李玉瑤,張婉怡,劉 喆,李美萱,付秀華,2
(1.長(zhǎng)春理工大學(xué)光電信息學(xué)院,長(zhǎng)春 130012;2.長(zhǎng)春理工大學(xué) 光電工程學(xué)院,長(zhǎng)春 130022)
S-on-1測(cè)量方式下薄膜激光損傷的累積效應(yīng)
李玉瑤1,張婉怡1,劉 喆1,李美萱1,付秀華1,2
(1.長(zhǎng)春理工大學(xué)光電信息學(xué)院,長(zhǎng)春 130012;2.長(zhǎng)春理工大學(xué) 光電工程學(xué)院,長(zhǎng)春 130022)
為了研究薄膜激光損傷機(jī)理及影響因素,基于平頂光束輻照測(cè)量的原理,采用1064nm的Nd∶YAG激光器,對(duì)電子束熱蒸發(fā)方式鍍制的HfO2薄膜在重復(fù)頻率激光作用下?lián)p傷的累積效應(yīng)進(jìn)行了理論分析和實(shí)驗(yàn)驗(yàn)證。運(yùn)用損傷閾值的測(cè)量原理,分析了1-on-1和S-on-1兩種測(cè)量方式的特點(diǎn),并分別開展了測(cè)量實(shí)驗(yàn)。采用二分法查找輻照激光能量,每個(gè)能量密度輻照20個(gè)測(cè)試點(diǎn),應(yīng)用零幾率損傷閾值和最小二乘法擬合確定測(cè)量結(jié)果。結(jié)果表明,對(duì)同種薄膜,1-on-1測(cè)量方式測(cè)得的損傷閾值為15.75J/cm2,S-on-1測(cè)量方式測(cè)得的損傷閾值為11.90J/cm2;從損傷閾值與損傷形貌兩方面的對(duì)比表明,S-on-1測(cè)量方式體現(xiàn)了典型的累積效應(yīng)。此結(jié)果對(duì)深入研究薄膜激光損傷的機(jī)理和影響因素具有重要意義。
薄膜;損傷閾值;測(cè)量方式;累積效應(yīng);二分查找;最小二乘法
隨著激光技術(shù)的發(fā)展,激光對(duì)薄膜的損傷成為制約其向高功率、高能量發(fā)展的“瓶頸”,也成為影響高功率激光薄膜元件可靠性、穩(wěn)定性和使用壽命的主要因素[1-4]。目前,國(guó)內(nèi)外對(duì)薄膜損傷的研究中,大多采用中心強(qiáng)度高、邊緣相對(duì)較弱的高斯光束輻照薄膜表面,光斑中心位置易導(dǎo)致薄膜的損傷,損傷部分的面積小于激光光斑面積,測(cè)量的損傷閾值比實(shí)際的閾值偏大[5-6]。從激光的角度而言,激光的波長(zhǎng)、脈沖寬度、能量密度與功率密度、光斑面積和偏振態(tài)等都對(duì)測(cè)量結(jié)果有影響[7-9];從薄膜本身的角度而言,膜層的材料、厚度、工藝、半波覆蓋層和膜層后處理等也對(duì)測(cè)量結(jié)果有影響[10-11]。
通常,薄膜激光損傷閾值與輻照的激光脈沖個(gè)數(shù)有關(guān),脈沖個(gè)數(shù)越多損傷閾值降低[12],多脈沖激光對(duì)薄膜的損傷可以解釋為熱累計(jì)損傷或非線性吸收損傷。目前,國(guó)內(nèi)外對(duì)激光薄膜損傷累積效應(yīng)的研究均基于高斯光束輻照測(cè)量,會(huì)導(dǎo)致?lián)p傷閾值的測(cè)量結(jié)果偏低[13-14]。本文中基于平頂光束輻照測(cè)量,研究累積效應(yīng)與作用次數(shù)的關(guān)系,采用1064nm的Nd∶YAG激光束對(duì)采用電子束熱蒸發(fā)方式鍍制的HfO2薄膜分別進(jìn)行1-on-1測(cè)量方式和S-on-1測(cè)量方式的測(cè)量實(shí)驗(yàn),分析薄膜激光的損傷機(jī)理。
圖1為薄膜激光損傷測(cè)量原理圖。激光束先通過能量調(diào)節(jié)裝置,再經(jīng)整形光學(xué)系統(tǒng)由高斯光束變換為平頂光束,然后經(jīng)聚焦光學(xué)系統(tǒng)后被分光鏡A以一定的分光比分成兩部分,一部分反射到被測(cè)樣品的表面,通過數(shù)碼顯微系統(tǒng)成像判識(shí)薄膜的損傷;另一部分透射到分光鏡B,再按照一定的分光比將光束分成兩部分,反射部分入射到能量探測(cè)器,計(jì)算出輻照到被測(cè)樣品表面的激光脈沖能量,透射部分入射到CCD探測(cè)面上,計(jì)算出輻照到被測(cè)樣品表面的光斑面積。根據(jù)激光脈沖能量和光斑面積計(jì)算出能量密度。應(yīng)用最小二乘法對(duì)不同輻照能量密度下樣品的損傷幾率進(jìn)行擬合,最終獲得樣品的激光損傷閾值。
