金剛石以優(yōu)異的性能在力學(xué)、光學(xué)、熱學(xué)和電子學(xué)（如半導(dǎo)體）等領(lǐng)域發(fā)揮著重要作用。然而，金剛石表面質(zhì)量會(huì)影響其在這些領(lǐng)域的應(yīng)用效果，因此通過高效拋光技術(shù)獲得高質(zhì)量表面一直是金剛石研究的重點(diǎn)內(nèi)容。金剛石拋光技術(shù)主要有機(jī)械拋光、熱化學(xué)拋光、激光拋光和化學(xué)機(jī)械拋光等，其中化學(xué)機(jī)械拋光（CMP）具有設(shè)備運(yùn)行成本低、工藝簡單、拋光后表面損傷小等優(yōu)點(diǎn)。

       近日，北方工業(yè)大學(xué)安康副教授聯(lián)合北京科技大學(xué)李成明研究團(tuán)隊(duì)在《人工晶體學(xué)報(bào)》2024年第10期發(fā)表了題為《金剛石化學(xué)機(jī)械拋光研究進(jìn)展》的綜述論文（第一作者：安康；通信作者：劉峰斌、李成明）。該論文在對幾種拋光方法進(jìn)行分析對比的基礎(chǔ)上，聚焦于CMP領(lǐng)域，對其發(fā)展歷程進(jìn)行了較詳盡的對比與分析。早期CMP技術(shù)雖在工藝和拋光效率上存在一定局限，但為后續(xù)技術(shù)的創(chuàng)新與優(yōu)化奠定了基礎(chǔ)；H2O2及其混合物的應(yīng)用，不僅增強(qiáng)了CMP過程中的化學(xué)反應(yīng)活性，提高了材料去除率，還有效降低了表面粗糙度，改善了金剛石表面質(zhì)量；光催化輔助化學(xué)機(jī)械拋光可使金剛石達(dá)到高表面質(zhì)量，但設(shè)備相對復(fù)雜，無法滿足大規(guī)模生產(chǎn)的需求，需要進(jìn)一步研究和優(yōu)化。此外，本文還對化學(xué)機(jī)械拋光的未來發(fā)展進(jìn)行了預(yù)測，為相關(guān)領(lǐng)域研究人員提供參考。

       論文題錄●●

       安康, 許光宇, 吳海平, 張亞琛, 張永康, 李利軍, 李鴻, 張旭芳, 劉峰斌, 李成明. 金剛石化學(xué)機(jī)械拋光研究進(jìn)展[J]. 人工晶體學(xué)報(bào), 2024, 53(10): 1675-1687.

AN Kang, XU Guangyu, WU Haiping, ZHANG Yachen, ZHANG Yongkang, LI Lijun, LI Hong, ZHANG Xufang, LIU Fengbin, LI Chengming. Research Progress in Chemical Mechanical Polishing of Diamond[J]. Journal of Synthetic Crystals, 2024, 53(10): 1675-1687.

       章節(jié)結(jié)構(gòu)

       0　引言

       1　金剛石的拋光方法

       2　金剛石的化學(xué)機(jī)械拋光

       2.1　早期化學(xué)機(jī)械拋光

       2.2　H2O2及其混合物化學(xué)機(jī)械拋光

       2.3　光催化輔助化學(xué)機(jī)械拋光

       3　金剛石化學(xué)機(jī)械拋光的原子級(jí)去除機(jī)理研究

       4　結(jié)語與展望

       文章導(dǎo)讀

       當(dāng)前，金剛石的表面拋光技術(shù)主要有機(jī)械拋光、熱化學(xué)拋光、激光拋光（laser polishing, LP）、離子束拋光（ion beam polishing, IBP）、等離子體輔助拋光（plasma assisted polishing, PAP）和CMP等。

機(jī)械拋光是最常用的金剛石拋光方法，拋光示意圖如圖1所示。拋光時(shí)，含有金剛石粉的拋光盤高速旋轉(zhuǎn)，通過樣品臺(tái)向下施加壓力，實(shí)現(xiàn)材料的去除。機(jī)械拋光會(huì)造成金剛石工件的表面損傷和亞表面損傷，拋光過程中的機(jī)械沖擊會(huì)導(dǎo)致拋光表面形成凹坑、亞表面裂紋和晶格損傷，亞表面裂紋和晶格損傷無法通過后續(xù)的拋光步驟消除，且難以被光學(xué)設(shè)備檢測。

