JP6689494B2 - Method for detecting carbon in silicon - Google Patents
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Description
本発明は、シリコン中の炭素検出方法に関する。 The present invention relates to a method for detecting carbon in silicon.
半導体集積回路等のデバイスの高密度化、高集積化に伴い、デバイス動作の安定化が頓に望まれてきている。特にリーク電流や酸化膜耐圧等の特性値改善は重要な課題である。しかし、不純物が集積回路基板であるシリコンウェーハ中に混入すると、その後作製したデバイスの安定動作は望めないことになる。例えば、半導体集積回路の製造工程において、ウェーハ中の炭素は、1×1015atоms/cm3以下の濃度であっても、デバイス特性に悪影響を及ぼすことが広く知られている。 2. Description of the Related Art As devices such as semiconductor integrated circuits have become higher in density and higher in integration, stabilization of device operation has been eagerly desired. In particular, improvement of characteristic values such as leak current and oxide film breakdown voltage is an important issue. However, if impurities are mixed into a silicon wafer which is an integrated circuit substrate, stable operation of a device manufactured thereafter cannot be expected. For example, in the manufacturing process of semiconductor integrated circuits, it is widely known that carbon in a wafer adversely affects device characteristics even at a concentration of 1 × 10 15 atms / cm 3 or less.
したがって、ウェーハ中の炭素を低く抑えるために、正確な濃度測定技術が必要とされている。通常、この目的で炭素濃度測定はフーリエ変換赤外分光法(Fourier Transform InfraRed Spectroscopy、FT−IR法)で測定される。この方法は、シリコンウェーハの赤外線吸収スペクトルから格子間炭素濃度を間接的に求める手法であり、簡便に測定できるため広く用いられている。 Therefore, accurate concentration measurement techniques are needed to keep the carbon in the wafer low. Usually, for this purpose, carbon concentration measurement is performed by Fourier transform infrared spectroscopy (FT-IR method). This method is a method for indirectly obtaining the interstitial carbon concentration from the infrared absorption spectrum of a silicon wafer, and is widely used because it can be easily measured.
しかし、FT−IR法による炭素濃度測定において、1014atоms/cm3台の濃度を精度良く測定することは極めて困難であるのが実情である。また、他の測定手法として挙げられるSIMS(Secondary Ion Mass Spectroscopy、二次イオン質量分析法)も同様である。 However, in the carbon concentration measurement by the FT-IR method, it is actually difficult to accurately measure the concentration of 10 14 atms / cm 3 unit. The same applies to SIMS (Secondary Ion Mass Spectroscopy), which is another measurement method.
その点、DLTS(Deep Level Transient Spectroscopy)法は、1013atоms/cm3台の濃度を測定できる可能性を持つ手法である。なお、DLTS法とは、測定対象に形成したショットキー接合部又はpn接合部に印加する逆バイアス電圧を操作し、その接合部に生じる空乏層の静電容量変化の温度依存性から深い不純物準位に関する情報を得る方法である。このDLTS法の測定結果は、例えばDLTS信号強度と測定温度のグラフで示される。グラフ上に形成されたピークが、ある深い不純物準位の存在を示す。また、そのピークの温度から大まかに深い不純物準位のエネルギーが判明し、そのピークの高さが理論的に深い不純物準位の密度を示す。 In that respect, the DLTS (Deep Level Transient Spectroscopy) method is a method having a possibility of measuring a concentration of 10 13 atms / cm 3 units. It is to be noted that the DLTS method is a method in which a reverse bias voltage applied to a Schottky junction or a pn junction formed on a measurement target is manipulated, and a deep impurity quasi-state is obtained from temperature dependence of a change in capacitance of a depletion layer generated at the junction. It is a way to get information about rank. The measurement result of the DLTS method is shown by, for example, a graph of DLTS signal strength and measurement temperature. The peak formed on the graph indicates the presence of some deep impurity levels. The energy of the deep impurity level is found from the temperature of the peak, and the height of the peak theoretically indicates the density of the deep impurity level.
ここで、例えば、非特許文献1に示される方法では、シリコン結晶中に存在する炭素関連準位E1、E2、E3がH−C、H−C−O複合体により形成される深い不純物準位であり、特に、E3の準位は、H−Cに起因する準位のため、酸素に影響を受けないとされている。 Here, for example, in the method disclosed in Non-Patent Document 1, the carbon-related levels E1, E2, and E3 present in the silicon crystal are deep impurity levels formed by the H—C and H—C—O composites. And in particular, the level of E3 is said to be unaffected by oxygen because of the level resulting from H—C.
とはいえ、このままでは1013atоms/cm3台の炭素濃度を高感度に測定できるとまでは言えない。そこで、いくつかの工夫が提案されている。例えば、特許文献1では、炭素関連不純物準位であるE1、E2、E3を合算することが有効であることを示している。 However, it cannot be said that the carbon concentration on the order of 10 13 atms / cm 3 can be measured with high sensitivity under this condition. Therefore, some ideas have been proposed. For example, Patent Document 1 shows that it is effective to add E1, E2, and E3, which are carbon-related impurity levels.
しかし、DLTS法によりシリコン中の炭素関連の不純物準位を測定すると、DLTS信号強度が安定しないという問題がある。この点、従来のいずれの文献にも低濃度に存在する炭素関連準位を高感度に安定して測定するための本質的解決策として、準位が不安定になる原因について言及してはいない。そのため、真の対策を施すことができておらず、本質的解決策を施した有効な手法とまでは至っていない。 However, when the carbon-related impurity level in silicon is measured by the DLTS method, there is a problem that the DLTS signal intensity is not stable. In this regard, none of the conventional documents mentions the cause of unstable levels as an essential solution for highly sensitive and stable measurement of carbon-related levels existing at low concentrations. . As a result, no real countermeasures have been taken and effective methods that have taken essential solutions have not been reached.
