Make custom button on Tab Bar rounded

Here is what I am trying to do:

Note: The screenshot is taken from an earlier version of iOS

What I have been able to achieve:

Code:

 override func viewWillAppear(animated: Bool) {
    // Creates image of the Button
    let imageCameraButton: UIImage! = UIImage(named: "cameraIcon")

    // Creates a Button
    let cameraButton = UIButton(type: .Custom)
    // Sets width and height to the Button
    cameraButton.frame = CGRectMake(0.0, 0.0, imageCameraButton.size.width, imageCameraButton.size.height);
    // Sets image to the Button
    cameraButton.setBackgroundImage(imageCameraButton, forState: .Normal)
    // Sets the center of the Button to the center of the TabBar
    cameraButton.center = self.tabBar.center
    // Sets an action to the Button
    cameraButton.addTarget(self, action: "doSomething", forControlEvents: .TouchUpInside)

    // Adds the Button to the view
    self.view.addSubview(cameraButton)
}

I did try to create a rounded button in the normal way, but this was the result:

Code Snippet for rounded button:

//Creation of Ronded Button
    cameraButton.layer.cornerRadius = cameraButton.frame.size.width/2
    cameraButton.clipsToBounds = true