JP3686910B2 - Etching method of silicon wafer - Google Patents

Description

【０００７】
このため、特に｛１００｝ウェーハをアルカリエッチングすると、ミクロな形状精度である表面粗さが悪化していた。
更に、従来のラッピングからエッチング後洗浄まで、２０有余の洗浄槽及びエッチング槽が配置されるため、取扱いが複雑になる上、薬液量、設備、時間、操作員等を多く要し、比較的エッチングコストが高くなる問題点もあった。
本発明の目的は、マクロな形状精度である平坦度とミクロな形状精度である表面粗さの双方を良好にするシリコンウェーハの化学エッチング方法を提供することにある。
【０００８】
【課題を解決するための手段】
請求項１に係る発明は、アルカリ水溶液にＣｕの硫酸塩又は塩化物、Ｎｉ又はＦｅの硝酸塩、硫酸塩又は塩化物である金属塩を添加して調製されたエッチング液を２０〜６０℃の温度にした後、このエッチング液にシリコンウェーハを浸漬してエッチングすることを特徴とするシリコンウェーハのエッチング方法である。
金属塩を含まないエッチング液にシリコンウェーハを浸漬した場合には、ラッピング後のダメージ及び粗さによって、このウェーハの表面電位は不均一になると推察される。しかし、本発明の金属塩を含んだアルカリ水溶液でエッチングすると、エッチング液中で金属塩の金属イオンがマイナス電位のウェーハ表面に付着するため、ウェーハの表面電位は均一化する。これは付着による表面電位の均一化とエッチング反応とが平衡状態を作り出し、見かけ上、表面電位の分布を均一にしたエッチングサイクルを実現したものと考えられる。この状態でエッチングが進行すると、ウェーハ表面は均一にエッチングされ、ウェーハのミクロな形状精度である表面粗さを劣化させない。このエッチングはアルカリエッチングであるため、反応律速で行われ、ウェーハのマクロな形状精度である平坦度を良好にする。なお、本明細書で「平坦度」とはシリコンウェーハのマクロな形状精度であって、「平坦度が良好である」とはウェーハの厚み分布が均一であることを意味し、「表面粗さ」とはシリコンウェーハのミクロな形状精度であって、「平坦度が良好である」とはウェーハの表面のラフネスアベレージ（Roughness average: Ｒａ）が小さいことを意味する。
【０００９】
【発明の実施の形態】
本発明のシリコンウェーハの化学エッチングは、ウェーハの平坦度と表面粗さの双方を良好にする必要があるときに行われる。例えばシリコン単結晶インゴットを切断して得た薄円板をラッピング（機械研磨）した後で、又はシリコンウェーハを酸素ドナー消去熱処理してポリッシング（機械的化学的研磨）した後で、或いは仕上った後で、行われる。好ましくはラッピング後に行われる。
本発明のエッチング液は、アルカリ水溶液にＣｕの硫酸塩又は塩化物、Ｎｉ又はＦｅの硝酸塩、硫酸塩又は塩化物である金属塩を添加して調製される。アルカリ水溶液としてはＮＨ₄ＯＨ水溶液、ＮａＯＨ水溶液、ＫＯＨ水溶液又はエチレンジアミン水溶液が挙げられる。このときＮＨ₄ＯＨ水溶液、ＮａＯＨ水溶液、ＫＯＨ水溶液又はエチレンジアミン水溶液は容積比でＮＨ₄ＯＨ／Ｈ₂Ｏ＝０．０１〜１．０、重量比でＮａＯＨ／Ｈ₂Ｏ＝０．２〜０．５、重量比でＫＯＨ／Ｈ₂Ｏ＝０．２〜０．５又は重量比でＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂／Ｈ₂Ｏ＝０．２〜０．５の組成を有する。ＮＨ₄ＯＨ（アンモニア水）の濃度は２０〜４０％の中から選択され、特に市販されている２９％濃度のＮＨ₄ＯＨが容易に入手できるため好ましい。ＮａＯＨ（水酸化ナトリウム液）の濃度は１０〜６０％の中から選択され、特に市販されている４８％濃度のＮａＯＨが容易に入手できるため好ましい。またＫＯＨ（水酸化カリウム液）の濃度は１０〜６０％の中から選択され、特に市販されている４８％濃度のＫＯＨが容易に入手できるため好ましい。更にＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂（エチレンジアミン液）の濃度は１０〜５０％の中から選択され、特に市販されている４０％濃度のＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂が容易に入手できるため好ましい。
【００１０】