圖1　機(jī)械拋光裝置示意圖

       熱化學(xué)拋光是以碳原子在熱金屬中的擴(kuò)散、金剛石轉(zhuǎn)化為石墨和金剛石的氧化為基礎(chǔ)的拋光技術(shù)；熱化學(xué)拋光時(shí)，金剛石膜在真空、氫氣或惰性氣體氣氛下，在加熱到750~1000 ℃的熱鐵光盤上轉(zhuǎn)動(dòng)摩擦，在高溫條件下，通過碳原子向鐵質(zhì)拋光盤擴(kuò)散來實(shí)現(xiàn)金剛石膜的平整化；激光拋光通過激光束照射到金剛石厚膜表面，使金剛石膜表面溫度升高，進(jìn)而使被加熱的金剛石表面碳原子氣化和石墨化，達(dá)到去除材料的目的；離子束拋光利用氧或具有較大濺射率的惰性氣體（Ar）離子，對金剛石膜進(jìn)行濺射刻蝕，可實(shí)現(xiàn)精細(xì)化拋光；等離子體輔助拋光同樣用于金剛石精細(xì)化拋光，可提高金剛石表面光潔度，Yamamura等[41]采用PAP技術(shù)對CVD單晶金剛石進(jìn)行拋光，先使用含水蒸氣的氬基等離子體對拋光板和單晶金剛石(100)表面進(jìn)行改性，后在10~52.6 kPa的拋光壓力下對單晶金剛石進(jìn)行拋光，結(jié)果表明，拋光后的表面粗糙度為0.13 nm，如圖2所示。

圖2　等離子體輔助拋光后的單晶金剛石(100)面AFM照片[41]

       激光拋光、離子束拋光和等離子體輔助拋光在粗拋和精拋領(lǐng)域有著各自的優(yōu)勢，但設(shè)備的使用成本明顯高于其他拋光方法。常見金剛石拋光特點(diǎn)如表1所示。

表1　常見金剛石拋光方法特點(diǎn)

       化學(xué)機(jī)械拋光（見圖3）是一種超精密拋光的加工方法，通過在機(jī)械拋光過程中加入氧化劑，氧化碳原子提高拋光速率，并且具有表面損傷小、粗糙度低、設(shè)備簡單、運(yùn)行維護(hù)成本低，拋光后的表面污染較輕等優(yōu)點(diǎn)，在金剛石拋光領(lǐng)域逐漸受到重視。

圖3　化學(xué)機(jī)械拋光裝置示意圖[43]

       早期化學(xué)機(jī)械拋光以高溫熔融鹽作為氧化劑進(jìn)行拋光。Yuan等[49]為了降低環(huán)境污染，選用K2FeO4作為氧化劑，在化學(xué)機(jī)械拋光前用不同尺寸的金剛石粉進(jìn)行機(jī)械研磨，使CVD金剛石膜的表面粗糙度降至1.042 nm，之后通過在K2FeO4加入金剛石、B4C、SiC、Al2O3等四種磨料配制拋光液，用H3PO4和NaOH來調(diào)節(jié)拋光液的pH值，拋光溫度為50 ℃。拋光結(jié)果表明，獲得的金剛石表面粗糙度為0.478 nm，無表面劃痕或凹坑，如圖4所示。

圖4　金剛石表面光學(xué)圖像[49]。(a)機(jī)械拋光；(b)化學(xué)機(jī)械拋光

       為了驗(yàn)證不同種類的氧化劑對化學(xué)機(jī)械拋光的影響，Yuan等[50]開始進(jìn)行對比試驗(yàn)，用金剛石粉進(jìn)行機(jī)械拋光后，表面粗糙度下降到48 nm，再用K2FeO4、KIO4、KMnO4和K2CrO7等不同氧化劑在50 ℃的條件下，進(jìn)行10組化學(xué)機(jī)械拋光對比試驗(yàn)，拋光機(jī)理如圖5所示，其中在化學(xué)機(jī)械拋光過程中使用K2FeO4拋光出的金剛石表面粗糙度為8.72 nm，具有最佳的表面質(zhì)量。

圖5　化學(xué)機(jī)械拋光機(jī)理示意圖[50]

