What major is beneficial for a long-term career goal in medical robotics/ robotics/ artificial intelligence? I have done my undergraduate degree in a Indian university in electronics and communication engineering and have done a job in one of my field of interest medical image processing and I worked in robotics while doing my degree projects. Now I am going to apply for masters in US universities but I am confused about what major I have to choose. My long term goal is to be a researcher in medical robotics/ robotics/ artificial intelligence also I can work with nano electronics but I have not done any work or not so much study in that before so I am a bit nervous to choose that. 

As after masters I will go for PhD so please help me to resolve which of the above field will be favorable. As I have doubts like medical image processing is not a vast and not so many funding is there for PhD students.
Robotics in itself is an inter-disciplinary field, which I assume is why you are asking the question and have no just found it online.  There are some programs specializing in robotics, but the main question is what part of robotics you want to work on.  Control, Design, Theory, etc..  Humanoid robotics is different than nano robotics.    

If your end goal is Nano electronics, look into schools that specialize in... Nano electronics.  In that field as well, you should figure out what aspect you are interested in.  Department of Physics will sometimes have these majors, or some inter-discipline schools may have a special Nano program.

Purdue has a Nanoelectronics field inside engineering: https://engineering.purdue.edu/NRL/Students/index.html

Carnegie Mellon Univ. has a Nanorobotics lab inside mechanical Engineering:
http://nanolab.me.cmu.edu/

Utah State has a bionanoelectronis lab in its Biological Engineering lab:
http://be.usu.edu/htm/research/research-labs

As you can see, "nano" is a hot topic, and can be approached by different fields.  First figure out what aspect your intersted in, and if you do not want to go directly to PHD, you can major is a more generalized aspect, either Mechanical engineering, Physics, Chemistry, or Electrical Engineering.

If your interested in Artificial Intelligence, then you may want to go into Computer Science. 

If you want build medical robotics, it may be useful to have some medical training ontop of your EE background, which may make you attractive to interdisciplinary labs.  Or you can do Mechanical Engineering.

Saying your interested in Robotics, is a bit like saying you want to "Build Things".  You want to build Sculpture, Houses, Sky Scrappers, Bridges, Circuit Boards. Etc..