ＮＨ₄ＯＨ／Ｈ₂Ｏ、ＮａＯＨ／Ｈ₂Ｏ、ＫＯＨ／Ｈ₂Ｏ又はＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂／Ｈ₂Ｏを容積比又は重量比で規定するのは、エッチングの特性を決定する第１の因子であり、主にエッチング速度、ウェーハの表面状態等を決定するためである。ここでＮＨ₄ＯＨ／Ｈ₂Ｏの容積比が０．０１未満、ＮａＯＨ／Ｈ₂Ｏ又はＫＯＨ／Ｈ₂Ｏの重量比がＯ．２未満、Ｈ₂ＮＣＨ₂ＣＨ₂ＮＨ₂／Ｈ₂Ｏの重量比が０．２未満では、本来のアルカリエッチングが行わず、所望のエッチング量が得られない。またＮＨ₄ＯＨ／Ｈ₂Ｏの容積比が１．０を超えるか、又はＮａＯＨ／Ｈ₂Ｏ又はＫＯＨ／Ｈ₂Ｏの重量比が０．５を超えるか、或いはＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂／Ｈ₂Ｏの重量比が０．５を超えると、エッチング過多となり、ウェーハの表面が荒れる。好ましい割合はＮＨ₄ＯＨ／Ｈ₂Ｏ＝０．０５〜０．５（容積比）、ＮａＯＨ／Ｈ₂Ｏ＝０．２５〜０．３５（重量比）、ＫＯＨ／Ｈ₂Ｏ＝０．２５〜０．３５（重量比）又はＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂／Ｈ₂Ｏ＝０．２５〜０．３５（重量比）である。
アルカリ水溶液に添加する金属塩としては、Ｃｕの硫酸塩又は塩化物，Ｎｉ又はＦｅの硝酸塩、硫酸塩又は塩化物が挙げられる。具体的には、ＣｕＳＯ₄、Ｎｉ(ＮＯ₃)₂、ＦｅＣｌ₂等が例示される。この場合、エッチング液はアルカリ水溶液がＮＨ₄ＯＨ水溶液であるときには０．０１〜１．０ｐｐｍ、好ましくは０．０５〜０．２ｐｐｍのＣｕ，Ｎｉ又はＦｅの金属イオンを含んで調製される。またアルカリ水溶液がＮａＯＨ水溶液又はＫＯＨ水溶液であるときには０．２〜０．５ｐｐｍ、好ましくは０．３〜０．４ｐｐｍのＣｕ，Ｎｉ又はＦｅの金属イオンを含んで調製される。更にアルカリ水溶液がＨ₂ＮＣＨ₂ＣＨ₂ＮＨ₂水溶液であるときには０．２〜０．５ｐｐｍ、好ましくは０．３〜０．４ｐｐｍのＣｕ，Ｎｉ又はＦｅの金属イオンを含んで調製される。
【００１１】
金属塩の添加物の濃度が上記下限値未満ではウェーハの表面電位を均一化するまでには至らず、上記上限値を超えるとエッチング後にウェーハ表面に残留する金属塩の量が多くなり過ぎ、洗浄して除去することが困難になる。
このように調製されたエッチング液は、エッチング時には２０〜６０℃に維持される。２０℃未満ではエッチング速度が極めて遅くなり、６０℃を超えるとウェーハの面粗さが増大する。この温度は好ましくは３０〜５０℃である。エッチング時間は全エッチング量が２０〜５０μｍ程度になるように決められる。このエッチング量は残留砥粒の多寡及び加工変質層の厚さによる。
【００１２】
本発明のエッチング液でシリコンウェーハをエッチングすると、次の式（２）の反応が行われる。
Ｓｉ＋２Ｈ₂Ｏ＋２ＯＨ^-→Ｓｉ(ＯＨ)₂(Ｏ^-)₂＋２Ｈ₂↑ ……（２）
図１に示すように、ラッピングしたシリコンウェーハは、ラッピング後洗浄を兼ねたエッチング前洗浄が行われ、上記エッチング液によるエッチングが行われ、エッチング後洗浄が行われた後、酸素ドナー消去のための熱処理が行われる。このエッチング前洗浄とエッチング後洗浄のために３〜６槽の洗浄槽が用いられ、エッチングには１〜４槽のエッチング槽が用いられる。エッチング槽を多槽にして、エッチング工程を数回に分けて行うと、化学反応により発生する反応熱を分散させることができ、この反応熱に起因したウェーハ表面におけるエッチング速度のばらつきを最小限に抑えることができる。
ここで、図２に示した従来のエッチングプロセスにおける洗浄槽と比べて、図１に示した洗浄槽の数がより少ないのは次の理由による。即ち、本発明では、従来のような混酸エッチングがエッチング工程中になく、反応速度が比較的小さいアルカリエッチングが主体であるため、液中で基板の表面電位をマイナスにし易い環境になる。従って、持込まれたパーティクル及びラッピングパウダーなどは、表面電位が同一となるため、除去し易い環境になり、エッチング前後の洗浄槽を減らすことができる。
エッチング前洗浄では洗浄液としてＫＯＨ又はＮａＯＨ水溶液などのアルカリ水溶液が用いられ、主としてサイズの大きなパーティクルが除去される。またエッチング後洗浄では洗浄液としてアンモニア過水が用いられ、主として小さなパーティクルが除去される。
【００１３】
【実施例】
次に本発明の実施例を比較例とともに説明する。
＜実施例１＞
直径６インチ、抵抗率８〜１１Ωｃｍのシリコン単結晶インゴットをスライスし、ベベリングし、その後両面をラッピングし、結晶面方位が（１００）のラッピングウェーハを用意した。エッチングはこのラッピングウェーハをＫＯＨ水溶液で洗浄した後、乾燥して行った。