本発明は、このような問題点に鑑みてなされたもので、シリコン中の炭素を高感度に又は安定して検出できる方法を提供することを課題とする。 The present invention has been made in view of such problems, and an object of the present invention is to provide a method capable of detecting carbon in silicon with high sensitivity or stably.
上記課題を解決するために、本発明のシリコン中の炭素検出方法は、シリコンにHNO 3 とHFを含む混酸によるエッチングを行い、そのエッチングを行ってから72時間〜120時間放置した後に、そのシリコン中に含まれる、炭素と水素を少なくとも含んだ複合体に起因した不純物準位をDLTS法により測定することを特徴とする。本発明によれば、従来の手法より高感度にシリコン中の炭素を検出することができる。 In order to solve the above-mentioned problems, the method for detecting carbon in silicon according to the present invention comprises etching silicon with a mixed acid containing HNO 3 and HF, and leaving the silicon for 72 hours to 120 hours after the etching. It is characterized in that the impurity level caused by the complex contained at least in carbon and hydrogen contained therein is measured by the DLTS method. According to the present invention, carbon in silicon can be detected with higher sensitivity than conventional methods.
また、本発明のシリコン中の炭素検出方法は、シリコンにHNO 3 とHFを含む混酸によるエッチングを行い、そのエッチングを行ってから144時間以上放置した後に、そのシリコン中に含まれる、炭素と水素を少なくとも含んだ複合体に起因した不純物準位をDLTS法により測定することを特徴とする。本発明によれば、従来の手法より安定してシリコン中の炭素を検出することができる。 Further, the carbon detection method in the silicon of the present invention, etched by mixed acid containing HNO 3 and HF in the silicon, after standing for more than 144 hours after performing the etching, contained in the silicon, carbon and hydrogen The impurity level resulting from the complex containing at least is measured by the DLTS method. According to the present invention, carbon in silicon can be detected more stably than conventional methods.
また、上記複合体は炭素と水素のみを含んだ複合体とするのが好ましい。これにより、炭素、水素以外の元素の濃度の影響で、DLTS測定値がばらついてしまうのを抑制できる。 Further, it is preferable that the above-mentioned complex is a complex containing only carbon and hydrogen. As a result, it is possible to suppress variations in DLTS measurement values due to the influence of the concentration of elements other than carbon and hydrogen.
以下、実施の形態について述べる。DLTS法は通常、ショットキー接合を形成する金属電極を測定試料表面に形成し、裏面にはオーミック接合を持つ金属電極を形成し、その2つの電極間に逆バイアスを印加し、生じた空乏層内の静電容量変化の温度依存性を取得すると、不純物が形成するエネルギー準位に応じた温度で静電容量変化がピークを形成する。そのピーク位置の静電容量変化から不純物準位密度を算出することができる。具体的に、n型シリコンウェーハにおいて、表面に形成するショットキー電極には金が用いられることが多く、裏面に形成するオーミック電極にはガリウムが用いられることが多い。いずれの電極も酸化膜の存在しない清浄なシリコン面に形成することが望ましく、その清浄面を得るためにHFによる表面酸化膜除去が行われる。 Hereinafter, embodiments will be described. In the DLTS method, a metal electrode forming a Schottky junction is usually formed on the surface of the measurement sample, a metal electrode having an ohmic junction is formed on the back surface, a reverse bias is applied between the two electrodes, and the resulting depletion layer is formed. When the temperature dependence of the capacitance change in the inside is acquired, the capacitance change forms a peak at a temperature according to the energy level formed by the impurities. The impurity level density can be calculated from the capacitance change at the peak position. Specifically, in an n-type silicon wafer, gold is often used for the Schottky electrode formed on the front surface, and gallium is often used for the ohmic electrode formed on the back surface. Both electrodes are preferably formed on a clean silicon surface without an oxide film, and the surface oxide film is removed by HF to obtain the clean surface.
本実施形態では、そのHF処理に先立ち、酸によるエッチング(湿式処理)を行い、その反応中にシリコンウェーハに深く水素を導入することが一つの特徴である。具体的に、使用する酸は、HNO3とHFを含む混酸であることが多くの場合有効であることが知られている。 One feature of the present embodiment is that etching (wet treatment) with an acid is performed prior to the HF treatment, and hydrogen is deeply introduced into the silicon wafer during the reaction. Specifically, it is known that it is often effective that the acid used is a mixed acid containing HNO 3 and HF.
この湿式処理により、シリコン中に水素が導入される。それは、水素の拡散が極めて速いため、室温における湿式処理においても十分にシリコン内に導入できるからである。 By this wet treatment, hydrogen is introduced into silicon. This is because the diffusion of hydrogen is so fast that it can be sufficiently introduced into silicon even in a wet process at room temperature.
しかし、このことは、導入された水素の不安定さを示すことになり、その安定性を調査することが、炭素関連準位の安定測定の鍵になると本発明者らは想到した。 However, this indicates the instability of the introduced hydrogen, and the present inventors have conceived that investigating the stability of hydrogen is the key to the stable measurement of carbon-related levels.
