around 2.7 millivolts. When V_CSB below 50 V when sputter cleaning, a contaminated layer on the transition surface could not be completely removed. When V_CSB300 V exceeded, the energy of sputter particles became large enough to cause damage to the transition surface, degrading the superconducting properties. Also, the Nb / Pb tunnel crossings subjected to sputter cleaning in an atmosphere of argon had - 7 vol. % C₂F₆ - 3 vol. % oxygen or an atmosphere of argon vol. % C₃F₈ - 5 vol. % nitrogen transition characteristics that were similar to those of the transitions described above in this example. Furthermore, the tunnel transitions subjected to sputter cleaning at a cathode voltage V_CSB from 50 to 250 V and using Nb-AI, Nb-N and Nb-Sn as a base electrode, respectively, similar excellent tunnel transition properties.

TABLE 5

Experiment number

<td></td><td> 61</td><td> 62</td><td> 63</td><td> 64</td><td> 65</td><td> 66</td><td> 67</td>

<td>Spurginating Vcsb (V)</td><td> 20</td><td> 50</td><td> 100</td><td> 150</td><td> 200</td><td> 300</td><td> 400</td>

<td>V / L characteristics</td><td>A-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel</td>

<td></td><td>particle</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>Rsq / Rnn</td><td>tunnel- type 3.7</td><td> 6,4</td><td> 12,2</td><td> 17,8</td><td> 16,0</td><td> 8,4</td><td></td>

<td>Vm (mV)</td><td> 0</td><td> 4,3</td><td> 19</td><td> 27</td><td> 30</td><td> 9,5</td><td> -</td>

Example 6

Nb-25.2 atomic% Ge, Nb-24.9 atomic% Sn, Nb-25.2 atomic% Ga, V-24.6 atomic% Si were prepared by an arc melting process to form a disc with a diameter of 100 millimeters. Its alloy was applied to a sapphire substrate, which was maintained at 600 ° to 900 ° by direct current magnetron sputtering in an argon atmosphere at a pressure of 6 to 27 Pa to form a film serving as a base electrode . The film had a thickness of 400 nm. Also, Nb and Nb-15 atomic% C were respectively applied to oxidized surfaces of silicon substrates by DC microwave sputtering in an argon and 20 volume% N₂ atmosphere at a pressure of 1.3 Pa to form an Nb-N film and Nb-CN film, respectively, serving as a base electrode. Each film had a thickness of 400 nm.

Pb-10 atomic% In was applied to an oxidized surface of a silicon substrate by direct current magnetron sputtering to form a Pb-ln film serving as a base electrode. Each of these base electrodes was sputtered in the same manner as described in Example 1. Sputter cleaning was performed in an atmosphere of a mixture gas of argon-10 volume% C for 20 minutes₂F₆ - 5 volume% oxygen at a pressure of 3.3 Pa at a cathode voltage V._CSB of 130 V, after which the cleaned surface of the base electrode was oxidized by plasma oxidation to form the oxide barrier thereon. Plasma oxidation was performed for 5 minutes in an atmosphere of argon and 4 volume% oxygen at a pressure of 3.3 Pa at a cathode voltage V_CSB of 50 V to form an oxide barrier thereon. Then, a lead film was applied to the oxide barrier to form a counter electrode.

Table 6 shows V / l characteristics of tunnel crossings numbers 68 through 74.

TABLE 6

<td rowspan="2"></td><td rowspan="2"> 68</td><td rowspan="2"> 69</td><td colspan="3">Experiment number</td><td rowspan="2"> 73</td><td rowspan="2"> 74</td>

<td> 70</td><td> 71</td><td> 72</td>

<td>Material of the base electrode</td><td>Nb-Ge</td><td>Nb-Sn</td><td>Nb-Ga</td><td>V-Si</td><td>Nb-N</td><td>Nb-CN</td><td>Pb-ln</td>

<td>Superconducting transition temperature of the base electrode Tc (K)</td><td> 21,5</td><td> 15,6</td><td> 17,7</td><td> 15,4</td><td> 15,9</td><td> 15,3</td><td> 7,8</td>

<td>V / L characteristics</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td>

<td></td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>^ SG ^ NN</td><td> 8,1</td><td> 12</td><td> 8,4</td><td> 10,3</td><td> 15,6</td><td> 11,6</td><td> 14,5</td>

<td>Vm (mV)</td><td> 18</td><td> 22</td><td> 17</td><td> 20</td><td> 29</td><td> 24</td><td> 26</td>

Example 7

Nb / Pb tunnel junctions were prepared in the same manner as described in Example 1 except that 99.99% purity Nb was applied to an oxidized surface of a silicon substrate maintained at 400 ° C by DC microwave sputter to form an Nb film serving as a base electrode. The Nb film was sputtered for 20 minutes in an atmosphere of a mixture gas of argon, 7.5 volume% CF₄ and 1 volume% oxygen at a cathode voltage V._CSB of 130 V. Thereafter, the cleaned surfaces of the Nb films were oxidized by plasma oxidation in an atmosphere of argon and oxygen under different conditions (total gas pressure, oxygen content, V_CSB, sputtering time) to form different oxide barriers. Then, a lead film was applied to the oxide barrier to provide a counter electrode.