ＮＨ₄ＯＨ／Ｈ₂Ｏ＝０．１（容積比）の組成を有する溶液にＣｕＳＯ ₄ を添加し、０．０８ｐｐｍのＣｕ²⁺イオンを含むエッチング液を用意した。エッチング液をエッチング槽に入れ、３０℃の温度に維持した後、このエッチング液に上記ラッピングウェーハを順次浸漬してエッチングを行った。全エッチング量が約３０μｍになるように、エッチング時間を決めた。エッチング後、アンモニア過水と塩酸過水を主要成分とする洗浄液で各ウェーハを洗浄し、乾燥した。
【００１４】
＜実施例２＞
実施例１と同一のラッピングウェーハについて、ＮＨ₄ＯＨ／Ｈ₂Ｏ＝０．５（容積比）の組成を有する溶液にＮｉ(ＮＯ₃)₂を添加し、０．８ｐｐｍのＮｉ²⁺イオンを含むエッチング液を用意し、実施例１と同様にエッチングした。エッチング前後の洗浄と乾燥は実施例１と同様に行った。
＜実施例３＞
実施例１と同一のラッピングウェーハについて、ＮＨ₄ＯＨ／Ｈ₂Ｏ＝０．８（容積比）の組成を有する溶液にＦｅＣｌ₂を添加し、０．５ｐｐｍのＦｅ²⁺イオンを含むエッチング液を用意し、実施例１と同様にエッチングした。エッチング前後の洗浄と乾燥は実施例１と同様に行った。
【００１５】
＜比較例１＞
実施例１と同一のラッピングウェーハについて、従来のＮＨ₄ＯＨ／Ｈ₂Ｏ＝０．２（容積比）の組成を有する溶液によるアルカリエッチングを行った。即ち、金属イオンを含まないエッチング液を単一のエッチング槽に入れ、８０℃に維持した後、このエッチング液に上記ラッピングウェーハを浸漬してエッチングを行った。全エッチング量が約３０μｍになるように、１０分間ラッピングウェーハを浸漬した。エッチング前洗浄はＫＯＨ水溶液で行い、エッチング後洗浄はアンモニア過水で行った。
【００１６】
＜実施例４＞
実施例１と同一のラッピングウェーハについて、ＫＯＨ／Ｈ₂Ｏ＝０．３５（重量比）の組成を有する溶液にＣｕＳＯ₄を添加し、０．７ｐｐｍのＣｕ²⁺イオンを含むエッチング液を用意し、実施例１と同様にエッチングした。エッチング前後の洗浄と乾燥は実施例１と同様に行った。
＜比較例２＞
実施例１と同一のラッピングウェーハについて、金属イオンを含まない、従来のＫＯＨ／Ｈ₂Ｏ＝０．６（重量比）の組成を有する溶液によるアルカリエッチングを比較例１と同様に行った。
＜比較例３＞
実施例１と同一のラッピングウェーハについて、従来の混酸による酸エッチングを行った。即ち、金属イオンを含まない、ＨＦ／ＨＮＯ₃／ＣＨ₃ＣＯＯＨ＝３／５／３（容積比）のエッチング液を用意し、このエッチング液を単一のエッチング槽に入れ、４０℃に維持した後、このエッチング液に上記ラッピングウェーハを浸漬してエッチングを行った。全エッチング量が約３０μｍになるように、７分３０秒間ラッピングウェーハを浸漬した。エッチング前洗浄及び後洗浄は比較例１と同様に行った。
【００２０】
＜比較評価＞
実施例１〜４と比較例１〜３のエッチング後の各ウェーハについて、各ウェーハに残留するＣｕ，Ｎｉ又はＦｅの量を測定した。また実施例１〜４と比較例１〜３のエッチング後の各ウェーハについて、各ウェーハを高平坦度のブロックプレートにワックスで貼付けて片面研磨した後、マクロな形状精度である平坦度（ＴＴＶ：Total Thickness Variation）とミクロな形状精度である表面粗さ（ＳＦＱＲ：Site Focal Quality Range）を測定した。その結果を表１に示す。
(a) ＴＴＶ
各ウェーハをそれぞれ真空吸着盤に吸着固定した後、ウェーハ表面の高さの最大値と最小値の差を求め、その値をＴＴＶとした。
(b) ＳＦＱＲ
各ウェーハを多数のサイトに分割し、各サイト内での基準面を設け、その基準面から各サイトでのプラス側とマイナス側の最大変化量を測定し、このプラス側の最大変化量の絶対値とマイナス側の最大変化量の絶対値との和をＳＦＱＲとした。
(c) 残留するＣｕ，Ｎｉ又はＦｅの量
各ウェーハの中央部にフッ酸と硝酸の混酸を滴下し、その液滴がウェーハの全表面に行渡るように液滴を巡らした後、その液滴を回収して原子吸光分析法で分析することにより、残留するＣｕ，Ｎｉ又はＦｅの量を求めた。表１において、残留金属とは、エッチング液に含まれていた金属（例えばＣｕ）がエッチングした後に残留する当該金属（Ｃｕ）を意味する。
【００２１】
【表１】

【００２３】
金属イオンを含むエッチング液を用いてエッチングした後では、残留金属の量は１×１０⁹原子／ｃｍ²以下であることが求められる。表１から明らかなように、実施例１〜実施例４の金属イオンを含むエッチング液のエッチング後の残留金属の量はすべて１×１０⁹原子／ｃｍ²以下である上、ＴＴＶ及びＳＦＱＲはそれぞれ１．０μｍ以下及び０．５μｍ以下の好ましい値を示した。これに対して比較例１〜比較例３ではＴＴＶ、ＳＦＱＲ及び残留金属（Ｃｕ）すべてについて実施例１〜実施例４より劣っていた。
【００２４】
【発明の効果】