Fig.1 Figure of measurement principle
Fig.2 Measurement mode
本文中對(duì)被測(cè)樣品分別進(jìn)行1-on-1和S-on-1兩種測(cè)量方式的實(shí)驗(yàn)。
1-on-1測(cè)量又稱為單脈沖損傷測(cè)量,是指同一能量密度的激光輻照在樣品上的多個(gè)測(cè)試點(diǎn),然后統(tǒng)計(jì)發(fā)生損傷的測(cè)試點(diǎn)個(gè)數(shù),計(jì)算損傷幾率。改變激光能量密度,再測(cè)出損傷幾率,根據(jù)零幾率損傷閾值獲得方法求取該樣品的損傷閾值[15]。采用1-on-1測(cè)量方式時(shí),每個(gè)測(cè)試點(diǎn)僅輻照一次,無論是否發(fā)生損傷,都移至下一個(gè)未被輻照的測(cè)試點(diǎn),如圖2a所示。
S-on-1測(cè)量又稱為多脈沖損傷測(cè)量,是重復(fù)激光脈沖能量輻照在樣品表面的累計(jì)損傷效應(yīng),即同一能量密度的多個(gè)脈沖輻照在同一測(cè)試點(diǎn)上進(jìn)行測(cè)量[16-17],如圖2b所示。
采用不同能量密度的激光對(duì)樣品進(jìn)行單脈沖輻照測(cè)量,每個(gè)能量密度輻照20個(gè)間距為2mm的測(cè)試點(diǎn),每個(gè)測(cè)試點(diǎn)僅輻照一次,通過控制樣品工作臺(tái)平移實(shí)現(xiàn)測(cè)試點(diǎn)的移動(dòng)。采用二分法查找輻照激光能量,找到損傷幾率非0%非100%的測(cè)量點(diǎn),然后按照能量遞增或遞減的順序進(jìn)行輻照測(cè)量,應(yīng)用零幾率損傷閾值確定測(cè)量結(jié)果。
在與被測(cè)薄膜表面等效的位置測(cè)得輻照在薄膜表面的光斑直徑為825μm,采用輻照激光能量密度計(jì)算軟件計(jì)算得到光斑面積為5.35×10-3cm2,并計(jì)算出每次輻照的激光能量密度。表1中列出了輻照的不同激光能量密度及其對(duì)應(yīng)的損傷幾率,*表示擬合可用數(shù)據(jù)。
Table 1 Measurement data of 1-on-1 measurement mode
應(yīng)用MATLAB對(duì)以上測(cè)量數(shù)據(jù)采用最小二乘法進(jìn)行擬合[18],如圖3所示。擬合得到其損傷閾值為15.75J/cm2。
Fig.3 Measurement results of 1-on-1 measurement mode
采用不同能量密度的激光對(duì)樣品進(jìn)行多脈沖輻照測(cè)量,每個(gè)能量密度輻照20個(gè)間距為2mm的測(cè)試點(diǎn),每個(gè)測(cè)試點(diǎn)輻照1000次,若輻照次數(shù)未達(dá)到1000次就已判定損傷發(fā)生,則通過控制2維運(yùn)動(dòng)工作臺(tái)的移動(dòng),對(duì)下一個(gè)測(cè)試點(diǎn)進(jìn)行多脈沖輻照測(cè)量。采用二分法查找輻照激光能量,找到損傷幾率非0%、非100%的測(cè)量點(diǎn),然后按照能量遞增或遞減的順序進(jìn)行輻照測(cè)量,應(yīng)用零幾率損傷閾值確定測(cè)量結(jié)果。
在與被測(cè)薄膜表面等效的位置測(cè)得輻照在薄膜表面的光斑直徑為890μm,采用輻照激光能量密度計(jì)算軟件計(jì)算得到光斑面積為6.22×10-3cm2,并計(jì)算出每次輻照的激光能量密度。表2中列出了輻照的不同激光能量密度及其對(duì)應(yīng)的損傷幾率,*表示擬合可用數(shù)據(jù)。
Table 2 Measurement data of S-on-1 measurement mode
