PDMS Soft lithography can be divided into the following main steps ：

1. Preparation of silylation mold
2. PDMS And curing agent
3. Degassing to remove bubbles
4. Pour... On the mold PDMS
5. PDMS bake
6. PDMS Peel off the mold
7. PDMS Cutting and punching
8. PDMS Bond

1. Preparation of silylation mold

When the mold is used for the first time , It must be made hydrophobic .

For mold , You can buy it from some enterprises or process it yourself .

in fact ,SU-8 The silicon of the mold is usually very hydrophilic , therefore ,PDMS Good affinity with silicon , This affinity is so great that it is impossible to peel from silicon PDMS, And what you can do is break the mold and go back to the beginning of the mold processing process .

therefore , The hydrophobic treatment of the mold is required .

There are several ways to make silicon hydrophobic . We use the functionalization of silane （ This function can realize permanent bonding on the surface ）, It can make the hydrophobic contact angle on the surface exceed 90 Over degrees . Gas or liquid can also be used to treat , This means that you can bond the silane function to the surface through gas or continuous water bath .

We can simply look at these two solutions . For humans , Silylation uses very dangerous products , It must be used under appropriate conditions, such as ventilated kitchen . If there are any questions , Please communicate with others in time .

Liquid silylation	Gas silylation
You can follow these steps to use some OTS(octadecyltrichlorosilane Octadecyltrichlorosilane ）	Silanization in gas , You can use the following steps Some volatilize at room temperature and atmospheric pressure TMCS(TriMethylChloroSilane, Trimethylchlorosilane )
1、 To the knot (crystallizer) Inside 50mL Dilute in cetane solution 0.5mL Of OTS	1、 Put the wafer in a Petri dish
2、 If possible , Please exhaust the air in the mold with a nitrogen gun	2、 In a dish , Add a few drops around the wafer TMCS
3、 Place the wafer in the mold and seal it with paraffin belly 15min	3、 Sealed Petri dish 15min
4、 Put the garden in pure cetane solution	4、 Open the Petri dish and wait 10min, Until all TMCS Evaporated
5、 Put the product circle in acetone solution	5、 Test the contact angle ： The mold is ready
6、 Put the round in an alcohol solution
7、 Dry wafers
8、 Test the contact angle ： The mold is ready

If the contact angle is not determined , Please check your test method and re execute .

Please don't try to PDMS Put it on the mold .

Silane reacts with air , therefore , Make sure it's really sealed TMCS or OTS Bottle and seal with paraffin film , For better security .

Liquid method is the most effective method , But more time and materials are needed to complete , The gas option is the most commonly used , With a little practice , You can get the same result as the liquid .

2. PDMS And curing agent

Once your mold is ready , We can process PDMS 了 .PDMS Is the name of the polymer . To make it harder , You must add a curing agent .

about PDMS Processing of ,PDMS The general ratio of and curing agent is 10：1（ weight ）. For some specific applications , This ratio can be changed , In order to make PDMS Softer or harder .

Please follow the following formula to process your PDMS：

(1) weighing PDMS( for example 60 g )

(2) Add curing agent （ for example 6 g ）

(3) Mix well PDMS And curing agent

It looks easier , But first, put PDMS, Then put the curing agent . If these two steps are reversed , It may lead to polymer crosslinking good . To mix the two components , You can also use automatic PDMS Mixer .

3. Degassing to remove bubbles

Due to strong mixing , You are preparing a mixture PDMS And curing agent , There will be a lot of bubbles , But these bubbles must be removed , Because if you don't remove bubbles , They will be trapped in PDMS In chip .

There are several ways to remove PDMS Bubbles in the mixture , You can use centrifuges or dryers and vacuum pumps . The second option is the simplest and most frequently used . actually , Centrifuges are suitable for small weight PDMS Go soak and say , It would be more appropriate .

Use a dryer to remove PDMS Bubbles in the mixture , Cover the glass cover and seal it . Use a vacuum pump or vacuum line for vacuum operation , If the vacuum is too strong or too fast , Then be careful , Don't make PDMS overflow .

Under the condition of maintaining vacuum , Usually 30min You can remove PDMS All bubbles in the mixture .

Once the bubbles are removed , You can turn off the vacuum , And bake PDMS.

In this step , Be careful , When the pressure is unstable , Do not open the dryer , Otherwise, your PDMS Could blow up .

4. PDMS Copy ： Pour... On the mold PDMS

Be sure to clean the mold before use , Use nitrogen or clean compressed air to remove SU-8 All dust and particles on the surface of the die .

once PDMS After degassing , You can put PDMS Cast on previously silylated SU-8 In the mold .

hold PDMS Put it in a container, such as a Petri dish , The size of the container will determine what you need PDMS And the thickness of the device you want .

about 4 inches In terms of the size of the base ,40 g Of PDMS Will get about 5mm Thick device . During pouring , If bubbles appear , You can use a steel needle to remove bubbles or put the wafer back into a vacuum vessel .

