Silisyumun anodik oksidasyonu ve katkı profili belirlenmesi
Anodic oxidation of silicon and determining the impurity profile
- Tez No: 14424
- Danışmanlar: PROF.DR. DURAN LEBLEBİCİ
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1991
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 94
Özet
ÖZET Katkı yoğunluğunun derinlikle değişimi, başka bir deyişle katkı profili, elektronik elemanların gerek Üre tilmeleri gerekse kullanılmaları aşamasında, eleman özel liklerinin iyileştirilebilmesi ve modellemenin gelişti rilebilmesi için, bilinmelidir. Günümüzde çeşitli pro fil çıkarma yöntemleri kullanılmaktadır. Bunlardan ba zıları kolay ve çabuk sonuç vermelerine rağmen çok paha lıdırlar. Silisyumu ince tabakalar halinde aşındırdık tan sonra tabaka direnci ölçümlerinden katkı yoğunluğu değerlerini belirleyerek profil çıkarmak, daha once ya pılan çalışmalarda da belirlendiği gibi, doğru sonuçlar veren ucuz ve kolay bir yöntemdir. Silisyumu çok ince tabakalar halinde dilimlemek, ancak üzerinde oksit büyü tüldükten sonra oksidin aşındın İması yi a kolayca mumkUn olabilmektedir. Burada temel alınan, oksit kalınlığının 0,44'u kadar bir silisyum tabakasının büyüyen oksidin o- luşumunu için harcanması dır. Böylece her oksitleme a- dımında oksidin 0,44'u kalınlığında bir silisyum tabaka sı da aşındırılmış olur. Profilin değişmemesi için ok sitleme işlemi, katkı iyonlarının difUze olamayacakları kadar duşuk sıcaklıklarda yapılmalıdır. Bu koşulların sağlandığı en uygun oksitleme yöntemi anodi zasyondur. Anodik oksitleme, tekrar edilebilirliği iyi, yüzey duz- gUnlUğU ve tabaka kalınlığı homojenliği adımlar ilerle dikçe bozulmayan, kristal yöneliminden ve katkılamadan bağımsız bir yöntemdir. Anodik oksidasyonla SO angst- romler mertebesinde bile silisyum tabakaları aşındırmak mumkUn olduğundan, yüzeyden itibaren, istenilen kalın lıkta tabakalar aşındırılarak jonksiyon düzlemine kadar kolayca inilebilir. Anodik oksidasyon sabit akımla veya sabit geri limle yapılabilir. BUyuyen oksit kalınlığının zamanla lineer olarak artması sonucu kalınlık kontrolü kolaylık la gerçekleştirilebildiğinden, bu çalışmada, sabit geri limle anodi zasyondan daha kullanışlı olan sabit akım a- nodizasyonu tercih edilmiştir. Büyüyen oksit aşındırıl dıktan sonra 4-nokta probuyla tabaka direnci ölçülerek elde edilen değerler jonksiyona yaklaşıldıkça arttığın dan bu yöntem“Artan Tabaka Direnci ölçümleri Yöntemi”olarak adlandırılır. Bu yöntemde profil dört aşamada çıkarılır: Anodik oksidasyon, oksidin aşı ndırıl ması, ta baka direnci ölçülmesi ve bu direnç değerlerinden katkı yoğunluğu hesaplanması. Profil, bu dört aşama her adım da tekrar edilerek çıkarılmıştır. CvD
Özet (Çeviri)
SUMMARY ANODIC OXIDATION OF SILICON AND DETERMINING THE IMPURITY PROFILE Electrical properties of semicoductor devices are effected primarily distribution of doping concen tration with depth. As known, doping concentration is dominant on the mobility of electron and holes. Quan tity of impurity ions in bulk can be calculated easily from doping concentration. Determination of the doping profile of a semiconductor device provides to handle better electrical properties and more accurate and con venient models of the device. Distribution of impurity ions from the surface of the silicon into the depth of the bulk can be deter mined using many different methods. Some of these are RBS CRutherford BackscatteringD, SIMS CSecondary Ion Mass Spectroscopy!), SR CSpreading Resistance!) and C-V C Capacitance-Voltage} measurements. These methods are used for sample profiling easily and fast, but, some of them have disadvantages; for example, SIMS, RBS and SR need very expensive softwares and when C-V measurements technique is used at once, a thin layer at silicon sur face have to be sucrified. In addition, some of them have limited profiling range and resolution, for ins tance, SR measurement method can be used in profiling for only the deep junctions, because of 0,1 fjm resolu tion, it has. Advantages and disadvantages of and a brief information about these methods are given in in terested section. In this work, Incremental Sheet Re sistivity Measurements Method is used for profiling do ped silicon. This method consists of repeated consu ming of silicon by removing the oxide grown by anodic oxidation, etching and measuring of the sheet resistan ce using a 4-point probe after each etching step. In the second chapter, oxidation of silicon is mentioned. Mostly used oxidation methods, measuring of oxide thickness and usage of silicon dioxide in semi conductor technology are briefly described. By the in formation given here, anodic oxidation can be compared with the other oxide growing methods in some respects. Cvi!)As can be seen silicon dioxide is in common use in in tegrated circuit technology. Sectioning of silicon by anodic oxidation for determining the impurity profile is the cheapest and simplest method explained in the third chapter. As known, 44 percent of the whole silicon dioxide thick ness is silicon. At the end of the removing process of silicon dioxide, original silicon surface at the be ginning of the oxidation is shifted down nearly 44 per cent of the oxide thickness. This percentage is vari ous i n anodi c oxi dati on depend! ng on some condi ti ons, for example, hardly, water concentration in electrolyte and electrolyte type. However, as written in some pa pers, 0,44 of the oxide thickness is silicon in an ano di zati on process when a special concentration of an ethylene glycol solution is used as electrolyte. This oxidation and etching procedure is done over and over again and at the end of each removal, sheet resistance of the doped silicon layer is measured then transformed into the doping concentration by calculation. In this method, accuracy and resolution can be made better than the others. It is very easy to grow oxide having mini mum resolution about SO