以上述べたように、本発明によれば、アルカリ水溶液に特定の金属塩を添加して調製されたエッチング液を２０〜６０℃の温度にした後、このエッチング液にシリコンウェーハを浸漬してエッチングすることにより、シリコンウェーハのマクロな形状精度である平坦度とミクロな形状精度である表面粗さの双方を良好にすることができる。特に、本発明の方法ではエッチング前後の洗浄槽及びエッチング槽が従来と比べて少なくて済み、エッチングコストを安価にすることができる利点もある。
【図面の簡単な説明】
【図１】本発明のエッチング工程及びエッチング前後の工程を示す図。
【図２】従来のエッチング工程及びエッチング前後の工程を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a chemical etching method for improving both flatness, which is macro shape accuracy of a silicon wafer, and surface roughness, which is micro shape accuracy. More particularly, the present invention relates to a method suitable for etching a silicon wafer after wrapping and cleaning a thin disk obtained by cutting a silicon single crystal ingot.
[0002]
[Prior art]
A silicon wafer is processed in the order of lapping, etching, and polishing a thin disk obtained by pulling up a silicon single crystal ingot and cutting the ingot. In recent years, as the degree of integration of devices has improved, extremely high shape accuracy is required for VLSI wafers.
Conventionally, as shown in FIG. 2, the lapped silicon wafer is cleaned after lapping, followed by pre-etch cleaning, etching, and post-etch cleaning, and then heat treatment for erasing oxygen donors. Here, the cleaning after lapping aims at removing lapping powder, lapping slurry and the like. In the cleaning after lapping, an alkaline cleaning liquid to which a surfactant is added or a cleaning liquid mainly composed of ammonia perwater is used, and silicon wafers are sequentially immersed in five cleaning tanks for cleaning.
The purpose of cleaning before etching is to remove organic substances and residues after lapping. Here, a cleaning liquid containing an alkaline aqueous solution as a main component is used, and the silicon wafers are sequentially immersed and cleaned in seven cleaning tanks. In addition, post-etch cleaning also means pre-cleaning for oxygen donor erasure, and cleanliness is required. For this reason, a cleaning liquid mainly composed of ammonia perwater is used, and silicon wafers are sequentially immersed in eight cleaning tanks. Washing.