図1にその確認実験結果を示す。3つの炭素関連準位E1、E2、E3のうち、酸素の影響を受けないE3準位に関し、混酸による湿式処理後、グラフの横軸に示す時間(hour)だけ放置し、その後、HFによる表面除去とショットキー電極形成処理を行い、DLTS測定を行った。図1に示すように、大まかには混酸による湿式処理直後から次第に準位密度が減少する方向が示されている。この大きな傾向は湿式処理で導入された水素の不安定さを示すと考えられる。一方、湿式処理を行ってからの放置時間が72〜96時間に準位密度のピークが見られる。現時点では、このピークが形成される理由については定かではないが、水素と炭素が結合して複合体に変化する反応が総合的に最大効率を有するのがこのピークと考えられる。また、このピークは、湿式処理直後での準位密度(つまり放置時間が0時間での準位密度)よりも大きくなっている。また、ピーク以降は、徐々に準位密度が低下していく。 Figure 1 shows the results of the confirmation experiment. Of the three carbon-related levels E1, E2, and E3, the E3 level that is not affected by oxygen is left to stand for the time (hour) shown on the horizontal axis of the graph after the wet treatment with mixed acid, and then the surface with HF. After removal and Schottky electrode formation processing, DLTS measurement was performed. As shown in FIG. 1, roughly, the direction in which the level density gradually decreases immediately after the wet treatment with mixed acid is shown. This large tendency is considered to indicate the instability of hydrogen introduced by the wet treatment. On the other hand, a peak of the level density is observed when the standing time after the wet treatment is 72 to 96 hours. At present, the reason why this peak is formed is not clear, but it is considered that this peak has the highest overall efficiency in the reaction in which hydrogen and carbon are bound and converted into a complex. Further, this peak is higher than the level density immediately after the wet treatment (that is, the level density when the standing time is 0 hour). Moreover, after the peak, the level density gradually decreases.
したがって、ある程度高濃度に炭素が存在し、その濃度を定量的に正確に安定して求めるためには、ピークを越えて準位密度が安定する、混酸による湿式処理を行ってから144時間以上経過するのを待ち、DLTS測定する方が望ましいことがわかる。この手法により、真のウェーハ中炭素濃度を求めるためには、他の手法で予め測定しておいたシリコン中炭素濃度と本手法により得られた準位密度との間に検量線を作成しておけばよい(詳細は後述)。 Therefore, carbon is present at a high concentration to some extent, and in order to quantitatively and accurately determine the concentration, the level density is stable beyond the peak, and 144 hours or more have passed since the wet treatment with mixed acid was performed. It can be seen that it is preferable to wait for the measurement to be performed and then perform the DLTS measurement. In order to obtain the true carbon concentration in the wafer by this method, a calibration curve is created between the carbon concentration in silicon previously measured by another method and the level density obtained by this method. You can leave it (details will be described later).
一方、定量的正確さより、微量炭素を検出するという定性的目的であれば、混酸による湿式処理を行ってからの放置時間が72〜120時間に表れるピークを用いることが有効である。このピーク値は、144時間以上経過した安定して測定できた炭素関連準位密度に対し、2倍以上の濃度を有するため、このピークの出現をもって炭素の存在を確認する定性分析であれば、有効な測定手法となりうる。しかし、このピーク濃度を用いて定量値とすることは、ピーク高さの持つ不安定性から避けるべき手法であると考えられる。 On the other hand, for the qualitative purpose of detecting a trace amount of carbon in terms of quantitative accuracy, it is effective to use a peak that appears at a standing time of 72 to 120 hours after performing wet treatment with a mixed acid. This peak value has a concentration twice or more the density of carbon-related levels that can be stably measured after 144 hours or more. Therefore, if the qualitative analysis confirms the presence of carbon by the appearance of this peak, It can be an effective measurement method. However, using this peak concentration as a quantitative value is considered to be a technique that should be avoided because of the instability of the peak height.
以下、本実施形態に係るシリコン中の炭素濃度の測定方法の詳細を説明する。先ず、シリコン中の炭素濃度の定量的評価の目的でDLTS測定する方法について説明する。 Hereinafter, details of the method for measuring the carbon concentration in silicon according to the present embodiment will be described. First, a method of measuring DLTS for the purpose of quantitatively evaluating the carbon concentration in silicon will be described.
(シリコン中の炭素濃度の定量的評価)
先ず、炭素濃度が異なる複数の第1シリコン結晶を準備する。具体的には、例えば、FZ法で引き上げたn型シリコン結晶インゴットを所定の厚さに切り出し、切り出したウェーハに粗研磨、エッチング及び研磨などを施して表面に鏡面加工がされた基板(ポリッシュドウェーハ)を準備する。なお、この基板は、例えばトランジスタ、ダイオード等の電子デバイスの形成用として作製された基板とすることができる。次に、基板からシリコン結晶を切り出して第1シリコン結晶を作製する。第1シリコン結晶の炭素濃度は、FT−IR法やSIMSにて測定可能な範囲(例えば、1×1015〜1×1016atоms/cm3)に調整するとよい。なお、第1シリコン結晶は、CZ法(チョクラルスキー法)で形成されたとしても良いし、FZ法(フローティングゾーン法)で形成されたとしても良いが、後述の第2シリコン結晶の結晶育成法と同じとする。また、第1シリコン結晶の炭素濃度は、SIMS、FT−IR法など、DLTS法以外の手法により予め測定、つまり既知としておく。
(Quantitative evaluation of carbon concentration in silicon)
First, a plurality of first silicon crystals having different carbon concentrations are prepared. Specifically, for example, an n-type silicon crystal ingot pulled up by the FZ method is cut into a predetermined thickness, and the cut wafer is subjected to rough polishing, etching, polishing, etc., and a substrate (polished Wafer). It should be noted that this substrate can be a substrate manufactured for forming an electronic device such as a transistor or a diode. Next, a silicon crystal is cut out from the substrate to produce a first silicon crystal. The carbon concentration of the first silicon crystal may be adjusted to a range measurable by the FT-IR method or SIMS (for example, 1 × 10 15 to 1 × 10 16 atms / cm 3 ). The first silicon crystal may be formed by the CZ method (Czochralski method) or may be formed by the FZ method (floating zone method). Same as law. Further, the carbon concentration of the first silicon crystal is measured in advance by a method other than the DLTS method such as SIMS and FT-IR method, that is, is known in advance.