If you choose the second option , Be careful , Prevent wafers from floating and PDMS Into the bottom of the wafer , Because once this happens , Release later PDMS when , It will be more difficult .

5. PDMS Copy :PDMS bake

once PDMS Mixed with curing agent , The cross-linking reaction has already begun , But need 24 Hours to get a hard enough device .

That's why molds and PDMS The reason for baking . Using the same tools , The time and temperature of baking vary with the laboratory and users . I I will try to give some suggestions for the success of this step .

Usually use a hot plate or oven to bake PDMS, Multiple molds can be baked at the same time in the oven .

For the setting of temperature and time , It depends on the specific instrument and equipment , High temperature baking can be carried out in the shortest time , It can also be baked at low temperature for a long time . Through the experimental test , Under different baking conditions , measurement PDMS Contact angle between layer surface and water , No change in contact angle was measured , therefore , Baking will not affect PDMS The chemical composition of , But it will affect the mechanical properties .

in fact ,PDMS After baking ,PDMS It will soften more or less , therefore , It will be easier to operate more or less . in any case ,PDMS Will eventually be developed with exactly the same mechanical properties , It may take longer .

Be careful not to bake too much PDMS, otherwise ,PDMS Will change “ The old ”, This means that there is a right PDMS When punching , It will be very difficult . Use the old one PDMS plate （ A few months later PDMS plate ） To do this , The same phenomenon will be observed .

The temperature must be selected according to the utensils of the mold , If you are using a Petri dish , The heating temperature should not exceed 9℃. I suggest you in the oven 80C Bake at a temperature of 2h. After baking and cooling , You can operate your PDMS 了 .

6. PDMS Peel off the mold

To be moulded and PDMS When it cools down , You can cut the substrate , Then peel PDMS In order to get your device .

A scalpel can be used to cut PDMS. In the process of cutting , Be careful , Keep the blade tangent to the die , Don't break the mold .

 

In the process of stripping , Just gently pull PDMS You can easily take it out . If PDMS Firmly adhere to the wafer , Maybe it's because you didn't silanize the mold or PDMS Not well made .

In this step , Be sure to wear gloves to operate , So as not to be in your PDMS Add fat fingerprints or other marks on the . Because these fingerprints or marks are difficult to clear Besides, it will cause great problems for subsequent processing .

7. PDMS Cutting and punching

here , We already have everything the device needs PDMS, You have to cut to get different chips . As before , You can use the scalpel to do a little .

 

In this step , If you want to add or remove liquid later , Then it must be in PDMS Several small holes are machined in the , That is to say PDMS Punch holes . Do that , The appropriate drilling machine can be selected according to the size of the conduit used . because PDMS It's a soft material , So when punching , The size of the hole needs to be machined slightly smaller than the size of the conduit , such , The catheter will remain firmly on the chip and will reduce leakage problems . Be sure to wear gloves during this step , Not in PDMS Leave fat marks on it , Otherwise it will be difficult to remove .PDMS Fat marks left on the And so on will have a certain impact on the subsequent processing process .

8. PDMS Bond

The last step is to bond your PDMS The device uses this to obtain a sealed channel or channel .PDMS Can be bonded and pasted to another piece PDMS Or on glass , The bonding methods or rules of both are the same . Before bonding , Every part must be well cleaned in order to remove all... From the surface dust 、 Grain . Isopropanol can be used (IPA) To clean your PDMS And glass pieces , You can even use ultrasonic cleaning fluid to clean the surface and PDMS Clean the inside of the hole .

For bonding PDMS, You need to activate PDMS Or the surface of glass , take PDMS Of Si-CH3 Functional groups are converted to Si-OH functional group , When you press the same functional group , Will create a powerful and permanent Si-O-Si key . So , The most common tool is to use plasma cleaning machine with oxygen or air function .

In order to produce better plasma , You must control the pressure in the chamber of the plasma cleaner . It is suggested that 500mTor Under the pressure of , Some devices can help You can achieve perfect plasma treatment, such as Equinox. In order to functionalize the surface , The plasma treatment time does not have to be very long ,2 Minutes of processing time can be compared with glass Or glass PDMS Form a strong bond . however , Under certain experimental conditions , The plasma treatment time may also be less than 2 minute .

 

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Reference resources

PDMS Chip processing process : Use SU-8 The mold is PDMS Lithographic reproduction of （ Soft lithography process ） - Wenhao Co., Ltd PDMS The process of lithography replication ： Tips and tricks PDMS It is a kind of polymer widely used in the field of microfluidics , It is often used to manufacture microfluidic devices such as lab on chip. Every year, , Many laboratories have begun to engage in microfluidic activities , And sometimes , They don't have good experimental equipment or know good experimental methods , The purpose of this article is to provide an overview of how to do PDMS Lithography replication provides basic knowledge ,PDMS Lithography replication is also known as soft lithography . Here are some tips and tricks for you , send PDMS Lithography copying is as easy as pressing a button .PDMS Lithography copy https://www.whchip.com/news/PDMSruanguangke.html