       H2O2是一種強(qiáng)氧化劑，使用H2O2溶液作為拋光液，在室溫下進(jìn)行化學(xué)機(jī)械拋光后，可得到原子級(jí)光滑的表面。Kubota等[54]隨后改進(jìn)了工藝，通過在H2O2溶液中用鐵板對金剛石進(jìn)行化學(xué)機(jī)械拋光，設(shè)置工作臺(tái)和樣品臺(tái)的轉(zhuǎn)速均為40 r/min，壓力為3.3 MPa，拋光15 h后，在3 μm×3 μm的面積上，得到Ra=0.784 nm，RMS=1.076 nm的表面，而在500 nm×500 nm的面積上可得到表面粗糙度為0.170 nm，RMS=0.310 nm。反應(yīng)機(jī)理如圖6所示，在Fe2+的催化下，H2O2會(huì)分解成具有強(qiáng)氧化性的·OH，在金剛石表面形成C=O和C—OH等化學(xué)鍵，磨料會(huì)在壓力的作用下與金剛石表面的C=O和C—OH結(jié)合，在剪切力作用下，金剛石表面的C—C鍵會(huì)發(fā)生斷裂，從而實(shí)現(xiàn)C原子去除。

圖6　基于Fenton反應(yīng)的化學(xué)機(jī)械拋光過程中碳原子的去除機(jī)理示意圖[55]

       為了驗(yàn)證Fe2+對拋光的影響，Yuan等[57]采用機(jī)械研磨和化學(xué)機(jī)械拋光相結(jié)合的方法，利用磨料顆粒和過渡金屬離子進(jìn)行室溫拋光。先進(jìn)行機(jī)械研磨，得到粗糙度約為0.2 μm的金剛石表面。配制質(zhì)量分?jǐn)?shù)為30%的H2O2溶液100 g、去離子水100 g、W0.5金剛石粉10 g、FeSO4水溶液100 g的拋光液，用于化學(xué)機(jī)械拋光處理，拋光時(shí)間為3 h，獲得表面粗糙度0.452 nm的超精密光滑金剛石表面。通過對比實(shí)驗(yàn)，發(fā)現(xiàn)相同條件下不含F(xiàn)e2+的拋光液拋光出的金剛石表面粗糙度為0.741 nm，證明了Fe2+的存在增強(qiáng)了拋光效果，如圖7所示。

圖7　金剛石表面顯微干涉圖像[57]

       Yuan等[55]對比了幾種基于Fenton反應(yīng)的拋光液對金剛石化學(xué)機(jī)械拋光的影響，分別是FeSO4+H2O2、Fe2(SO4)3+H2O2和Fe·OH+H2O2，結(jié)果表明用Fe2(SO4)3+H2O2試劑拋光金剛石，在868 μm×868 μm范圍內(nèi)，可得到最低的表面粗糙度0.076 nm，去除率最高可達(dá)752 nm/h。Fe2(SO4)3+H2O2作為拋光劑拋光效果最好的原因是H2O2被快速消耗，金剛石不能被完全氧化，而Fe3+需要消耗H2O2生成Fe2+，然后生成·OH，反應(yīng)速率較慢，因此能夠?qū)饎偸L時(shí)間氧化（見圖8）。

圖8　氧化前、后金剛石表面SEM照片[55]

       金剛石的帶隙能為5.45 eV，可以在波長小于225 nm的紫外照射下激發(fā)產(chǎn)生空穴和電子對，并立即與大氣中的氧和水分子結(jié)合，成鍵反應(yīng)產(chǎn)生大量的O原子和·OH，使金剛石表面氧化。研究人員基于這一理論，提出了光催化輔助化學(xué)機(jī)械拋光（photocatalytic assisted chemical mechanical polishing, PCMP）法，Anan等[60]用紫外光（UV）輻照拋光單晶金剛石（見圖9），用石英拋光盤對Ib型單晶金剛石進(jìn)行拋光，紫外光可以透過石英拋光盤照射在金剛石表面。拋光前在235 nm×309 nm范圍內(nèi)樣品的表面粗糙度為1.35 nm，經(jīng)過2 h的UV拋光，樣品表面粗糙度達(dá)到0.19 nm，而非UV照射拋光的金剛石表面粗糙度僅為0.74 nm，并在單晶金剛石的(100)面和(110)面均證實(shí)了紫外輻照的有效性。

圖9　高速水平主軸紫外線拋光機(jī)[60]

