6601期末必考点
1.光刻（Lithography）
三个主要过程：Coating $\rightarrow$ Expose $\rightarrow$ Developing
1.1 Light Intensity
Mercury Arc Lamp: V Light Wavelength大概在300-400nm之间时，也就是g-line、h-line、i-line光刻胶，用的是深汞灯(Hg lamp)进行光刻，光刻强度低(low intensity)，因此所需曝光时间长。
Excimer laser Source(准分子激光源): 而当UV Light Wavelength下降到300以下时，就是KrF光刻胶，用的是Deep UV(DUV)，也就是深紫外灯，其拥有更高的光刻强度(high intensity)，则所需曝光时间就很短.
1.2 Resists
如上图所示，左边是正性光刻胶，被UV光照的地方溶解，而有掩膜(Mask)遮盖的地方，也就是没有光照的光刻胶不溶解；而右边则是负性光刻胶，其工作机理与正光刻胶相反，被光照的光刻胶不溶解，而被掩膜遮盖的位置则溶解。
1.3 Eight Steps of Lithography
step 1: Vapour Prime
① Wafer dehydration bake；② HMDS adhesion clean and dry
step 2: Spin Coat
① Make PR to expected Thickness
另外，Resist layer thickness depends on the viscosity of resist and is inversely proportional to the square root of the spin speed, $t{\frac{\alpha}{\omega_2}}$.
Thinner Resist: ${I_R}\simeq{[\frac{viscosity\times{solid\,content}}{\sqrt{spin\,speed}}]}$，Spin speed越快越好，如果转速越慢，表面的光刻胶就会越厚。
step 3: Soft Bake
① Partial evaporation of resist solvents; ② Improves resist-to-wafer adhesion; ③ Improves etch resistance
短时间的Pre-bake可以防止由于残留在PR中的的solvent过多而导致的UV light reaching the PAC；
Over-bake则会增加sensitivity to UV light，甚至可能破坏PAC和减少solubility of the PR。
step 4: Alignment and Exposure
① Transfer pattern to wafer; ② Active PAC
step 5: Post-Exposure Bake
① Evaporate all solvnent
还能减少驻波(standing wave)效应的影响
step 6: Developing
① Remove the soluble portion with a solvent; ② Patterning
step 7: Hard Bake
① Evaporate solvnent from step6 adhesion; ② Etch resistance
step 8: Develop Inspect
① Check the quality of process to ensure the pattern is transfered.
1.4 Printing
注意：在Contact Printing和Proximity Printing中，掩膜被称作mask，但是在Projection Printing中，则被成为reticle，但实际上两者是一样的。
另外，在Contact Printing中，mask和photoresist是直接接触的；而在Proximity Printing中，mask和wafer并没有直接接触，这一改动可以prevent dust particles。
1.5 Minimum Linewidth / Resolution
Resolution for the proximity printer:
$W_{min}\simeq{\sqrt{k_1\lambda{g}}}$ ， $\lambda=\frac{c}{f}$， $E=h\frac{c}{f}$
$k_1$ is a constant
$\lambda$ is Wavelength of the exposure source
$g$ is Gap between the mask and the wafer surface
and more larger resolution is better.
1.6 Numerical Aperture
NA is a measure of the ability to collect light
$NA=nsin\theta$
Where $n=$refraction index (the medium in which the system is immersed, in this $n=1$ for air)
So n is for air or water not for LENS.
$NA=sin\theta\,{\simeq}{\frac{\frac{d}{2}}{f}}={\frac{d}{2f}}$
1.7 Depth of Focus
Depth of focus:
$\sigma=\pm{\frac{\frac{W_{min}}{2}}{tan\theta}}\simeq\pm{\frac{\frac{k\lambda}{2NA}}{sin\theta}}$
$\sigma=\pm{\frac{\frac{k\lambda}{2NA}}{\frac{NA}{n}}\simeq\pm{\frac{nk_2{\lambda}}{(NA)^2}}}$
$\lambda$越小或者$NA$越大，则$\sigma$越小；$\sigma$越小越好
1.8 光强度图
左边是Contact Printeing，右边是Proximity Printing。
上面的光强度图要求要会画，三种光刻模式都要会。
1.9 Modulation transfer function (MTF)
We hope projection printer shows no diffraction, like contact printer light and dark edge is very sharp.
$MTF=\frac{I{max}-I{min}}{I{max}+I{min}}$，MTF is for optical system(Lens) and bigger the better.
2 光刻胶技术（resist technology）
2.1 Contrast curve
A good resist, Dose critical is 50,
You shine 51 dose, all resist is soluble;
You shine 49 dose, nothing happened.
一个重要的参数：
Sensitivity: $D_{100}(D_f)$
Chemical contrast $\gamma$ $\rightarrow$ slope of curve defined as:
$\gamma=\frac{1}{log{10}\frac{(D{100})}{D_{0}}}$
2.2 Critical Modulation transfer function (CMTF)
从上面的 $\gamma$ 可以推导出CMTF：
$CMTF=\frac{D_f-D_0}{D_f+D_0}=\frac{10^{\frac{1}{\gamma}}-1}{10^{\frac{1}{\gamma}}+1}$
$MFT$ 的数值是越高越好，但是 $CMTF$ 却是越低越好。而且一般来说 $MTF\geqslant{CMTF}$ is required for the resist to resolve the aerial image.
2.3 Components of Resist
① Resin（树脂）
Give resist mechanical and chemical properties.
② Sensitiser（光敏）
Photo active compound (PAC/ PAG) .
③ Solvent（溶剂）
Keeps resist in liquid state；
Allows spin coating of the resist.
④ Additives（添加剂）
Improve Etch resistivity and implant blocking capability.
2.4 Advantages and Disadvantages of Resists
注意：正光刻胶是在光照中被溶解，而负光刻胶是在光照中被固化。
现在工业中更喜欢使用正光刻胶，因为有更好的Resolution。
2.5 Chemically Amplified DUV Resist（化学放大深紫外光刻胶）
关键是CA Chemical Amplify:
DUV makes PAG Photo Acid Generator to generate acid, PEB diffusion acid
2.6 Standing wave effect
解释：Input light reflected and diffusion with reflected light, standing wave formed Zigzag or swing curve
如何减少驻波效应:
① Dye PR more absorption
② PEB flat the zigzag（PEB also helps by smoothing out the zigzag due to resist thermal reflow）
③ ARC anti-reflective
