Çeşitli lazer parametrelerinin silikon malzeme delme sürecine etkisi
Effect of various laser parameters to the process of drilling silicon materials
Yazar: AYŞEGÜL SAYDAM
Danışman: YRD. DOÇ. DR. ALİ FUAT ERGENÇ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Mekatronik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Fizik ve Fizik Mühendisliği = Physics and Physics Engineering ; Mekatronik Mühendisliği = Mechatronics Engineering ; Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Ana malzemesi silisyumdan (Si) oluşan silikonun atomik yapısı, silikonu ideal bir yarı iletken haline getirir. Hem ucuz olması, hem de iletkenlik özellikleri nedeniyle, günümüzde ise birçok alanda; özellikle de ileri teknoloji sanayilerinde, bütünleşik devrelerde, MEMS (mikro-elektromekanik sistem) üretiminde, opto-elektronik bileşenlerde, dedektör ve sensörlerde ve güneş pillerinde yaygın olarak kullanılmaktadır.
Bu silikon tabaka, üstünde ya da içinde mikro cihazların yapılandırıldığı bir alt tabaka olarak işlev görür. İyon katkılanması, aşındırma, çeşitli materyallerin kaplanması (deposition) ve fotolitografik desen oluşturma gibi birçok işleme maruz kalır. Son olarak da her bir silikon mikro-cihaz dilimlenir ve paketlenir.
Silikon tabakaların, yukarıda bahsedilen bütünleşik devrelerde, MEMS üretiminde ya da güneş pillerindeki gibi kullanımlarında bu tabakalara mikro-delikler açmak gerekir. Bu mikro-delikler çeşitli yöntemlerle açılabilir; kuru ya da ıslak aşındırma, kum püskürterek temizleme ve lazer işleme gibi. Aşındırma süreçleri hem yavaş, hem de pahalı ve maske gerektirirken, kum püskürtmede ise temiz oda süreci ile uyumsuz, minimum delik çapı sınırlı olup, kenarlar da konik olmaya meyillidir. Üretimde delme sürecinin hızlı olması ve delik kalitesinin iyi olması nedeniyle, lazerle delme işlemleri son zamanlarda en çok tercih edilen yöntemlerden biri haline gelmiştir.
Bu çalışmamızda mikro-cihaz yapımında kullanılan, silikon tabakalara farklı lazer türleriyle delik delme süreci, delik çap ve kalitesine etki eden lazer parametreleri ve bu sürecin optimize edilmesinde, parametrelerin etkisi deneysel ve teorik olarak araştırılmıştır.
Silikon yüzeye delik açma işlemi için, Nd: YAG diyot pompalı ve Nd: YAG fiber lazer kullanılmıştır. Tek bir lazer türü ile çalışırken; atım genişliği, lazerin ortalama gücü gibi lazer parametreleri birbirinden bağımsız olarak değiştirilip, delikler optik mikroskop ile incelenerek, delik çapları kaydedilmiştir.
Çalışmalarda lazerle delme yöntemlerinden; kuyu delme burgusu olarak adlandırılan 'trepan' delme ve içini doldurarak delme ya da diğer bir deyişle kazıyarak delme yöntemleri kullanılmıştır. Kullanılan bu yöntemlerde içini kazıma yöntemi, diğer yönteme göre daha temiz, daha yuvarlak, daha hassas deliklere ulaşılmasını sağlamıştır. 
Diğer parametreler değiştirilmeden, lazerin ortalama gücü arttırıldıkça, mikro-delik çaplarının da arttığı gözlemlenmiştir. Yine diğer parametreler değiştirilmeden, atım genişliklerinin azaltılması ile mikro-delikler etrafında ısıdan daha az etkilenmiş bölgeler oluşmuş, dolayısıyla da daha kaliteli mikro-delikler elde edilmiştir.

Abstract:
The atomic structure and the cost efficiency of the silicon materials utilized in the integrated circuits, solar batteries, detectors and the production of micro-electromechanical systems (MEMS) which has many fields of application such as biomedicine, biology, biochemistry, automotive, aerospace and photonics makes it an ideal semiconductor  material. What makes silicon ideal is its electrical, optical and mechanical properties connected to its atomic structure which makes it semi conductor.
The micro-electromechanical systems may contain other materials compatible with the silicon such as silicon based aluminum, silicon nitrite, silicon carbide, silicon oxide.
According to application and production processes, different types of silicon may be chosen as mono-crystalline, poly-crystalline or amorphous. 
The silicon materials are subjected to many processes such as ion contribution, etching, deposition of various materials and creating a photolithographic patterns in the production of these micro devices.