次に、DLTS測定に先立ち、各第1シリコン結晶の表面又は裏面又は両面に対して酸によるエッチング処理(湿式処理)を実施する。湿式処理としては、例えば、HNO3とHFを含む混酸液処理が挙げられる。混酸液処理は、酸液を所定液温にして第1シリコン結晶の表面をエッチングした後、第1シリコン結晶を純水でリンス処理するものである。混酸液処理としては、例えば、液温30℃で第1シリコン結晶の表面を50μm以下エッチングした後、3分間、純水で第1シリコン結晶をリンス処理する。なお、混酸液を構成する各酸(HNO3、HF等)の濃度や、混合比率(体積比又は質量比)は適宜に設定すればよい。また、混酸液処理の時間やエッチング量は適宜に設定すればよい。 Next, prior to the DLTS measurement, an acid etching process (wet process) is performed on the front surface, the back surface, or both surfaces of each first silicon crystal. Examples of the wet treatment include mixed acid liquid treatment containing HNO 3 and HF. In the mixed acid solution treatment, the acid solution is heated to a predetermined temperature to etch the surface of the first silicon crystal, and then the first silicon crystal is rinsed with pure water. As the mixed acid solution treatment, for example, the surface of the first silicon crystal is etched to 50 μm or less at a liquid temperature of 30 ° C., and then the first silicon crystal is rinsed with pure water for 3 minutes. The concentration of each acid (HNO 3 , HF, etc.) that constitutes the mixed acid solution and the mixing ratio (volume ratio or mass ratio) may be set appropriately. Further, the time of the mixed acid solution treatment and the etching amount may be set appropriately.
この湿式処理を実施した後、第1シリコン結晶を一定時間放置する。具体的には、混酸による湿式処理を行ってから144時間以上放置する。144時間以上放置することで、後に実施するDLTS測定により、湿式処理を行ってから72〜120時間の間に現れる準位密度のピークの影響を抑制した形で準位密度を得ることができる(図1参照)。つまり、安定した準位密度を得ることができる。さらに安定した準位密度を得るためには、湿式処理後、192時間以上放置するのが好ましい。図1に示されるように、湿式処理後、192時間以上放置すると、準位密度の時間経過に対する変化度合いがより小さくなるためである。このとき、湿式処理後の放置時間の上限値は特にないが、生産性などを考慮して適宜決定すればよい。 After carrying out this wet treatment, the first silicon crystal is left standing for a certain time. Specifically, it is left for 144 hours or more after performing wet treatment with mixed acid. By leaving it for 144 hours or more, the level density can be obtained by the DLTS measurement performed later while suppressing the influence of the peak of the level density that appears during 72 to 120 hours after the wet treatment ( (See FIG. 1). That is, a stable level density can be obtained. In order to obtain a more stable level density, it is preferable to leave it for 192 hours or more after the wet treatment. This is because, as shown in FIG. 1, when the wet treatment is left for 192 hours or more, the degree of change in level density with time is further reduced. At this time, there is no particular upper limit for the standing time after the wet treatment, but it may be appropriately determined in consideration of productivity and the like.
ここで、湿式処理を行ってから144時間以上放置に関して、例えば、湿式処理を2月1日の午後12時に実施した場合には、2月7日の午後12時をもって湿式処理から144時間の放置となる。したがって、湿式処理を2月1日の午後12時に実施した場合には、少なくとも2月7日の午後12時まで第1シリコン結晶を放置する。 Here, with regard to leaving for 144 hours or more after performing the wet treatment, for example, when the wet treatment is performed at 12:00 pm on February 1, it is left for 144 hours after the wet treatment at 12:00 pm on February 7. Becomes Therefore, when the wet treatment is performed at 12:00 pm on February 1, the first silicon crystal is left at least until 12:00 pm on February 7.
次に、湿式処理から144時間以上放置した各第1シリコン結晶に対してDLTS法により炭素関連の不純物準位の密度を測定する。具体的には、炭素と水素とを少なくも含んだ複合体の不純物準位の密度を測定する。炭素と水素とを少なくも含んだ複合体の中でも特に炭素、水素以外の元素(酸素等)を含んでいない複合体(つまり、炭素、水素のみの複合体(H−C複合体))の準位密度を測定するのが好ましい。酸素等の他の元素の濃度の影響で、DLTS信号がばらついてしまうのを抑制するためである。第1シリコン結晶の導電型がn型の場合には、非特許文献1に示された炭素関連準位E1、E2、E3のうち、H−C複合体に起因した準位E3を測定するのが好ましい。なお、3つの不純物準位E1、E2、E3は、DLTS法でn型シリコン結晶を測定することにより検出される約0.11〜0.15eVの範囲に形成される炭素関連の不純物準位であって、準位E1のエネルギーが0.11eV、準位E2のエネルギーが0.13eV、準位E3のエネルギーが0.15eVである。 Next, the density of carbon-related impurity levels is measured by the DLTS method for each first silicon crystal left for 144 hours or more after the wet treatment. Specifically, the density of impurity levels of a complex containing at least carbon and hydrogen is measured. Among the composites containing at least carbon and hydrogen, a composite containing no elements (oxygen, etc.) other than carbon and hydrogen (that is, a composite containing only carbon and hydrogen (HC composite)) It is preferable to measure the unit density. This is to prevent the DLTS signal from varying due to the effect of the concentration of other elements such as oxygen. When the conductivity type of the first silicon crystal is n-type, of the carbon-related levels E1, E2, and E3 shown in Non-Patent Document 1, the level E3 due to the H—C complex is measured. Is preferred. The three impurity levels E1, E2, and E3 are carbon-related impurity levels formed in the range of about 0.11 to 0.15 eV detected by measuring an n-type silicon crystal by the DLTS method. Therefore, the energy of the level E1 is 0.11 eV, the energy of the level E2 is 0.13 eV, and the energy of the level E3 is 0.15 eV.
