Mikro Ölçekte Elektrosprey Sistemi Üretimi Ve Karakterizasyonu

Abstract

Kapiler bir tüpün içerisinde bulunan iletken sıvıya yüksek voltaj uygulandığında kapilerin ucunda oluşan elektrik alandan dolayı sıvı koni şekline dönüşür. Eşik voltajı aşıldığında koni şeklindeki sıvının tepesinden yüklü sıvı parçacıklar ayrılmaya başlar. Bu olaya elektrosprey adı verilmektedir. Tek bir kapiler uçta oluşturulan elektrosprey yeterince verimli olmadığı için pratik olarak kullanımı azdır. Bundan dolayı ortak bir sistem içerisinde bir anda birden fazla elektrosprey oluşturma ihtiyacı oluşmuştur. Bu anlamda bugüne kadar yapılan çalışmalarda 3 farklı türde çoklu elektrosprey sistemi geliştirilmiştir. Bunlar lineer dizilimli, multi-jet modu ve düzlemsel dizilimli elektrosprey sistemleridir. Bu tez çalışmasında hedeflenen çoklu düzlemsel elektrosprey sisteminin parçalarından nozül ve ekstraktör elektrodunun RİDA (Reaktif İyonlar ile Derin Aşındırma) yöntemi ile üretimidir. Bu anlamda çalışmanın ilk bölümlerinde elekstrosprey oluşumu teorik ve matematiksel detayına girmeden anlatılmıştır. Ardından elektrospreyin, aşındırma yöntemlerinin, RİDA yönteminin ve elektrosprey sisteminin üretim prosesleri ile alakalı tarihten günümüze yapılan çalışmalar ve elde edilen gelişmeler konusunda bilgiler verilmiştir. Ardından elektrosprey sisteminin üretiminde kullanılan aşındırma işleminden detaylı olarak bahsedilmiştir. Bu başlık içerisinde ıslak ve kuru aşındırma yöntemleri karşılaştırılmış; bunun yanında kuru aşındırma adı altında bulunan siyah silisyum yöntemi, kriyojenik aşındırma ve Bosch prosesi’nin avantajları ve dezavantajları üzerinde durulmuştur. Yapılan bu karşılaştırmalar sonucunda kuru aşındırma yöntemlerinden Bosch prosesi’nin (diğer adı RİDA) çoklu elektrosrprey sistemi üretimi için diğerlerine nazaran uygun olduğu ortaya çıkarılmıştır. RİDA işleminde kullanılan işlem parametrelerinin aşındırma prosesine etkileri detaylı olarak anlatılmıştır. RİDA parametreleri: RFcoil, RFbias, basınç, aşındırma ve pasivasyon gaz debileri, altlık sıcaklığı, DC bias voltajı, aşındırma ve pasivasyon adım süreleridir. Bütün bu parametrelerin altlık ve maskenin aşındırma hızına, profil anizotropisine, yüzey pürüzlülüğü ve uniformluğuna, seçiciliğine ve kırılma mukavemetine etkileri anlatılmıştır. Bu çıkarımlar sonucunda mikro ölçekte çoklu elektrosprey sisteminin tasarımı ve kullanım alanları açısından ne tür sınırlamaları olduğu, bu sınırlamalara karşı nasıl bir tasarımın ve tasarım kriterlerinin olması gerektiği araştırılmıştır. Bu anlamda söz konusu tasarım kriterlerine uygun olarak elektrosprey sistemi parçalarından nozül ve ekstraktör elektrodunun RİDA yöntemi ile üretiminden bahsedilmiştir. Son bölümde literatür çalışmalarından elde edile veriler ışığında oluşturulan proses adımlarından ve bu prosese uygun olarak deneysel ve üretim çalışmalarından elde edilen sonuçlardan bahsedilmiştir. Yapılan çalışmalar neticesinde 250 nozül/cm2 yoğunlukta düzlemsel elektrosprey sisteminin parçalarının üretimi başarılmıştır.

