phantomjs.js

driver.FindElement(By.Id("Button_To_Click")).Click();

var wait = new WebDriverWait(driver, TimeSpan.FromSeconds(15)); //just make it longer then you think the Ajax call will take

//if this ajax call is seen before 15 seconds are up, the driver will stop waiting and create the ajaxItem object
var ajaxItem = wait.Until(driver => driver.FindElement(By.Id("Data_Holder_Name")));