[0003]
As the etching, either acid etching or alkali etching is employed. In this etching process, the remaining abrasive grains and the work-affected layer are applied to a silicon wafer that has been processed with high precision in both flatness and thickness by polishing both surfaces simultaneously in the lapping process using an etching tank consisting of 1 to 2 tanks. In order to eliminate the above, the surface of the wafer is chemically removed by about 20 to 50 μm.
Acid etching does not have selective etching properties with respect to a silicon wafer, has a small surface roughness, improves micro shape accuracy, and has an advantage of high etching efficiency. As an etching solution for this acid etching, an etching solution containing a ternary element obtained by diluting a mixed acid of hydrofluoric acid (HF) and nitric acid (HNO ₃ ) with water (H ₂ O) or acetic acid (CH ₃ COOH) is mainly used. Yes. The above-mentioned advantage can be obtained by acid etching because the etching proceeds based on the diffusion-controlled condition with the above-mentioned etching solution, and under this diffusion-controlled condition, the reaction rate depends on the crystal surface orientation, crystal defects, etc. It is thought that the diffusion on the crystal surface has the main effect.
Alkali etching, on the other hand, has excellent flatness, improved macro shape accuracy, little metal contamination, and no harmful by-products such as NOx in acid etching and no handling risk. KOH or NaOH is used as an etching solution for this alkali etching. It is considered that the above characteristics can be obtained by alkaline etching because this etching basically proceeds based on surface reaction rate-determining conditions.
[0004]
[Problems to be solved by the invention]
However, when an etching solution having the above composition is used for both acid etching and alkali etching, there is a problem that it is very difficult to control the wafer shape accuracy.
In other words, acid etching improves the surface roughness of the silicon wafer, but as the acid etching progresses, the outer periphery of the wafer is eroded, the flatness, which is macro shape accuracy, is impaired, and the cost of the chemical solution is high. There was a drawback that it was difficult to control and maintain the composition of the etchant.
[0005]
Alkali etching improves the flatness of the silicon wafer, while it is anisotropic etching, so it has a problem of inferior surface roughness, which is microscopic shape accuracy. As described above, this alkali etching is reaction-limited, the etching rate strongly depends on the crystal plane orientation of the silicon wafer, and has the following equation (1):
[0006]
[Expression 1]

[0007]
For this reason, particularly when {100} wafers are subjected to alkali etching, the surface roughness, which is microscopic shape accuracy, is deteriorated.
Furthermore, from the conventional lapping to post-etch cleaning, 20 cleaning tanks and etching tanks are arranged, so that handling is complicated, and a large amount of chemicals, equipment, time, and operators are required, and etching is relatively easy. There was also a problem that the cost increased.
An object of the present invention is to provide a chemical etching method for a silicon wafer that improves both the flatness, which is macro shape accuracy, and the surface roughness, which is micro shape accuracy.
[0008]
[Means for Solving the Problems]
According to the first aspect of the present invention, there is provided an etching solution prepared by adding a metal salt which is a sulfate or chloride of Cu, a nitrate of Ni or Fe, a sulfate or a chloride to an alkaline aqueous solution at a temperature of 20 to 60 ° C. Then, the silicon wafer is etched by immersing and etching the silicon wafer in the etching solution.
When a silicon wafer is immersed in an etching solution that does not contain a metal salt, the surface potential of the wafer is assumed to be non-uniform due to damage and roughness after lapping. However, when etching with an alkaline aqueous solution containing a metal salt of the present invention, the metal ion of the metal salt is adhered to the wafer surface minus potential in the etching solution, the surface potential of the wafer is uniform. This is considered that the uniformization of the surface potential due to adhesion and the etching reaction created an equilibrium state, and apparently realized an etching cycle in which the distribution of the surface potential was uniform. When etching proceeds in this state, the wafer surface is uniformly etched, and the surface roughness, which is the micro shape accuracy of the wafer, is not deteriorated. Since this etching is alkaline etching, it is performed at a reaction rate-determining condition, and the flatness, which is the macro shape accuracy of the wafer, is improved. In the present specification, “flatness” means macro shape accuracy of a silicon wafer, and “good flatness” means that the thickness distribution of the wafer is uniform, and “surface roughness” "" Means the microscopic accuracy of the silicon wafer, and "good flatness" means that the roughness average (Ra) of the surface of the wafer is small.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The chemical etching of the silicon wafer of the present invention is performed when it is necessary to improve both the flatness and surface roughness of the wafer. For example, after lapping (mechanical polishing) a thin disk obtained by cutting a silicon single crystal ingot, or after polishing (mechanical chemical polishing) or finishing a silicon wafer by oxygen donor erasing heat treatment And done. Preferably it is performed after lapping.