Table 7-1 shows a total gas pressure in the plasma oxidation and V / L characteristics of tunnel crossings number 75 to 80.

Table 7-2 shows an oxygen content (volume%) in the plasma oxidation and V / l characteristics of tunnel crossings numbers 81 to 86.

Table 7-3 shows the cathode voltage V._CSB in the plasma oxidation and V / l characteristics of tunnel crossings numbers 87 to 92.

Table 7-4 shows the sputtering time in the plasma oxidation and V / l characteristics of tunnel junctions numbers 93 to 95.

As can be seen from Tables 7-1 through 7-4, the Josephson transitions subjected to plasma oxidation at a total pressure of 0.67 Pa to 1 and 20 minutes are with 0.80 Pa with the oxygen content of 4 to 20 volume% with a cathode voltage V._CSB from 40 to 150 V of the tunnel type.

TABLE 7-1

Experiment number

76 77 78 79 80

Oxidation:

V_CSB 100 V time 7 s Argon + 8 volume% 0₂

<td>total gas pressure (Pa)</td><td> 0,40</td><td> 0,67</td><td> 1,3</td><td> 4,0</td><td> 8,0</td><td> 13</td>

<td>V / L characteristics</td><td>Short circuit</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>one-particle</td>

<td></td><td>ting</td><td>type</td><td>type</td><td>type</td><td>type</td><td>tunnel type</td>

<td>Rsg / Rnn</td><td> -</td><td> 8,5</td><td> 11</td><td> 17</td><td> 14</td><td> 10</td>

<td>Vm (mV)</td><td> —</td><td> 14</td><td> 18</td><td> 30</td><td> 22</td><td> 2</td>

TABLE 7-2

<td rowspan="2"></td><td rowspan="2"> 81</td><td colspan="5">Experiment number</td>

<td> 82</td><td> 83</td><td> 84</td><td> 85</td><td> 86</td>

<td>Oxidation: V_CSB 100 V time 7 s</td><td> 2</td><td> 4</td><td> 8</td><td> 12</td><td> 20</td><td> 30</td>

<td>Argon + O₂ mixed gas oxygen content (Vol.%) V / L characteristics</td><td>Short circuit</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td>

<td></td><td>ting</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>Rsg ^ Rnn</td><td> -</td><td> 9,8</td><td> 17</td><td> 16</td><td> 15</td><td> 12</td>

<td>Vm (mV)</td><td> —</td><td> 19</td><td> 30</td><td> 28</td><td> 20</td><td> 0</td>

TABLE 7-3

Experiment number

<td></td><td> 87</td><td> 88</td><td> 89</td><td> 90</td><td> 91</td><td> 92</td>

<td>Oxidation:</td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Argon + 8 Vol. %O₂</td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>at 4.0 Pa</td><td></td><td></td><td></td><td></td><td></td><td></td>

<td>Cathode voltage V_CSB</td><td> 20</td><td> 40</td><td> 80</td><td> 100</td><td> 150</td><td> 300</td>

<td>V / L characteristics</td><td>Short circuit</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Bridge-</td>

<td></td><td>ting</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

<td>Rsg ^ Rnn</td><td> -</td><td> 14</td><td> 20</td><td> 17</td><td> 11</td><td> -</td>

<td>Vm (mV)</td><td> —</td><td> 21</td><td> 32</td><td> 30</td><td> 18</td><td> —</td>

TABLE 7-4

<td rowspan="2"></td><td rowspan="2"> 93</td><td rowspan="2"> 94</td><td colspan="4">Experiment number</td>

<td> 95</td><td> 96</td><td> 97</td><td> 98</td>

<td>Oxidation: Argon + 8% O₂ at 4.0 Pa Cathode voltage V_CSB 80 Volt plasma oxidation time (min)</td><td> 1</td><td> 3</td><td> 7</td><td> 10</td><td> 20</td><td> 30</td>

<td>V / L characteristics</td><td>Short circuit</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td><td>Tunnel-</td>

<td></td><td>ting</td><td>type</td><td>type</td><td>type</td><td>type</td><td>type</td>

TABLE 7-4 (continued)

<td rowspan="2"></td><td rowspan="2"> 93</td><td rowspan="2"> 94</td><td colspan="4">Experiment number</td>

<td> 95</td><td> 96</td><td> 97</td><td> 98</td>

<td>Rsg / Rnn</td><td> —</td><td> 10</td><td> 20</td><td> 22</td><td> 16</td><td> 12</td>

<td>Vm (mV)</td><td> —</td><td> 17</td><td> 32</td><td> 34</td><td> 25</td><td> 0</td>