It is required to open microholes to silicon layers for the implementations on integrated circuits or production of micro-electromechanical systems and solar cells. Such microholes may be drilled with various processes such as dry or wet etching, laser drilling or sandblasting. Yet, one of these methods, the etching method is both expensive and slow and requires masks. The sandblasting method also has some disadvantages such as having taper sides or limited minimum diameter of microholes.
The choice of laser, parameters of laser to be utilized in the process of laser drilling or the thickness and properties of the material to be utilized and the location of the material in regards to focus affect the process of drilling.
Various defects utilized for the structure and geometry of the microholes obtained in the process of drilling indicate the microhole quality. These defects consists of the terms of taper, microcracks, spatter and recast layer. For example, locating the material in regards to the focus may affect the size, depth and  taper of the microholes obtained. In a same way, the laser power may affect the size, depth and taper of the microholes obtained  and  the creation of defects stated above. The pulse width is more effective for creating thermal damage at the perimeter of microholes. The smaller the pulse width, the smaller the damage given to the aimed area by the laser beam as known as from the femtolasers (10-15s). The pulse width is also a parameter significantly effective over the microholes. 
The drilling methods affecting the drilling process has effects over the diameter and quality of the microholes. For example, the single shot drilling method does not produce microholes with good definition, the amount of defects stated  above is increased. The taper is smaller and thicker materials may be drilled with the percussion drilling method. The drilling methods of trepanning and helical trepanning are more effective for thicker materials and able to obtain bigger and smoother holes.
The choice of laser shall have effective results of the materials utilized. For example, the literature contains many studies conducted with infrared lasers, UV lasers and ultrashort lasers over silicon materials.
In addition to these methods, the type, thickness and smoothness of the material utilized have impacts over the method of drilling; for example the type of laser utilized on a thick material or choice of parameters has a significance. For example, decreasing the power of the laser gradually while drilling a thick material is effective for some of the hole properties such as taper and depth. In case a hole is desired to be opened thoroughly, the power of the laser has a significant impact. The factors such as type and thickness of the material were not examined in this study.
With the consideration of all these factors affecting the period of drilling, the experiments and impacts of each parameter over the microholes were discussed in many studies. The purpose is to render the process of drilling to its most efficient way and obtain holes in aimed depths and sizes with lesser defects.
The combination of different laser parameters were experimented in experimental studies and the process is desired to be become effective with the optimal parameter combination. Reaching such combinations requires long and systematic studies, the knowledge of these parameter values and properties of the laser utilized has tens this process.
Along with the development of micro-electromechanical (MEMS) systems in recent years, silicon materials are preferred thanks to their properties stated above. The laser process became one of the most popular methods for many applications with the consideration of advantages of causing minor damage to the perimeter of the aimed area to be processed, being able to process even the hardest materials in regards to time and costliness and being able to obtain holes in desired diameters and depths. There are many types of lasers in the industry, the choice of laser conducted in the most suitable way for the properties of the materials increases these advantages even more. The parameters of the laser affecting the process of drilling, methods of drilling, the defaults affecting the quality of the drill hole are researched theoretically. 
In experimental studies, Nd: YAG fiber with an average power of 20 Watt and      Nd: YAG diode pump laser were utilized. In this section, how the average laser power which is one of the laser parameters affecting the process of drilling, affects the diameter and quality of the microholes. The parameters values such as average power, pulse frequency, scan speed of the lasers we have utilized may be adjusted in a finite way and thus eased the ability to obtain aimed microholes.
The experiments were conducted systematically, the average power was decreased in determined intervals and tests were conducted over two sample materials of same properties for each power value. The diameters of the microholes obtained were measured vertically and horizontally with a microscope and the relation between the average laser power and diameters of the holes was examined. In addition, the effect of pulse width which has a very significant impact on this process, over the diameter and quality of microhole were examined experimentally. The experiments realized independently from each other with the utilization of different laser parameters and silicon materials of different thicknesses the parameter value which is desired to be studied was changed and other parameters were kept stabile. 
The repetition of this process for both lasers and having coherent results obtained was beneficial for understanding the parameters of average power and pulse width affecting the drilling process and microholes. 
Contrast to the utilization of silicon materials of different thicknesses in our study, the effect of the change of material thickness over the microholes was not examined. The parameters such as pulse frequency, scan speed, location of the material by the focus which may have effect over this process was not examined in this study.
The effect of drilling methods to the diameter and quality of the hole was discussed theoretically and in addition to the trepanning method of the experiments, the method of drilling by incising the aimed figure was utilized for the experiments. The effect of the methods utilized over the quality of microholes was not discussed in detail, yet it was observed that method of incising was more effective for obtaining holes with more smooth and decent holes by the observation of the results with an optical microscope.