真空技术基础-7-2012

1、q 真空的基本知识真空的基本知识q 稀薄气体的基本性质稀薄气体的基本性质q 真空的获得真空的获得q 真空的测量真空的测量第四章 薄膜制备技术 为了使被研究的样品不被周围气氛所污染，为了使被研究的样品不被周围气氛所污染，获取获取“原子清洁原子清洁”的表面，薄膜制备和衬底表面形的表面，薄膜制备和衬底表面形成过程往往是在真空或超高真空中进行的。目前，成过程往往是在真空或超高真空中进行的。目前，人们所广泛使用的薄膜制备系统都具有真空系统。人们所广泛使用的薄膜制备系统都具有真空系统。F 真空：真空： 低于一个大气压的气体空间。低于一个大气压的气体空间。F 真空单位：真空单位： 帕斯卡（帕斯卡（Pasca

2、l）：）：Pa 托（托（Torr）：）：1Torr=133.322Pa 旧的单位：旧的单位： Torr mmHg bar atm 1、 毫米汞柱（mmHg）：1mmHg=133.3Pa。2、 托（Torr）：1Torr=133.3Pa。3、 毫巴（mbar）：1mbar=7.510-1Torr=102Pa. Atmospheric: 760 Torr Low Vacuum: 1 to 1x10-3 Torr 真空的划分真空的划分 Medium Vacuum: 1x10-3 to 1x10-5 Torr High Vacuum (HV): 1x10-6 to 1x10-8 Torr Ultra-

3、High Vacuum (UHV): 200 C to remove adsorbed gasesSimple, inexpensive, oil-free沸石 扩散泵扩散泵(Diffusion Pumps)依靠被抽气体向蒸汽流扩散进行工作的. Momentum transfer to gas molecules through collision with directed jet of oil molecules Require cooling water, backing pump 10-3 to 10-7 Torr (to 10-9 Torr with LN2 cooling)q Ad

4、vantages Robust（高效） High pumping speed for relatively low cost. No vibration or noise.q Disadvantages Oil as a pumping medium, high risk of back-streaming oil, cold traps（冷阱）required Potential for serious vacuum problems 涡轮分子泵涡轮分子泵 Turbo Molecular PumpsMolecules mechanically pumped by collision with

5、 angled high speed turbine blades (rotor). Several rotor arranged in a series spinning at 30,000-60,000 rpm.Rotor tangential velocity is on the order of the average thermal velocity of molecules. Atmosphere to 10-10 TorrOil/grease/electromagnetic bearingsMost common HV/UHV pump.qAdvantages Correctly

6、 operated they do not back-stream oil into the vacuum system at any time. They can be started and stopped in a few minutes time. qDisadvantage Turbo pump can be noisy and they induce vibration. Turbo pumps are expensive. 离子泵离子泵 Ion PumpsHigh voltage between anode and cathode (5 kV)Electrons are capt

7、ured in anode and spiral due the to magnetic field (longer path-length).Gas molecules are ionized by collisions with electrons and are accelerated to cathode.Ions embedded in cathode material (titanium) and sputter titanium atoms from surface.Sputtered Ti atoms act as getter for reactive gases.10-4

8、Torr to 10-11 TorrqAdvantages Clean, oil-free. No moving parts, no vibrations, quiet. Low power consumption and relatively long operating livesqDisadvantage Do not pump noble gases well. Requires “regeneration” of Ti every 4-6 years. Heated Ti filament evaporates Ti film onto cooled surface. Ti gett

9、ers reactive gases by reaction. Operate at 10-8-10-11 Torr Inexpensive, reliable Periodic operation - not primary pumping mechanism 升华泵升华泵 Titanium Sublimation Pumps 真空材料真空材料 Outgassing rates （除气速率） Producing “virtual” leaks （产生气体虚漏） Mechanical stability（机械稳定性） Temperature stability（温度稳定性） Conductiv

