I have a Canon 700D and YN-568EX ii external flash that I am using in slave mode. But whenever I try to fire the built-in flash pops up automatically and fires itself. Is there any option to use only external flash?
Yes. Use radio triggers.
If you want to avoid firing the pop-up flash completely, then use a radio transmitter in the hotshoe that triggers a radio receiver attached to your off-camera flash, usually by the foot or the sync port. The Yongnuo YN-622C triggers are great for remote controlling with full TTL and high-speed sync (HSS), and the YN-685 has one built-in (the YN-568EX II, otoh, does not). Or you can use simple manual-only triggers, like the RF-603II or RF-605, if you don't need to remotely control the power, TTL, or HSS. (See also the Flash Havoc gear guides).
Radio triggers are favored among strobists more because radio increases the reliability and range of remote triggering, and eliminates the line-of-sight requirements optical triggering methods have.
The pop-up flash is unavoidable with optical triggering because the pop-up flash's light is how the camera communicates with the flash to tell it to fire. If you're using Canon's wireless TTL scheme (Sc mode on your YN-568EX II), there are multiple pre-flashes to send the flash settings you've made on the camera to the flash before the main burst. If you're using the "dumb" S1/S2 modes, then the pop-up flash burst itself is what's triggering the remote flash to fire, either immediately (S1) or after single TTL-metering pre-flash (S2).
However. If you're using the Canon wireless scheme which allows for setting the power of the remote flash, HSS, and TTL, you can also tell the camera to only send the settings and the fire command, and not to fire on the main burst.
In the menus go to Camera1 → Flash Control.
Go to Built in flash settings.
Set Wireless func. to show the icon for only the speedlight:
(If you're shooting Nikon, the process is similar, and the setting you're looking for the pop-up flash's power output is - -).
This should eliminate any light from the pop-up contributing to the image you take at normal subjects distances.
The only caveat is that if you're shooting at macro distances, there may still be some light, as the signal for the remote flashes to fire may spill onto the subject. At this point, you may have to come up with some way to flag off the pop-up's light so that it still reaches the remote flash, but doesn't hit your subject. You can also try covering the pop-up with an IR-pass filter, or, as I stated at the top, resort to radio triggers.
I have the same camera as you.
Under built-in flash settings in the menu you need to select the "custom wireless function" and select only the external flash (and NOT the easy wireless flash setting). By default both of them fire as you have noticed, so make sure that the icon there shows only the external flash.
Notice that even when you select only the external flash, you will still see the built-in flash fire. That would be a "control" flash which has very low power (needed just to control the external flash) so it does not really affect your exposure.
The icons I am talking about are seen here http://www.learn.usa.canon.com/resources/blogs/2013/20131217_chan_incameraflash_blog.shtml (scroll down to "The built-in pop-up flash" paragraph)
See also your manual at chapter 8 page 234 that explains what you are asking.
No, there is no way to use your YN568ex II as a slave flash unless you also use the pop up flash on your 700D. (unless you buy a wireless radio transmitter/receiver)
Your flash actually has 4 modes where it can be remotely triggered but all of them require a flash of light from another source which is connected to the camera.
Four remote operating modes:
• Sc (Canon ETTL Remote)
• Sn (Nikon iTTL Remote)
• S1 (optical slave – manual power – triggered by first flash)
• S2 (optical slave – manual power – ignores TTL pre-flash – triggered by second flash)
You need to read the manuals for both the camera and flash to understand how they can be used together as a wireless optical Slave/Master.
The simplest way just to get started is the use your camera flash in ETTL mode and the YN568 II in manual mode set to S2 slave mode.
Much more complex is to use your camera flash set to Master mode and then use the flash in Sc ETTL slave mode. The camera and flash will communicate using coded light pulses. It is possible to set the camera flash so that it contributes almost no light to the scene but there will still be a visible flash in order to trigger the slave flash.
Here is a basic video that describes the basic steps to get the Canon wireless system working for you: Yongnuo 468II or YN468II - Quick Off Camera Flash Setup with T5i(700D)
Here is an even more detailed video discussing more complex ways to use the Canon wireless system: Yongnuo Flash - YN468II, 568 and 565 (Long discussion of flash and flash options)
For you to use any flash in Slave mode there must be a master flash to control it! Your camera is popping up the built in flash because it senses you need flash to take the shot, you've enabled automatic flash, and the camera has nothing attached to the hot shoe so it has no other way of communicating with the YN 568EX II.
It sounds like what you really want to do is use a single flash off camera. The only way to do this with the YN568EX II is to either use a set of wireless radio triggers or use a Canon ST-E2 transmitter or Yongnuo equivalent.
The YN568EX II has a built in optical receiver, and so there needs to be an optical Master on the camera's hot shoe (or connected via a radio trigger that makes it appear to the camera to be mounted on the hot shoe) to control the YN568EX II in Slave mode. The ST-E2 uses near-infrared, rather than visible light, to communicate with any Canon E-TTL compatible optical slave flash including your YN568EX II. Absent a Master flash, optical transmitter (or wireless radio transmitter) mounted on your hot shoe, the camera will pop up the built-in flash to use it to communicate with the slave flash.
For manual control only you can use just about any set of manual radio triggers. For various levels of control - from controlling only the manual flash power, to also controlling such things as flash head zoom, to controlling every aspect of the flash just as if it were attached directly to the camera's hot shoe there are various radio triggers available.
For full E-TTL functionality, the most budget conscious solution is probably a Yongnuo YN622C-TX on the hot shoe and a YN-622C on the flash. The YN622C can be used as either a transmitter or receiver, so a set of two of them would also work. But the YN622C-TX also has an LCD panel that allows you to visually see the current settings, and the cost of a single YN622C-TX is about the same as a single YN-622C. Similarly, a set of two YN-622Cs cost about the same as a set with a single YN622C-TX and a single YN-622C. If you later want to add more flashes, using the YN622C-TX on the camera makes it much easier to control groups and ratios than using a YN-622C on the camera in the transmitter role.