應(yīng)用MATLAB對(duì)以上測(cè)量數(shù)據(jù)采用最小二乘法進(jìn)行擬合,如圖4所示。擬合得到其損傷閾值為11.90J/cm2。
圖5中為采用1-on-1測(cè)量方式且能量密度為22.75J/cm2時(shí)的損傷形貌,由于輻照激光為平頂光束,因此膜層表面除缺陷部分外,損傷區(qū)域與光斑輻照區(qū)域基本符合。
Fig.4 Measurement results of S-on-1 measurement mode
Fig.5 Damage morphology of 1-on-1 measurement mode
采用S-on-1測(cè)量方式能量密度為14.5J/cm2時(shí),脈沖個(gè)數(shù)分別為1,10,100和1000時(shí)的損傷形貌如圖6所示。隨著脈沖作用次數(shù)的增加,損傷越來越嚴(yán)重,尤其在缺陷存在區(qū)域首先發(fā)生損傷。
Fig.6 Damage morphology of S-on-1 measurement mode
通過以上測(cè)量實(shí)驗(yàn)可知,采用S-on-1測(cè)量方式測(cè)得的損傷閾值比1-on-1測(cè)量方式大,S-on-1測(cè)量方式采用同一激光能量密度對(duì)同一測(cè)試點(diǎn)進(jìn)行多脈沖輻照測(cè)量,隨著脈沖次數(shù)的增加損傷越來越嚴(yán)重,體現(xiàn)了典型的累積效應(yīng)。
系統(tǒng)的測(cè)量誤差主要來源于激光輻照能量的測(cè)量誤差、光斑面積的測(cè)量誤差、測(cè)量結(jié)果的擬合誤差和損傷判識(shí)誤差等。
該誤差主要包括:兩塊分光鏡的分光誤差為1%;能量探測(cè)器的分辨率為1μJ;平頂光束的平頂度為93.66%;薄膜表面接收激光輻照,將吸收的激光能量轉(zhuǎn)化成熱能,導(dǎo)致薄膜表面溫度升高,能量探測(cè)器測(cè)量的激光能量大于薄膜實(shí)際損傷所需能量,導(dǎo)致測(cè)量誤差的產(chǎn)生。
光斑面積的測(cè)量是在樣品表面的等效位置進(jìn)行的,因此CCD靶面與樣品表面位置等效的偏離是誤差的主要來源。為了解決這一問題,在設(shè)計(jì)聚焦光學(xué)系統(tǒng)時(shí),采用長(zhǎng)焦距結(jié)構(gòu)以增大系統(tǒng)的焦深,使CCD靶面在一定的調(diào)節(jié)范圍內(nèi)測(cè)得的光斑與樣品表面光斑的大小相近。焦深的表達(dá)式為:
式中,λ為工作波長(zhǎng),f′為系統(tǒng)焦距,n′為像空間介質(zhì)折射率,Um′為像方最大孔徑角,D為系統(tǒng)入瞳直徑。對(duì)于一定波長(zhǎng)的激光,當(dāng)入射口徑一定時(shí),焦深近似與焦距的平方成正比。
從CCD角度來說,引起光斑面積測(cè)量誤差的主要因素有CCD本身的分辨率誤差、CCD的積分誤差以及CCD的背景噪聲。
采用最小二乘法進(jìn)行曲線擬合時(shí)會(huì)產(chǎn)生一定的擬合誤差,同時(shí)測(cè)量時(shí)選取能量等級(jí)的多少對(duì)擬合精度產(chǎn)生一定影響。為了減小擬合誤差,盡量多選擇測(cè)量激光能量等級(jí),尤其是損傷幾率為20%~60%之間的能量對(duì)擬合結(jié)果影響最大。
基于平頂激光束輻照測(cè)量,采用1-on-1和S-on-1兩種不同測(cè)量方式對(duì)電子束熱蒸發(fā)方式鍍制的HfO2薄膜激光損傷進(jìn)行了研究,從損傷閾值與損傷形貌兩方面驗(yàn)證了S-on-1測(cè)量方式體現(xiàn)了典型的累積效應(yīng),最后從激光輻照能量測(cè)量、光斑面積測(cè)量和測(cè)量結(jié)果擬合等方面進(jìn)行了誤差分析。
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CumulativeeffectofthinfilmlaserdamageunderS-on-1measurementmode