10、ity（热导率） Chemical inertness（化学惰性） Weldability（可焊接性）真空材料应考虑的问题：真空材料应考虑的问题：辅助材料：辅助材料： Oxygen free high-purity copper (OFHC无氧高导电性铜) Be-Cu alloy Tantalum（钽）, Molybdenum（钼）, Tungsten （钨） Teflon (gassy)（聚四氟乙烯，特氟龙） MACOR (machinable glass composite) Alumina（氧化铝） Quartz, pyrex (gassy耐热玻璃) mu-metal magnetic

11、shielding (Co, Ni, Fe) Molybdenum disulfide（二硫化钼） (lubricant)mu：机组避免使用高蒸气压材料：避免使用高蒸气压材料： Zn, Cd: especially be careful of fasteners, bolts （紧固件、螺栓） Brass（黄铜） Certain solders（焊料） Any type of grease or oils（油脂）系统和管道的连接：系统和管道的连接：氟橡胶氟橡胶密封圈密封圈Pump Down（抽气）抽气） Typically follows a well-defined sequence acc

12、ording to the types of pumps on the vacuum system For UHV systems, typically requires a few hours to reach a medium vacuum after a vent to airBake Out Heat the chamber to temperatures between 100oC and 200oC for 1 - 2 days. Rapidly remove adsorbed gases from the chamber walls at high temperatures in

13、 order to lower the outgassing rates at room temperature. Generally it takes another day to cool and recover base pressure. 热电偶真空计热电偶真空计Heat filament with a constant current.Measure filament temperature with thermocouple.Gas molecules collide with and cool the filament.Voltage increases to keep fila

14、ment at constant current.Atm to 10-4 TorrFast, simple, inexpensive. 热电偶真空计热电偶真空计热电偶真空计的规管主要由加热灯丝C与D（铂丝）和用来测量热丝温度的热电偶A与B（铂铑或康铜-镍铬）组成。热电偶热端接热丝，冷端接仪器中的毫伏计，从毫伏计中可以测出热偶电动势。1）测量的压强是被测容器的真实压强；2）能连续测量，并能远距离读数；3）结构简单，容易制造；4）即使突然遇到气压急剧升高，也不会烧毁。热电偶真空计的优点：热电偶真空计的优点：1）标准曲线因气体种类而异，故对于空气测量得到的标准曲线不能直接用于其他气体；2）由于热惯性

15、，压强变化使热丝温度的改变将滞后，读数亦必定滞后；3）受外界的影响很大，所以规管必须安装在不易受热辐射或对流热的位置；4）老化现象严重，必须经常校准。热电偶真空计的缺点：热电偶真空计的缺点： 电离真空计规管电离真空计规管具有足够能量的电子在运动中与体分子碰撞，可能引起分子的电离，产生正离子及电子。而电子在一定的“飞行”路程中与分子碰撞次数，又正比于分子的密度，一定温度下也正比于气体压强，故产生的正离子数也正比于压强。由此可见，电离现象是与压强有关的现象，可作为一种真空测定的依据。热阴极电离真空计是测量极高真空的仪器，测量范围为0.1110-5 Pa1）测出的是总压强2）反应迅速，可连续读数，还

16、可以远距离控制；3）可测很低的压强。一般的电离计就可测量到510-5 Pa；4）规管小，易于连接到被测量处；5）一般电离真空计的校准曲线范围宽，通常在101 105 Pa之间；6）对机械振动不敏感；热阴极电离真空计的原理热阴极电离真空计的原理热阴极电离真空计的优点热阴极电离真空计的优点1）灵敏度与气体的种类有关；2）压强大于10-1 Pa时，灯丝易于烧毁，但一旦真空系统突然漏气，如不设置专门保护线路，规管灯丝往往立即烧毁；3）在工作时产生化学清除作用及电清除作用，造成压强的改变，影响测量准确度；4）玻璃壳、电极的放气，会导致产生测量误差；热阴极电离真空计的缺点：热阴极电离真空计的缺点： 质谱仪真空计质谱仪真空计 Quadrupole mass spectrometer - RGA (residual gas analyzer) 10-4 to 10-14 torr Total pressure mode integrates all ion intensities Partial pressure mode indicates residual vacuum composition Highly accurate,