DLTS測定では、第1シリコン結晶の表面及び裏面に対してHFによる表面酸化膜除去処理を実施した後、表面に例えばAuを蒸着してショットキー電極とするとともに、その裏面には例えばGaを塗布してオーミック電極を作製する。そして、2つの電極間に逆バイアス(例えば‐5V)を印加し、所定温度範囲(例えば30〜300Kの範囲)で掃引して、炭素関連の準位密度を測定する。 In the DLTS measurement, the front surface and the back surface of the first silicon crystal are subjected to a surface oxide film removal treatment by HF, and then Au is vapor-deposited on the front surface to form a Schottky electrode, and Ga is applied on the back surface thereof. Then, an ohmic electrode is produced. Then, a reverse bias (for example, −5 V) is applied between the two electrodes, and sweeping is performed within a predetermined temperature range (for example, a range of 30 to 300 K) to measure the carbon related level density.
次に、各第1シリコン結晶から得られた炭素関連の準位密度と、各第1シリコン結晶の炭素濃度とに基づいて、炭素関連の準位密度と、シリコン中の炭素濃度との相関関係を示した検量線を導出する。なお、炭素関連の準位密度と、シリコン中の炭素濃度とがほぼ比例の関係となる場合には、第1シリコン結晶から得られた炭素関連の準位密度を、第1シリコン結晶の炭素濃度で除算することで、シリコン中の炭素が不純物準位を形成する割合である準位形成率を導出しても良い。この準位形成率も、炭素関連の準位密度とシリコン中の炭素濃度との相関関係を示した指標となる。 Next, based on the carbon-related level density obtained from each first silicon crystal and the carbon concentration of each first silicon crystal, the correlation between the carbon-related level density and the carbon concentration in silicon A calibration curve showing is derived. When the carbon-related level density and the carbon concentration in silicon have a substantially proportional relationship, the carbon-related level density obtained from the first silicon crystal is used as the carbon concentration of the first silicon crystal. It is also possible to derive the level formation rate, which is the rate at which carbon in silicon forms an impurity level, by dividing by. This level formation rate is also an index showing the correlation between the level density related to carbon and the carbon concentration in silicon.
次に、炭素濃度が未知の第2シリコン結晶を準備する。第2シリコン結晶の作製法は第1シリコン結晶と同様である。 Next, a second silicon crystal whose carbon concentration is unknown is prepared. The method for producing the second silicon crystal is similar to that for the first silicon crystal.
この第2シリコン結晶に対して、第1シリコン結晶に対して実施したのと同様の湿式処理を実施し、その湿式処理から144時間以上放置する。このとき、第2シリコン結晶に対する湿式処理からの放置時間は、第1シリコン結晶に対する湿式処理からの放置時間と同じとする。すなわち、例えば、第1シリコン結晶に対する放置時間を192時間とした場合には、第2シリコン結晶に対する放置時間も192時間とする。 A wet treatment similar to that performed on the first silicon crystal is performed on the second silicon crystal, and the wet treatment is left for 144 hours or more. At this time, the leaving time of the second silicon crystal after the wet treatment is the same as the leaving time of the first silicon crystal after the wet treatment. That is, for example, when the leaving time for the first silicon crystal is 192 hours, the leaving time for the second silicon crystal is also 192 hours.
次に、144時間以上放置した第2シリコン結晶に対してDLTS法により炭素関連の不純物準位の密度を測定する。このとき、先に導出した検量線又は準位形成率が、H−C複合体に起因した準位E3に基づいて得られた場合には、第2シリコン結晶に対しても準位E3の密度を測定する。つまり、検量線又は準位形成率を構成する不純物準位と同じ不純物準位を第2シリコン結晶から測定する。なお、第2シリコン結晶に対するDLTS測定の手順は、第1シリコン結晶に対するDLTS測定の手順と同じである。 Next, the density of carbon-related impurity levels is measured by the DLTS method with respect to the second silicon crystal left for 144 hours or more. At this time, when the previously derived calibration curve or level formation rate is obtained based on the level E3 caused by the H—C complex, the density of the level E3 also with respect to the second silicon crystal. To measure. That is, the same impurity level as the impurity level constituting the calibration curve or the level formation rate is measured from the second silicon crystal. The DLTS measurement procedure for the second silicon crystal is the same as the DLTS measurement procedure for the first silicon crystal.
次に、第2シリコン結晶の炭素関連の準位密度と、第1シリコン結晶から得られた検量線又は準位形成率(シリコン中の炭素濃度と炭素関連の準位密度との相関関係)とに基づいて、第2シリコン結晶中の炭素濃度を求める。以上により、シリコン結晶中の炭素濃度をDLTS法により求めることができる。 Next, a carbon-related level density of the second silicon crystal and a calibration curve or level formation rate obtained from the first silicon crystal (correlation between carbon concentration in silicon and carbon-related level density) Based on the above, the carbon concentration in the second silicon crystal is determined. From the above, the carbon concentration in the silicon crystal can be obtained by the DLTS method.