The etching solution of the present invention is prepared by adding a metal salt which is a sulfate or chloride of Cu, a nitrate, sulfate or chloride of Ni or Fe to an alkaline aqueous solution. Examples of the alkaline aqueous solution include NH ₄ OH aqueous solution, NaOH aqueous solution, KOH aqueous solution, and ethylenediamine aqueous solution. At this time, NH ₄ OH aqueous solution, NaOH aqueous solution, KOH aqueous solution or ethylenediamine aqueous solution has a volume ratio of NH ₄ OH / H ₂ O = 0.01 to 1.0 and a weight ratio of NaOH / H ₂ O = 0.2 to 0. 5. It has a composition of KOH / H ₂ O = 0.2 to 0.5 by weight ratio or H ₂ NCH ₂ CH ₂ NH ₂ / H ₂ O = 0.2 to 0.5 by weight ratio. The concentration of NH ₄ OH (ammonia water) is selected from 20 to 40%, and particularly a commercially available 29% concentration of NH ₄ OH is preferable because it is easily available. The concentration of NaOH (sodium hydroxide solution) is selected from 10 to 60%, and a commercially available 48% concentration of NaOH is particularly preferred because it is readily available. Further, the concentration of KOH (potassium hydroxide solution) is selected from 10 to 60%, and a commercially available 48% concentration of KOH is particularly preferable because it can be easily obtained. Further, the concentration of H ₂ NCH ₂ CH ₂ NH ₂ (ethylenediamine liquid) is selected from 10 to 50%, and is particularly preferable because a commercially available 40% concentration of H ₂ NCH ₂ CH ₂ NH ₂ is easily available. .
[0010]
Defining NH ₄ OH / H ₂ O, NaOH / H ₂ O, KOH / H ₂ O or H ₂ NCH ₂ CH ₂ NH ₂ / H ₂ O by volume ratio or weight ratio determines the etching characteristics. The first factor is mainly for determining the etching rate, the surface state of the wafer, and the like. Here, the volume ratio of NH ₄ OH / H ₂ O is less than 0.01, and the weight ratio of NaOH / H ₂ O or KOH / H ₂ O is O.D. If the weight ratio is less than _{2 and} the weight ratio of H ₂ NCH ₂ CH ₂ NH ₂ / H ₂ O is less than 0.2, the original alkali etching is not performed, and a desired etching amount cannot be obtained. Further, the volume ratio of NH ₄ OH / H ₂ O exceeds 1.0, the weight ratio of NaOH / H ₂ O or KOH / H ₂ O exceeds 0.5, or H ₂ NCH ₂ CH ₂ NH _When the weight ratio of ₂ / H ₂ O exceeds 0.5, etching becomes excessive and the surface of the wafer becomes rough. Preferred ratios are NH ₄ OH / H ₂ O = 0.05 to 0.5 (volume ratio), NaOH / H ₂ O = 0.25 to 0.35 (weight ratio), KOH / H ₂ O = 0.25. is 0.35 (weight ratio) or _{H 2 NCH 2 CH 2 NH 2} / H 2 O = 0.25~0.35 ( weight ratio).
Examples of the metal salt added to the alkaline aqueous solution include Cu sulfate or chloride , Ni or Fe nitrate, sulfate or chloride. _{Specifically, C uSO 4, Ni (NO} 3) 2, FeCl 2 , and the like. In this case, the etching solution is prepared to contain 0.01 to 1.0 ppm, preferably 0.05 to 0.2 ppm of Cu, Ni or Fe metal ions when the alkaline aqueous solution is an NH ₄ OH aqueous solution. When the alkaline aqueous solution is a NaOH aqueous solution or a KOH aqueous solution, it is prepared containing 0.2 to 0.5 ppm, preferably 0.3 to 0.4 ppm of Cu, Ni or Fe metal ions. Further, when the aqueous alkaline solution is an aqueous H ₂ NCH ₂ CH ₂ NH ₂ solution, it is prepared containing 0.2 to 0.5 ppm, preferably 0.3 to 0.4 ppm of Cu, Ni or Fe metal ions.
[0011]
If the concentration of the metal salt additive is less than the above lower limit value, the surface potential of the wafer will not be uniformed. If the concentration exceeds the above upper limit value, the amount of metal salt remaining on the wafer surface after etching will increase too much and cleaning will occur. And difficult to remove.