The conductive liquid at the tip of the capillar deform into cone shape when the potential difference applied between liquid and collector electrode. If the potential difference reaches to threshold, charged droplets are spread from the tip of the cone. This phenomenon is called as electrospray. The sizes of droplets related to diameter of the capillary and liquid flow rate. It is not practical and to use a single electrospray, since the generated flow rate is not efficient for any applications. Therefore, it is necessary to use multiplexed electrospray sources simultaneously in a compact system. From the first studies to the present different approaches were brought for electrospray systems and 3 different multiplexed electrospray systems have been developed. Those are: linear arrayed, multi-jet mode, planar arrayed electrospray systems. When the capillars are ordered linearly, it is called as linear arrayed systems. Maximum 1000 nozzle/cm linear arrayed electrospray system is reported up until today. If the tip of a stainless steel nozzle (electrospray source) grooved, multiple electrosprays occur on each grooves on the nozzle. This is named as multi-jet mode. The nozzles fabricated and arrayed hexagonally (mostly) on a planar material, then it refers to planar electrospray systems. Beside the progressive improvements of electrospray technologies, new micro fabrication techniques have been achieved and developed in 1900’s. The plasma etching has a specific and important role among others for electrospray systems. In this technique reactive ions and neutral species can be accelerated to the surface of bulk sample in plasma environment. At the end of 1900’s there was a revolutionary method achieved which is called as Bosch process. In this method 5-15 seconds passivation and etching steps changes one after the other constantly. In passivation step thin film is deposited on the sample surface, thin film is removed and the surface etched during etching step. Within Bosch process or DRIE (Deep Reactive Ion Etching) it is possible to produce micro scaled and highly anisotropic features with narrow widths. It can be possible to fabricate high packing density multiplexed electrospray systems with DRIE technology. But as much it is getting denser the fabrication is getting more sophisticated and harder. In the related literature fabrications and characterizations of 100, 250, 1000 and 11000 nozzle/cm2 packing density planar electrospray systems were reported. It seems it will possible to produce much denser systems in the immediate future. Electrospray can be used for different applications such as: space propulsion systems for microsatellites, micro combustors, mass spectroscopy for chemical analysis, thin film deposition, electrospinnig applications for reinforcing the composite materials with micro and nano fibers and etc. The objects of this thesis study are to produce the nozzle and extractor electrode, which are the main pieces of the planar multiplexed electrospray system. First there is wide literature study is shown in the beginning of the first section after that there is literature summary shown which mention about the milestones, developments and informative datas about electrospray history, different electrospray systems, areas of usage, electrospray manufacturing processes, etching process, DRIE method for miniaturizing from the first studies to present. Thereafter the electrospray phenomenon was explained without detailed theoretical information and mathematical model. In the following part, detailed information was given to understand the etching technologies that are used to manufacture the pieces of electrospray system. Beside that the preparation steps such as mask selection, mask designing, optic lithography were explained. Under this title comparisons are made between wet etching and dry etching method. Wet etching was performed in a solution bath that is including different ratios of etching chemicals with water and the sample. Thus the etching occurs on the sample surface where the solution contact. Plasma etching was performed in a reactor with high vacuum. Reactive ions and species were delivered in the chamber; the plasma occurs with the help of two different RF sources. The ions react and sputter the sample surface, thus the etching happens. On the other hand the same comparisons are made between 3 different dry etching methods. Those methods are cryogenic plasma etching, black silicon and Bosch process. Cryogenic etching is carried out below minus 98$^\circ$C temperatures. In black silicon method, the passivation and etching take place at the same time with specific gas ratios. Bosch process was performed with constantly alternating passivation and etching steps. The advantages and disadvantages are discussed. After the comparisons, Bosch process was found suitable for manufacturing of micro scaled electrospray system. After that, DRIE parameters, such as RF$_{coil}$, RF$_{bias}$, chamber pressure, SF$_6$ flow rate, C$_4$F$_8$ flow rate, etching and passivation times, DC bias voltage observed in depth. Their effects on Si and photoresist etch rates, profile anisotropy, uniformity, surface roughness, selectivity ratio between wafer and mask and profile fracture strength are explained in depth. In the following section, first the manufacturing steps were shown and the parts of planar multiplexed electrospray system were acquainted. In the result of these inferences, it was researched that what sort of design criterias are appropriate against the restrictions. For this purpose different approaches were discussed. In this sense it is mentioned about manufacturing and bonding of two pieces of electrospray system the extractor and nozzle electrode with DRIE and precision bond aligning methods in accordance with design criterias. The DRIE fabrication process and different manufacturing methods were shown in figures step by step. The results of experimental and manufacturing process that is created from the data’s acquired from the literature study is shown in the last section. First the experimental plan are shown after that all the preparation steps are such as mask designing, hard mask deposition, photoresist coating, photolithography with used facilities and parameters are indicated. Before entering the results, the encountered challenges and problems were explained during experimental studies. Experimental studies was began from a reference DRIE recipe from literature studies. After the SEM characterizations, related DRIE parameters were changed in the following experiments. This method was implemented for nozzle and extractor electrode fabrication process. In consequence of the studies, the planar multiplexed electrospray system was produced with the packing density of 250 nozzle/cm$^2$. Cromium mask is used as photomask for lithography, <100> and <110> crystal oriented single side and double side polished silicon with 500-550 $\mu$m thickness was used as wafer material. AZ4533 with 3 $\mu$m thickness was used as photoresist and SiO$_2$ with 1 $\mu$m thickness was used as hard mask material. Experimental studies, characterizations and fabrication processes were performed in UNAM (National Nanotechnology Research Center) and ITU-MEMS (Istanbul Technical University-Micro Electro Mechanics) clean rooms laboratories.