(シリコン中の炭素濃度の定性的評価)
次に、シリコン中の炭素濃度の定性的評価の目的でDLTS測定する方法について説明する。
(Qualitative evaluation of carbon concentration in silicon)
Next, a method for DLTS measurement for the purpose of qualitatively evaluating the carbon concentration in silicon will be described.
先ず、炭素濃度の評価対象となるシリコン結晶を準備する。このシリコン結晶はCZ法で製造されたとしても良いし、FZ法で製造されたとしても良い。 First, a silicon crystal that is an object of carbon concentration evaluation is prepared. This silicon crystal may be manufactured by the CZ method or the FZ method.
次に、準備したシリコン結晶に対して、上記第1シリコン結晶に対して実施したのと同様の湿式処理を実施する。 Next, the prepared silicon crystal is subjected to the same wet treatment as that performed on the first silicon crystal.
この湿式処理を実施した後、シリコン結晶を一定時間放置する。具体的には、湿式処理から72〜120時間放置する。図1で説明したように、湿式処理からの放置時間が72〜120時間に準位密度のピークが現れるので、シリコン結晶を湿式処理から72〜120時間間放置することで、後のDLTS測定により、このピーク付近の準位密度を測定できる。つまり、高感度に準位密度を測定できる。 After carrying out this wet treatment, the silicon crystal is left to stand for a certain period of time. Specifically, the wet treatment is left for 72 to 120 hours. As described with reference to FIG. 1, since the peak of the level density appears at the time of leaving 72 to 120 hours after the wet treatment, by leaving the silicon crystal for 72 to 120 hours after the wet treatment, the DLTS measurement performed later. , The level density near this peak can be measured. That is, the level density can be measured with high sensitivity.
ここで、湿式処理から72〜120時間放置に関して、例えば湿式処理を2月1日の午後12時に実施した場合には、2月4日の午後12時〜2月6日の午後12時まで放置することを意味する。 Here, regarding the 72 to 120 hours left after the wet treatment, for example, when the wet treatment is performed at 12:00 pm on February 1, it is left until 12:00 pm on February 4 to 12:00 on February 6 Means to do.
次に、湿式処理から72〜120時間放置したシリコン結晶に対してDLTS法により炭素関連の不純物準位の密度を測定する。この準位密度の測定は、上述の第1シリコン結晶に対する準位密度の測定と同様である。 Next, the density of carbon-related impurity levels is measured by the DLTS method with respect to the silicon crystal left to stand for 72 to 120 hours after the wet treatment. The measurement of the level density is similar to the above-described measurement of the level density for the first silicon crystal.
そして、得られた準位密度に基づいて、シリコン結晶中の炭素濃度を評価する。例えば、DLTS法により炭素関連の準位密度を測定できたということは、シリコン結晶中に微量ながらも炭素が存在することを意味するので、準位密度に基づいてシリコン結晶中の炭素の存在を確認評価する。 Then, the carbon concentration in the silicon crystal is evaluated based on the obtained level density. For example, the fact that the carbon-related level density can be measured by the DLTS method means that a small amount of carbon exists in the silicon crystal. Therefore, the presence of carbon in the silicon crystal based on the level density Confirm and evaluate.
このように、本実施形態では、DLTS測定に先立って、シリコン結晶に対して酸による湿式処理を実施するので、シリコン結晶中に水素を導入できる。この導入により、シリコン結晶中の不純物準位のうち、炭素と水素を含んだ複合の準位を活性化でき、DLTS測定により、この準位を高感度に測定できる。 As described above, in this embodiment, since the silicon crystal is subjected to the wet treatment with acid prior to the DLTS measurement, hydrogen can be introduced into the silicon crystal. With this introduction, of the impurity levels in the silicon crystal, a composite level containing carbon and hydrogen can be activated, and this level can be measured with high sensitivity by DLTS measurement.
また、DLTS測定は、湿式処理から144時間以上放置した後に、又は湿式処理から72〜120時間放置した後に行うので、安定して又は高感度に炭素関連の準位密度を測定できる。すなわち、湿式処理から144時間以上放置した後にDLTS測定を行った場合には、時間経過に対する炭素関連の準位密度の変化が小さい時にその測定を行うことができるので、精度の高い準位密度を得ることができる。つまり、シリコン中の炭素濃度を精度よく測定できる。 Further, since the DLTS measurement is performed after leaving the wet treatment for 144 hours or more, or after leaving the wet treatment for 72 to 120 hours, the carbon-related level density can be stably or highly sensitively measured. That is, when the DLTS measurement is performed after leaving for 144 hours or more after the wet treatment, the measurement can be performed when the change in the carbon-related level density over time is small, so that the level density with high accuracy can be obtained. Obtainable. That is, the carbon concentration in silicon can be accurately measured.
一方、湿式処理から72〜120時間放置した後にDLTS測定を行った場合には、準位密度のピーク付近でその測定を行うことができるので、シリコン中の炭素を高感度に検出でき、シリコン中の炭素が微量(例えば炭素濃度が1013atоms/cm3台)であっても、その炭素の存在を確認できる。 On the other hand, when DLTS measurement is carried out after being left for 72 to 120 hours after the wet treatment, the measurement can be carried out near the peak of the level density, so that carbon in silicon can be detected with high sensitivity and in the silicon. The presence of carbon can be confirmed even if the amount of carbon is very small (for example, the carbon concentration is 10 13 atms / cm 3 unit).
以下、本発明の実施例及び比較例を挙げて具体的に説明するが、本発明はこれらに限定されるものではない。 Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited thereto.