The etching solution thus prepared is maintained at 20 to 60 ° C. during etching. If it is less than 20 ° C., the etching rate is extremely slow, and if it exceeds 60 ° C., the surface roughness of the wafer increases. This temperature is preferably 30-50 ° C. The etching time is determined so that the total etching amount is about 20 to 50 μm. This etching amount depends on the amount of residual abrasive grains and the thickness of the work-affected layer.
[0012]
When a silicon wafer is etched with the etching solution of the present invention, the reaction of the following formula (2) is performed.
Si + 2H ₂ O + 2OH ⁻ → Si (OH) ₂ (O ⁻ ) ₂ + 2H ₂ ↑ (2)
As shown in FIG. 1, the lapped silicon wafer is subjected to pre-etching cleaning that also serves as cleaning after lapping, etching with the above-described etching solution is performed, and post-etching cleaning is performed, followed by oxygen donor erasing. Heat treatment is performed. For this pre-etching cleaning and post-etching cleaning, 3 to 6 cleaning tanks are used, and for etching, 1 to 4 etching tanks are used. If the etching process is performed in several times with multiple etching tanks, the reaction heat generated by the chemical reaction can be dispersed, and variations in the etching rate on the wafer surface due to this reaction heat can be minimized. Can be suppressed.
Here, the reason why the number of cleaning tanks shown in FIG. 1 is smaller than that in the conventional etching process shown in FIG. 2 is as follows. That is, in the present invention, since conventional mixed acid etching is not performed during the etching process and alkali etching with a relatively low reaction rate is mainly performed, an environment in which the surface potential of the substrate is easily made negative in the liquid is obtained. Accordingly, since the surface potential of the brought particles, wrapping powder, and the like becomes the same, it becomes an environment that can be easily removed, and the cleaning tank before and after the etching can be reduced.
In the pre-etching cleaning, an alkaline aqueous solution such as a KOH or NaOH aqueous solution is used as a cleaning solution, and large particles are mainly removed. In the post-etching cleaning, ammonia overwater is used as a cleaning liquid, and mainly small particles are removed.
[0013]
【Example】
Next, examples of the present invention will be described together with comparative examples.
<Example 1>
A silicon single crystal ingot having a diameter of 6 inches and a resistivity of 8 to 11 Ωcm was sliced and beveled, and then both surfaces were lapped to prepare a lapping wafer having a crystal plane orientation of (100). Etching was performed by washing the lapping wafer with an aqueous KOH solution and then drying it.
NH ₄ OH / H solution was added CuSO ₄ with ₂ composition of O = 0.1 (volume ratio) was prepared containing non-image etching solution Cu ²⁺ ions 0.08 ppm. Put et etching liquid to e etching bath, was maintained at a temperature of 30 ° C., were etched the lapping wafers to the error etching liquid sequentially immersed in. The etching time was determined so that the total etching amount was about 30 μm. After the etching, each wafer was cleaned with a cleaning liquid mainly composed of ammonia perwater and hydrochloric acid and then dried.
[0014]
< Example 2 >
For the same lapping wafer as in Example 1, Ni (NO ₃ ) ₂ was added to a solution having a composition of NH ₄ OH / H ₂ O = 0.5 (volume ratio), and 0.8 ppm of Ni ²⁺ ions were added. The etching liquid containing was prepared and it etched like Example 1. FIG. Cleaning and drying before and after etching were performed in the same manner as in Example 1.
<Example 3 >
For the same lapping wafer as in Example 1, FeCl ₂ was added to a solution having a composition of NH ₄ OH / H ₂ O = 0.8 (volume ratio), and an etching solution containing 0.5 ppm of Fe ²⁺ ions was added. Prepared and etched as in Example 1. Cleaning and drying before and after etching were performed in the same manner as in Example 1.
[0015]
< Comparative Example 1 >
The same lapping wafer as in Example 1 was subjected to alkali etching with a solution having a composition of conventional NH ₄ OH / H ₂ O = 0.2 (volume ratio). That is, put an etching solution containing no metal ions in a single etching bath, was maintained at 80 ° C., it was etched by immersing the lapping wafer in this etching solution. The lapping wafer was immersed for 10 minutes so that the total etching amount was about 30 μm. Cleaning before etching was performed with a KOH aqueous solution, and cleaning after etching was performed with ammonia perwater.
[0016]
< Example 4 >
For the same lapping wafer as in Example 1, CuSO ₄ was added to a solution having a composition of KOH / H ₂ O = 0.35 (weight ratio), and an etching solution containing 0.7 ppm of Cu ²⁺ ions was prepared. Etching was performed in the same manner as in Example 1. Cleaning and drying before and after etching were performed in the same manner as in Example 1.