angstrom by anodic oxidation. In the other oxide growing methods the thickness cont rol is not easy as much as anodic oxidation. Moreover, as mentioned in this chapter, oxidation process have to be done at room temperature because the impurity profi le of the sample changes at high temperatures by the time with respect to diffusion mechanism. At this po int it can be seen that the anodic oxidation is the most proper and practical method. Moreover, the anodic oxidation is independent of er i stal orientation and im purity concentration. Surface flatness and thickness uniformity of consuming layer for extracting impurity concentration with depth and reproduceability are not decayed by increasing the number of the steps. Another adventage of the anodi zati on is that, controlling of growing oxide thickness can be easily done after making the calibration of the system. It has only one di sad- ventage that if it is desired to make resolution high, the number of reproduc-able steps increases dramatically with depth which will be profiled. However, this method gives more satisfying results than the others. Studies about anodic oxidation of silicon, that were done up till today, exposed that the constant cur rent anodi zati on is more preferable than the constant voltage anodi zati on. Because in this method, oxide thickness increases linearly with time as a result of easy controllability. In this work, the constant cur rent anodi zati on is examined and so the thickness is controlled efficiently. CviiDIn the third chapter, properties of anodic oxi des, usage of anodic oxidation in semiconductor techno logy and growing mechanism of anodic oxidation are exp lained, a comparison of anodic and thermal oxides and a brief discussion about parameters which influence anodization are also given by the side of the anodic oxidation of silicon. Costant current and constant voltage anodization are described. In constant current anodization, electric field on the growing oxide remains constant during the anodization. It means that incre ase an amount of dV in potential accompanies to an amo unt of the dx increment in thickness. For that reason the thickness -vol t age curves in constant current anodi zation is linear. In costant voltage anodization cur rent effiency, current density and electric field on the oxide are functions of the time. Anodic oxides can be comparable to the thermal oxides by anneal ling. Usually these oxides are not stoichiometric, means that one silicon atom does not react two oxygen atoms exactly. This ratio is general ly about 1,8 in anodic oxidation. That's why the ano dic oxides are more amorphous than the thermal oxides and density of the anodic oxides are lower than the ot hers because of the porous structure of them. Electri cal isolation properties of anodic oxides are also wor se than the thermal oxides. Although it has these bad properties it is used in many different areas of semi conductor technology for isolation in bipolar applica tions, converting of silicon nitride films into the si licon dioxide, stopping the etching of micromechanic membrans, as a doping source, etc.. Silicon has to has two holes for oxidizing in an oxidant electrlyte. Therefore two oxygen atoms can set up two bonds with silicon easily and a silicon di oxide molecule is created. Oxidation is carried out by the movements of the anions from the cathode to the a- node. Oxygen which oxidize the silicon comes from hyd- roxyl ions and these hydroxyl ions come from water in the electrolyte. The basic reactions include hydroxide can be expressed as follows; Si + 3h+> Si2“*”C1D Si2* + 3COH D> SiCOHD C3D 2 SiCOHD> SiO + H C3D 2 2 2 The whole chemical reaction of the anodic oxidation is written as; CviiiDSi + 2h2+ + 2H O>SiO + 2H+ + H C4Z> 2 2 2 In the fourth chapter, impurity profiling met-hods are explained mentioned above. Incremental sheet re sistivity measurements are defined clearly here, four point probe, sheet resistivity and transform into do ping profile by calculation of concentration values us ing sheet resistivities are described. An expression is found for calculation of the concentration values; pCxD = 0.4343 R CxDC5} ^ s d log R CxD s dx where R CxD is the sheet resistance of the layer at a distance amount of x from the surface. After measure ments of sheet resistance values for every sectioning step, by calculation of the resistivity and by using Irvin Curves the profile is determined point by point. The fifth chapter has explanations of the anodi - zation cell, electronic configuration, choice of elect rolyte and anodi zati on technique. Anodi zati on cell consists of a beaker and a teflon holder. A voltage source is converted to a current source for constant current anodi zation. During anodi zati on the current and the cell voltage were measured continiously. As e- lectrolyte ethylene glycol + 0,04 N KNO + 2,5 % water is used. Anodi zati on is carried out for 3-5-7-9 mA/cm current densities. In the sixth chapter the experimental work was done is mentioned. For calibration experiments uniform doped wafers are used. Calibration was carried out by p and n type wafers. For profiling, a p -n junction was formed on an n type substrate and a n -p junction was formed on an p type substrate by using borosilica film and phosphorosilica film as spin-on dopant sources. Before oxidation the wafers were cleaned carefully and dried. Calibration curve and a doping profile deter mined are given in fig..1 and 2. As can be seen in the figures anodic oxidation and profiling were done successfully. CixDNet Forming Voltage (V) 260 240- 220- 200 180 160 HO 120 100 80 60 40 20 H 100 200 300 400 500 600 700 800 900 1000 1100 1200 Oxide Thickness (A0) Fig. 1. Calibration curve. CxZ>10 15 20 25 30 Number of Steps Fig. 2. Doping profile of the p+ side of a P+-n junction. Cxi D
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