(比較例1)
FZ法により、直径6インチ、方位<100>、n型、抵抗率が10Ωcmで炭素を0.03ppma程度含むシリコン結晶棒を引き上げた。このシリコン結晶棒を加工してシリコンウェーハとした。このシリコンウェーハを2群に分け、1群は何もせず、もう1群は、HNO3とHFの混酸により表層をエッチング(湿式処理)した。この湿式処理は、混酸の液温を30℃として、シリコン結晶の表面を3μmエッチングした後、シリコン結晶を3分間、純水でリンスする処理とした。その後、ただちに2群のシリコンウェーハともHFによる酸化膜除去を施し、Auによるショットキー電極とGaによるオーミック電極をそれぞれ表裏面に形成後、DLTS測定を行った。そのDLTS測定では、電極間に−5V(逆バイアス)を印加し、温度30〜300Kの範囲で掃引してH−C複合体の準位E3の密度を測定した。
(Comparative Example 1)
A silicon crystal rod having a diameter of 6 inches, an orientation of <100>, an n-type, a resistivity of 10 Ωcm, and a carbon content of about 0.03 ppma was pulled by the FZ method. This silicon crystal rod was processed into a silicon wafer. This silicon wafer was divided into two groups, one group did nothing, and the other group had its surface layer etched (wet treatment) with a mixed acid of HNO 3 and HF. In this wet treatment, the temperature of the mixed acid was set to 30 ° C., the surface of the silicon crystal was etched by 3 μm, and then the silicon crystal was rinsed with pure water for 3 minutes. Immediately thereafter, the oxide film was removed by HF from the two groups of silicon wafers, and Schottky electrodes made of Au and ohmic electrodes made of Ga were formed on the front and back surfaces, respectively, and then DLTS measurement was performed. In the DLTS measurement, −5 V (reverse bias) was applied between the electrodes, the temperature was swept in the range of 30 to 300 K, and the density of the level E3 of the H—C complex was measured.
その結果、混酸処理を行った試料では、8×1011atoms/cm3程度の炭素関連準位E3の密度が得られたのに対し、混酸処理を行わなかった試料では、2×1010atoms/cm3程度の炭素関連準位E3の密度が得られ、混酸処理がE3準位活性化の効果をもつことがわかった。なお、混酸処理を行った試料から得られた準位密度(8×1011atoms/cm3程度)は、図1の放置時間が0時間における点として示している。 As a result, the density of the carbon-related level E3 of about 8 × 10 11 atoms / cm 3 was obtained in the sample subjected to the mixed acid treatment, whereas it was 2 × 10 10 atoms in the sample not subjected to the mixed acid treatment. A density of carbon-related level E3 of about / cm 3 was obtained, and it was found that the mixed acid treatment had an effect of activating the E3 level. The level density (about 8 × 10 11 atoms / cm 3 ) obtained from the sample subjected to the mixed acid treatment is shown as a point when the standing time in FIG. 1 is 0 hour.
(実施例1)
FZ法により、直径6インチ、方位<100>、n型、抵抗率が10Ωcmで炭素を0.03ppma程度含むシリコン結晶棒を引き上げた。このシリコン結晶棒を加工してシリコンウェーハとした。このシリコンウェーハに対し、HNO3とHFの混酸による表層エッチングを施した。この湿式処理は、混酸の液温を30℃として、シリコン結晶の表面を3μmエッチングした後、シリコン結晶を3分間、純水でリンスする処理とした。その湿式処理から72時間、シリコンウェーハを放置した。その後、HFによる酸化膜除去を施し、Auによるショットキー電極とGaによるオーミック電極をそれぞれ表裏面に形成後、DLTS測定を行った。そのDLTS測定では、電極間に−5V(逆バイアス)を印加し、温度30〜300Kの範囲で掃引してH−C複合体の準位E3の密度を測定した。
(Example 1)
A silicon crystal rod having a diameter of 6 inches, an orientation of <100>, an n-type, a resistivity of 10 Ωcm, and a carbon content of about 0.03 ppma was pulled by the FZ method. This silicon crystal rod was processed into a silicon wafer. This silicon wafer was subjected to surface layer etching with a mixed acid of HNO 3 and HF. In this wet treatment, the temperature of the mixed acid was set to 30 ° C., the surface of the silicon crystal was etched by 3 μm, and then the silicon crystal was rinsed with pure water for 3 minutes. The silicon wafer was left for 72 hours after the wet treatment. After that, the oxide film was removed by HF, and a Schottky electrode made of Au and an ohmic electrode made of Ga were formed on the front and back surfaces, respectively, and then DLTS measurement was performed. In the DLTS measurement, −5 V (reverse bias) was applied between the electrodes, the temperature was swept in the range of 30 to 300 K, and the density of the level E3 of the H—C complex was measured.
その結果、1.8×1012atoms/cm3程度の炭素関連準位E3の密度が得られ、混酸処理直後に電極形成とDLTS測定を行った試料よりも、2倍以上のE3準位密度を得ることができ、炭素存在の定性的判断を確実に行うことができた。なお、この準位密度(1.8×1012atoms/cm3程度)は、図1の放置時間が72時間における点として示している。 As a result, a density of the carbon-related level E3 of about 1.8 × 10 12 atoms / cm 3 was obtained, and the density of the E3 level was more than twice that of the sample on which the electrode formation and the DLTS measurement were performed immediately after the mixed acid treatment. It was possible to obtain a qualitative judgment of the existence of carbon. Note that this level density (about 1.8 × 10 12 atoms / cm 3 ) is shown as a point when the standing time in FIG. 1 is 72 hours.