       在這一基礎(chǔ)上，有學(xué)者將光催化輔助CMP與Fenton反應(yīng)相結(jié)合，用于提高金剛石表面質(zhì)量，Yang等[63]以納米金剛石（nanodiamond, ND）和CuFe層狀雙氫氧化物（layered double hydroxide, LDH）為原料制備了用于可見光-Fenton反應(yīng)的催化劑，將質(zhì)量分?jǐn)?shù)為20%的H2O2溶液混合ND/LDH催化劑作為拋光液，在拋光過程中開啟可見光源，ND/LDH催化劑在可見光照射下，發(fā)生Fenton反應(yīng)產(chǎn)生具有強(qiáng)氧化性的·OH，單晶金剛石表面的碳原子經(jīng)過·OH氧化，再經(jīng)過機(jī)械作用的處理，得到光滑的表面，實(shí)驗(yàn)裝置如圖10所示。

圖10　實(shí)驗(yàn)裝置示意圖[63]

       納米TiO2在受到一定波長（387.5 nm）的紫外光照射后，處于價(jià)帶的電子就會(huì)被激發(fā)躍遷到導(dǎo)帶上，同時(shí)在價(jià)帶上產(chǎn)生帶正電的空穴，空穴可以將吸附在TiO2顆粒表面的OH-和H2O氧化，生成具有強(qiáng)氧化性的·OH，原理如圖11所示。

圖11　TiO2紫外光催化反應(yīng)的原理[66]

       光催化輔助化學(xué)機(jī)械拋光可提高金剛石表面質(zhì)量，達(dá)到納米級(jí)粗糙度。但相比傳統(tǒng)的化學(xué)機(jī)械拋光技術(shù)，設(shè)備復(fù)雜度較高，無法滿足大規(guī)模生產(chǎn)的需求，需要進(jìn)一步地研究和優(yōu)化，以提高其實(shí)際應(yīng)用能力。表2對化學(xué)機(jī)械拋光常用氧化劑成分進(jìn)行了總結(jié)。

表2　文獻(xiàn)中化學(xué)機(jī)械拋光常用氧化劑成分總結(jié)

       分子動(dòng)力學(xué)（molecular dynamics, MD）模擬可以通過高時(shí)間和空間分辨率可視化材料去除的細(xì)節(jié)，是一種適合研究原子級(jí)材料去除機(jī)理的方法。Yang等[77]和Liu等[78]均采用分子動(dòng)力學(xué)方法研究了金剛石在金剛石(100)面上摩擦滑動(dòng)時(shí)，金剛石晶體表層非晶態(tài)碳的形成和演化過程，模型如圖12所示，模擬結(jié)果表明，在摩擦作用下，金剛石sp3雜化碳向sp2雜化發(fā)生轉(zhuǎn)變，沿[100]方向滑動(dòng)的非晶化速率大于沿[110]方向滑動(dòng)的非晶化速率。

圖12　金剛石晶體機(jī)械拋光MD模型[78]

       Guo等[58]用分子動(dòng)力學(xué)模擬研究了SiO2磨料在H2O2/純H2O水溶液中化學(xué)機(jī)械拋光金剛石過程中的原子去除機(jī)理，模型如圖13所示。金剛石表面的C與·OH、O和H結(jié)合成為C—OH、C—O、C—H鍵，在機(jī)械拋光的作用下，金剛石表面的C原子以CO、CO2或C鏈的形式被快速去除，從原子尺度上解釋了化學(xué)機(jī)械拋光過程中C原子的去除過程，并進(jìn)一步研究壓力對材料去除率的影響，發(fā)現(xiàn)壓力越大金剛石表面吸附的·OH越多，越有利于C原子的去除。

圖13　SiO2磨料在H2O2/純H2O水溶液中拋光過程MD示意圖[58]

       結(jié)語與展望

       當(dāng)前金剛石正以每年數(shù)億美元的市場規(guī)模擴(kuò)大應(yīng)用范圍，表面質(zhì)量是影響其應(yīng)用的重要因素。已有多種拋光技術(shù)應(yīng)用于金剛石平整化過程，本綜述對其優(yōu)缺點(diǎn)進(jìn)行了總結(jié)分析?；瘜W(xué)機(jī)械拋光具有較高去除率、高表面質(zhì)量、低加工成本等優(yōu)勢，是一種高效的拋光方法，尤其是H2O2及相關(guān)加工方法的使用，不僅使金剛石表面粗糙度達(dá)到亞納米級(jí)，可以獲得超光滑且低損傷的表面，而且降低了化學(xué)污染。未來，實(shí)現(xiàn)金剛石大面積、無亞表面損傷的拋光依舊是其在半導(dǎo)體、熱沉等領(lǐng)域獲得應(yīng)用的重要基礎(chǔ)。

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