<Comparative example 2 >
For the same lapping the wafer as in Example 1, no metal ions were alkaline etching with a solution having a composition of conventional KOH / H ₂ O = 0.6 (weight ratio) in the same manner as in Comparative Example 1.
<Comparative Example 3 >
The same lapping wafer as in Example 1 was subjected to conventional acid etching with a mixed acid. That is, an etching solution containing no metal ions and having HF / HNO ₃ / CH ₃ COOH = 3/5/3 (volume ratio) was prepared, and this etching solution was put in a single etching bath and maintained at 40 ° C. Thereafter, etching was performed by immersing the lapping wafer in this etching solution. The lapping wafer was immersed for 7 minutes and 30 seconds so that the total etching amount was about 30 μm. Pre-etching cleaning and post-cleaning were performed in the same manner as in Comparative Example 1 .
[0020]
<Comparison evaluation>
For each wafer after etching in Examples 1 to 4 and Comparative Examples 1 to 3 , the amount of Cu, Ni or Fe remaining on each wafer was measured. Moreover, about each wafer after the etching of Examples 1-4 and Comparative Examples 1-3 , after sticking each wafer on the block plate of high flatness with wax and carrying out one side polishing, flatness (TTV: The surface roughness (SFQR: Site Focal Quality Range) which is total shape variation (micro) and micro shape accuracy was measured. The results are shown in Table 1 .
(a) TTV
After each wafer was sucked and fixed on the vacuum suction disk, the difference between the maximum value and the minimum value of the wafer surface height was determined, and the value was defined as TTV.
(b) SFQR
Divide each wafer into a number of sites, provide a reference plane in each site, measure the maximum amount of change on the positive side and the negative side at each site from the reference surface, and calculate the absolute value of the maximum change on the positive side. The sum of the value and the absolute value of the maximum amount of change on the negative side was defined as SFQR.
(c) Remaining amount of Cu, Ni or Fe A mixed acid of hydrofluoric acid and nitric acid is dropped at the center of each wafer, and the liquid is circulated so that the liquid drops spread over the entire surface of the wafer. Drops were collected and analyzed by atomic absorption spectrometry to determine the amount of residual Cu, Ni or Fe. In Table 1, the residual metal means the metal (Cu) remaining after the metal (for example, Cu) contained in the etching solution is etched.
[0021]
[Table 1]

[0023]
After etching using an etching solution containing metal ions, the amount of residual metal is required to be 1 × 10 ⁹ atoms / cm ² or less. As is apparent from Table 1, the amounts of residual metals after etching of the etching solutions containing the metal ions of Examples 1 to 4 are all 1 × 10 ⁹ atoms / cm ² or less, and TTV and SFQR are respectively Preferred values of 1.0 μm or less and 0.5 μm or less were shown. In contrast to the specific Comparative Examples 1 to Comparative Example 3 TTV, SFQR and residual metal (Cu) were all inferior to Examples 1 to 4 for.
[0024]
【The invention's effect】
As described above, according to the present invention, an etching solution prepared by adding a specific metal salt to an alkaline aqueous solution is brought to a temperature of 20 to 60 ° C., and then a silicon wafer is immersed in the etching solution and etched. By doing so, both the flatness, which is the macro shape accuracy of the silicon wafer, and the surface roughness, which is the micro shape accuracy, can be improved. In particular, in the method of the present invention, the number of cleaning tanks and etching tanks before and after etching can be reduced as compared with the prior art, and there is an advantage that the etching cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a diagram showing an etching process of the present invention and a process before and after etching.
FIG. 2 is a view showing a conventional etching process and processes before and after the etching.

Claims (3)

An etching solution prepared by adding a metal salt which is a sulfate or chloride of Cu, a nitrate of Ni or Fe, a sulfate or a chloride to an alkaline aqueous solution is brought to a temperature of 20 to 60 ° C. A method for etching a silicon wafer, comprising immersing and etching a silicon wafer. The alkaline aqueous solution is an NH ₄ OH aqueous solution, an NaOH aqueous solution, a KOH aqueous solution or an ethylene diamine aqueous solution, and a volume ratio of 20 to 40% NH ₄ OH / H ₂ O = 0.01 to 1.0, and a weight ratio of NaOH / _{H 2 O = 0.2~0.5, H 2} NCH 2 CH 2 NH with KOH / H ₂ O = 0.2~0.5 or weight ratio in weight ratio ₂ / H ₂ O = _0.2~0 The etching method according to claim 1, which has a composition of .5. The Cu, Ni or etching method according to claim 1 or 2, wherein is prepared containing the metal ions of Fe d etching solution at a rate of 0.01~1.0ppm in an aqueous alkaline solution.

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