(実施例2)
湿式処理からの放置時間を192時間としたこと以外は、実施例1と同様にして、炭素関連準位E3の密度を測定した。
(Example 2)
The density of the carbon-related level E3 was measured in the same manner as in Example 1 except that the standing time after the wet treatment was 192 hours.
その結果、1.3×1011atoms/cm3程度の炭素関連準位E3の密度が得られ、比較例1に示すような混酸処理を行わなかった試料の2倍弱程度のE3準位活性化効率を有しながら、安定したE3準位密度を得ることができ、炭素存在の定量的判断を確実に行うことができた。なお、この準位密度(1.3×1011atoms/cm3程度)は、図1の放置時間が192時間における点として示している。 As a result, a carbon-related level E3 density of about 1.3 × 10 11 atoms / cm 3 was obtained, and the E3 level activity was about twice as low as that of the sample not subjected to the mixed acid treatment as shown in Comparative Example 1. It was possible to obtain a stable E3 level density while maintaining the efficiency of conversion, and to make a quantitative determination of the presence of carbon with certainty. Note that this level density (about 1.3 × 10 11 atoms / cm 3 ) is shown as a point when the standing time in FIG. 1 is 192 hours.
(実施例3)
湿式処理からの放置時間を96時間としたこと以外は、実施例1と同様にして、炭素関連準位E3の密度を測定した。
(Example 3)
The density of the carbon-related level E3 was measured in the same manner as in Example 1 except that the standing time after the wet treatment was 96 hours.
その結果、1.2×1012atoms/cm3程度の炭素関連準位E3の密度が得られ、混酸処理直後に電極形成とDLTS測定を行った試料よりも大きいE3準位密度を得ることができ、炭素存在の定性的判断を確実に行うことができた。なお、この準位密度(1.2×1012atoms/cm3程度)は、図1の放置時間が96時間における点として示している。 As a result, a density of carbon-related level E3 of about 1.2 × 10 12 atoms / cm 3 was obtained, and an E3 level density higher than that of the sample for which electrode formation and DLTS measurement were performed immediately after the mixed acid treatment was obtained. It was possible to make a qualitative judgment of the existence of carbon. The level density (about 1.2 × 10 12 atoms / cm 3 ) is shown as a point when the standing time in FIG. 1 is 96 hours.
(実施例4)
湿式処理からの放置時間を336時間としたこと以外は、実施例1と同様にして、炭素関連準位E3の密度を測定した。
(Example 4)
The density of the carbon-related level E3 was measured in the same manner as in Example 1 except that the standing time after the wet treatment was 336 hours.
その結果、1.0×1011atoms/cm3程度の炭素関連準位E3の密度が得られ、比較例1に示すような混酸処理を行わなかった試料よりも大きいE3準位活性化効率を有しながら、安定したE3準位密度を得ることができ、炭素存在の定量的判断を確実に行うことができた。なお、この準位密度(1.0×1011atoms/cm3程度)は、図1の放置時間が336時間における点として示している。 As a result, a density of the carbon-related level E3 of about 1.0 × 10 11 atoms / cm 3 was obtained, and the E3 level activation efficiency higher than that of the sample not subjected to the mixed acid treatment as shown in Comparative Example 1 was obtained. While having it, a stable E3 level density could be obtained, and the quantitative determination of the presence of carbon could be reliably performed. The level density (about 1.0 × 10 11 atoms / cm 3 ) is shown as a point when the standing time in FIG. 1 is 336 hours.
また、実施例2、4で得られた各準位密度の差は、0.3×1011atoms/cm3程度であり、湿式処理からの放置時間が192時間以降では、120時間より前に比べて、時間経過に対する準位密度の変化が小さい。したがって、湿式処理からの放置時間を144時間以上(特に192時間以上)とすることで、炭素関連の準位密度が安定しているといえる。 Further, the difference in level densities obtained in Examples 2 and 4 is about 0.3 × 10 11 atoms / cm 3 , and after the standing time after the wet treatment of 192 hours, before 120 hours. In comparison, the change in level density over time is small. Therefore, it can be said that the carbon-related level density is stable by setting the standing time after the wet treatment to 144 hours or longer (particularly 192 hours or longer).
なお、本発明は上記実施形態に限定されるものではない。上記形態は例示であり、本発明の特許請求の範囲に記載された技術的思想と実質的に同一な構成を有し、かつ同様な作用効果を奏するものは、いかなるものであっても本発明の技術的範囲に包含される。 The present invention is not limited to the above embodiment. The above embodiments are mere examples, and any of the embodiments having substantially the same configuration as the technical idea described in the scope of claims of the present invention and exhibiting the same operational effect It is included in the technical scope of.
例えば、上記実施例では、H−C複合体の準位E3の密度をDLTS測定した例を示したが、H、Cを少なくとも含む準位E3以外の不純物準位(H−C−O複合体など)の密度をDLTS測定して、その測定値に基づいてシリコン中の炭素濃度を評価しても良いし、特許文献1のように、H−C複合体、H−C−O複合体の準位E1、E2、E3の密度をDLTS測定して、得られた密度の合計値に基づいて、シリコン中の炭素濃度を評価しても良い。 For example, in the above example, an example of DLTS measurement of the density of the level E3 of the H-C complex was shown. However, impurity levels other than the level E3 containing at least H and C (H-C-O complex It is also possible to evaluate the carbon concentration in silicon based on the measured value by DLTS measurement of the density of the H.C. composite and the H.C.O. The density of the levels E1, E2, and E3 may be measured by DLTS, and the carbon concentration in silicon may be evaluated based on the total value of the obtained densities.
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