LIYuyao1,ZHANGWanyi1,LIUZhe1,LIMeixuan1,F(xiàn)UXiuhua1,2
(1.College of Optical and Electronical Information Changchun University of Science and Technology, Changchun 130012, China; 2.College of Opto-electronics Engineering, Changchun University of Science and Technology, Changchun 130022,China)
In order to study the damage mechanism and influencing factors of laser thin films, the principle of radiation measurement based on flat top beam was proposed. The cumulative damage effect of HfO2films processed by electron beam thermal evaporation under the repeated frequency laser irradiation was studied by using 1064nm Nd∶YAG laser. After theoretical analysis and experimental verification, the characteristics of two measuring methods, 1-on-1 andS-on-1, were analyzed based on the measuring principle of damage threshold. The irradiation laser energy was seeked by dichotomy. Each energy density was irradiated at 20 test points. Zero probability damage threshold and least square method were used to fit and determine the measurement results. The results show that for the same kind of film, the damage threshold measured by 1-on-1 method is 15.75J/cm2and the damage threshold measured byS-on-1 method is 11.90J/cm2. The comparison of damage threshold and damage morphology shows thatS-on-1 measurement method reflects the typical cumulative effect. The study is of great importance for the study of laser damage mechanism and influencing factors of thin film.
thin films;damage threshold;measurement mode;cumulative effect;binary search;least square method
1001-3806(2018)01-0039-04
吉林省科技支撐計(jì)劃資助項(xiàng)目(20150204047GX);吉林省教育廳“十三五”科學(xué)技術(shù)研究資助項(xiàng)目(2015578)
李玉瑤(1982-),女,博士,主要從事光學(xué)設(shè)計(jì)、光學(xué)儀器及激光測(cè)試等方面的研究。
E-mail:liyuyao1983@163.com
2017-02-13;
2017-03-17
TN247;O484.4+1
A
10.7510/jgjs.issn.1001-3806.2018.01.008